SYSTRA
Training Material
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Contents
1
1.1
1.2
1.2.1
1.2.2
1.3
1.4
1.5
1.6
1.6.1
Introduction to GSM .................................................................. 8
Module Objectives ....................................................................... 8
Introduction .................................................................................. 9
Background and Requirements ................................................... 9
Advantages of GSM ................................................................... 11
Evolution of GSM ....................................................................... 11
Open Interfaces of GSM ............................................................ 14
GSM Technical Specifications ................................................... 16
Introduction to GSM Review ...................................................... 17
Review Questions ...................................................................... 17
2
2.1
2.2
2.3
2.3.1
2.3.1.1
2.3.2
2.4
2.4.1
2.4.2
2.4.3
2.4.4
2.4.5
2.5
2.5.1
2.5.2
2.6
2.7
2.7.1
2.7.2
2.7.3
2.7.4
2.7.5
2.7.6
2.7.6.1
2.8
2.8.1
2.8.2
2.8.3
2.8.4
2.8.5
2.8.6
2.8.7
2.8.8
2.9
Traffic Management ................................................................. 19
Module Objectives ..................................................................... 19
Introduction ................................................................................ 20
Mobility Functions ...................................................................... 24
Registration and Database ........................................................ 24
The Subscriber Identity Module ................................................. 25
Location Update ......................................................................... 27
Call Set-Up in a GSM Network .................................................. 29
Network Switching Subsystem (NSS) ........................................... 37
Locating the Subscriber ............................................................. 39
Base Station Subsystem (BSS) .................................................... 42
A Mobile Terminated Call and Paging ....................................... 46
Mobile Originated Call ................................................................ 48
Location Update ......................................................................... 50
Types of Location Update .......................................................... 50
Procedures ................................................................................. 53
Handover ................................................................................... 54
Charging .................................................................................... 60
What to Charge?........................................................................... 60
Subscription Charge .................................................................. 60
Renting of Service ...................................................................... 60
Charge for use of the network ..................................................... 61
Whom to Charge?......................................................................... 62
Charging Procedure in GSM ...................................................... 64
Distributed Charging .................................................................. 67
Network Architecture .................................................................. 68
NSS - Network Switching Subsystem ........................................ 69
BSS - Base Station Subsystem ................................................. 69
Network Management Subsystem ............................................. 71
Authentication Principle ............................................................. 74
Security Algorithms .................................................................... 75
Ciphering/Speech Encryption .................................................... 78
IMEI Checking ............................................................................ 79
User Confidentiality .................................................................... 79
Services ..................................................................................... 80
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2.9.1
2.9.2
2.9.3
2.9.3.1
2.9.3.2
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2.9.3.3
2.9.4
2.9.5
2.10
2.11
2.11.1
What Are Services? ......................................................................
Classification Of Services ..........................................................
Teleservices ...............................................................................
Speech (Telephony) and Emergency Calls ...............................
Short Message Services: Mobile Originated, Mobile
Terminated and Cell Broadcast .................................................
Facsimile Transmission (T61 and T62).........................................
Bearer Services .........................................................................
Supplementary Services ............................................................
Summary of the Learning Points ................................................
Traffic Management Review ......................................................
Review Questions ......................................................................
3
3.1
3.2
3.2.1
3.2.2
3.3
3.3.1
3.3.2
3.3.3
3.3.4
3.4
3.4.1
3.4.2
3.4.3
3.4.4
3.5
3.5.1
3.5.2
3.5.3
3.5.4
3.5.5
3.6
3.7
3.7.1
Signalling .................................................................................. 99
Module Objectives ..................................................................... 99
Introduction .............................................................................. 100
Standard Messages ................................................................. 101
Implementation and Evolution .................................................. 102
Common Channel Signalling System No.7................................. 104
Message Transfer Part (MTP) .................................................... 105
Telephone User Part (TUP) ........................................................ 106
Signalling Connection and Control Part (SCCP)......................... 107
Summary .................................................................................. 108
Other Applications of SS7 in GSM Networks ........................... 109
Base Station Subsystem Application Part (BSSAP) ................... 110
Mobile Application Part ............................................................ 111
Transaction Capabilities Application Part (TCAP) ...................... 111
Summary .................................................................................. 112
SS7 Layers in GSM Elements ................................................. 113
Protocol Stack in MSC .............................................................. 113
Protocol Stack in HLR .............................................................. 114
Protocol Stack in BSC.............................................................. 115
Other Signalling Protocols in GSM .......................................... 116
Summary .................................................................................. 117
Summary of the Learning Points .............................................. 118
Signalling Review ..................................................................... 119
Review Questions .................................................................... 119
4
4.1
4.2
4.3
4.3.1
4.3.2
4.3.2.1
4.3.2.2
4.3.2.3
4.3.2.4
4.3.3
Transmission .......................................................................... 122
Module Objectives ................................................................... 122
Introduction to Radio and Terrestrial Transmission ................. 123
Transmission Through the Air Interface................................... 128
Physical and Logical Channels ................................................ 129
Logical channels ...................................................................... 133
Broadcast Channels ................................................................. 135
Common Control Channels ...................................................... 136
Dedicated Control Channels .................................................... 137
Traffic Channels (TCH) ............................................................... 137
Time Slots And Bursts ............................................................. 138
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4.4
4.4.1
4.4.2
4.4.3
4.5
4.5.1
4.5.2
4.5.3
4.6
4.7
4.7.1
Problems and Solutions of the Air Interface .............................
Multipath propagation ..............................................................
Shadowing ...............................................................................
Propagation Delay ...................................................................
Terrestrial Transmission ..........................................................
Base Transceiver Station .........................................................
Transmission between BSC and BTS......................................
The Concept of Multiplexing ....................................................
Summary of the Learning Points ..............................................
Transmission Review ...............................................................
Review Questions ....................................................................
140
140
145
146
147
147
148
149
152
153
153
5
5.1
5.2
5.3
5.3.1
5.3.2
5.4
5.5
5.5.1
Network Planning ...................................................................
Module Objectives ...................................................................
Introduction ..............................................................................
Radio Network .........................................................................
Dimensioning Cells ..................................................................
Frequency Reuse .....................................................................
Summary of the Learning Points ..............................................
Network Planning Review ........................................................
Review Questions ....................................................................
155
155
156
161
161
163
165
165
165
6
6.1
6.2
6.3
6.4
6.5
6.5.1
6.6
6.6.1
6.7
6.7.1
6.8
6.8.1
6.9
6.9.1
6.9.1.1
6.9.1.2
6.9.1.3
6.9.1.4
6.10
6.11
6.11.1
Nokia Implementation ............................................................
Module Objectives ...................................................................
Introduction ..............................................................................
Network Architecture ................................................................
DX 200 Platform .......................................................................
DX 200 MSC/VLR Architecture ................................................
Functional Units in MSC/VLR ..................................................
DX 200 HLR/AC/EIR ................................................................
Functional Units in HLR ...........................................................
DX 200 BSC .............................................................................
Functional Units in DX 200 BSC ..............................................
Nokia NMS/2000 ......................................................................
Functional Units in NMS/2000 .................................................
Nokia BTS .................................................................................
Nokia BTS Families .................................................................
Nokia 2nd generation BTS. ..........................................................
Nokia Talk Family BTSs...........................................................
Nokia PrimeSite .......................................................................
Nokia MetroSite .......................................................................
Summary of the Learning Points ..............................................
Nokia Implementation Review .................................................
Review Questions ....................................................................
167
167
168
168
170
173
174
176
177
177
179
180
181
182
182
183
184
185
187
188
189
189
7
7.1
7.2
7.2.1
Next Step ................................................................................ 192
Module Objectives ................................................................... 192
Introduction .............................................................................. 193
High-Speed Circuit Switched Data (HSCSD) .............................. 195
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7.2.2
7.2.3
7.2.4
7.3
7.3.1
7.3.2
7.3.3
7.3.4
7.3.5
7.3.6
7.4
7.5
7.5.1
General Packet Radio Service (GPRS) ...................................... 196
Enhanced Data rates over GSM Evolution (EDGE) .................... 198
The Wireless Application Protocol (WAP)................................... 199
3rd Generation Mobile Systems................................................. 200
Frequency Allocation for 3rd Generation Systems.................... 201
The Universal Mobile Telephone System (UMTS...................... ) 202
Code Division Multiple Access (CDMA)...................................... 203
W-CDMA (Wideband CDMA)...................................................... 205
3G Network Architecture .......................................................... 206
3G Mobile Data Terminals ....................................................... 207
Summary of Learning Points .................................................... 208
Next Step Review ..................................................................... 209
Review Questions .................................................................... 209
8
8.1
8.2
8.2.1
8.2.2
8.2.2.1
8.4.1.1
8.4.1.2
8.4.2
8.4.2.1
8.4.3
8.4.4
8.5
8.5.1
8.5.2
8.5.3
8.5.4
8.5.5
8.5.6
8.6
8.7
8.8
8.9
8.9.1
Intelligent Network ................................................................. 210
Module Objectives ................................................................... 210
Introduction .............................................................................. 210
History ...................................................................................... 212
Basic telephone network.......................................................... 213
Example of introducing new service in the network and
problems it faces ...................................................................... 213
Concept of a central service provider ...................................... 214
Conceptual view of IN call ........................................................ 215
Intelligent Network Conceptual Models..................................... 216
Physical Entities ....................................................................... 217
Nokia Implementation of IN.......................................................... 218
SSP and IP (Service Switching Point and Intelligent
Peripheral) in MSC................................................................... 219
Software ................................................................................... 219
Intelligent Peripheral ................................................................ 219
SCP - Service Control Point ..................................................... 220
Core INAP ................................................................................ 220
SMP - Service Management Point ........................................... 221
SCE - Service Creation Environment ....................................... 222
IN Service Examples ................................................................ 223
New and existing services provided within IN.............................. 223
Flexible Billing ........................................................................... 224
Reachability/Mobility ................................................................ 224
Customised User Groups ......................................................... 224
Customised services for mobile user ......................................... 224
Final conclusive example:............................................................ 225
Summary of the Learning Points ............................................... 225
References ................................................................................ 226
IN Glossary ............................................................................... 227
Intelligent Network Review ....................................................... 229
Review Questions .................................................................... 229
9
9.1
SYSTRA Course Review ........................................................
Review Questions ....................................................................
8.2.2.2
8.2.2.3
8.3
8.3.1
8.4
8.4.1
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Acronyms .................................................................................
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1
Introduction to GSM
1.1
Module Objectives
At the end of the module the student will be able to:
8 (244)
•
Describe the evolution of the GSM network
•
List four advantages of GSM over analogue networks
•
Name and describe two open interfaces of GSM networks
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1.2
Introduction
1.2.1
Background and Requirements
At the beginning of the 1980s it was realised that the European
countries were using many different, incompatible mobile phone
systems. At the same time, the needs for telecommunication services
were remarkably increased. Due to this, CEPT (Conférence Européenne
des Postes et Télécommunications) founded a group to specify a
common mobile system for Western Europe. This group was named
“Groupe Spéciale Mobile” and the system name GSM arose.
This abbreviation has since been interpreted in other ways, but the most
common expression nowadays is Global System for Mobile
communications.
Figure 1.1 GSM – Global System for Mobile communications
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At the beginning of the 1990s, the lack of a common mobile system
was seen to be a general, world-wide problem. For this reason the GSM
system has now spread also to the Eastern European countries, Africa,
Asia and Australia. The USA, South America in general and Japan had
made a decision to adopt other types of mobile systems which are not
compatible with GSM. However, in the USA the Personal
Communication System (PCS) has been adopted which uses GSM
technology with a few variations.
During the time the GSM system was being specified, it was foreseen
that national telecommunication monopolies would be disbanded. This
development set some requirements concerning the GSM system
specifications and these requirements are built into the specifications as
follows:
•
There should be several network operators in each country. This
would lead to competition in tariffs and service provisioning and
it was assumed to be the best way to ensure the rapid expansion
of the GSM system; the prices of the equipment would fall and
the users would find the cost of calls reducing.
•
The GSM system must be an open system, meaning that it should
contain well-defined interfaces between different system parts.
This enables the equipment from several manufacturers to coexist
and hence improves the cost efficiency of the system from the
operator's point of view.
•
GSM networks must be built without causing any major changes
to the already existing Public Switched Telephone Networks
(PSTN).
In addition to the commercial demands above, some other main
objectives were defined:
•
•
•
•
•
•
•
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The system must be Pan European.
The system must maintain a good speech quality.
The system must use radio frequencies as efficiently as possible.
The system must have high / adequate capacity.
The system must be compatible with the ISDN (Integrated
Services Digital Network).
The system must be compatible with other data communication
specifications.
The system must maintain good security concerning both
subscriber and transmitted information.
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1.2.2
Advantages of GSM
Due to the requirements set for the GSM system, many advantages will
be achieved. These advantages can be summarised as follows:
1.3
•
GSM uses radio frequencies efficiently, and due to the digital
radio path, the system tolerates more intercell disturbances.
•
The average quality of speech achieved is better than in analogue
cellular systems.
•
Data transmission is supported throughout the GSM system.
•
Speech is encrypted and subscriber information security is
guaranteed.
•
Due to the ISDN compatibility, new services are offered
compared to the analogue systems.
•
International roaming is technically possible within all countries
using the GSM system.
•
The large market increases competition and lowers the prices
both for investments and usage.
Evolution of GSM
One key factor for the success of GSM was that the standardisation
work was not completed after 1989. It was initially decided that GSM
will evolve over time. With improvements in computing and radio
access technology, GSM will offer continuous improvement and more
services. In 1995 the “Phase 2” recommendations were frozen. The
GSM 900 and GSM 1800 specifications were merged and additional
supplementary services were defined, the short message service was
improved and improvements in radio access and SIM cards were
introduced.
After the Phase 2 recommendations, GSM continues to evolve at full
speed. Many new features are being introduced to GSM and the number
of improvements is so large that together they are called "Phase 2+"
features. These Phase 2+ features are frozen at regular intervals under
what are known as "Releases".
The following list highlights some important years in the short history
of GSM.
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1982
CEPT initiated a new cellular system. The European
Commission (EC) issued a directive which required member
states to reserve frequencies in the 900MHz band for GSM to
allow for roaming.
1985
CEPT made decision on time schedule and action plan.
1986
CEPT tested eight experimental systems in Paris.
1987
Memorandum of Understanding (MoU). Allocation of the
frequencies.
- 890-915 uplink (from mobile to base station)
- 935-960 downlink (from base station to mobile)
1988
European Telecommunications Standard Institute (ETSI) was
created includes members from administrations, industry and
user groups.
1989
Final recommendations and specifications for GSM Phase 1.
1990
Validation systems implemented and the 1st GSM World
congress in Rome with 650 participants.
1991
First official call in the world with GSM on 1st July.
1992
Worlds first GSM network launched in Finland. By December
there were 13 networks operating in 7 areas. Australian
operators were first non-European signatories of the GSM
MoU. New frequency allocation for GSM 1800 (DCS 1800).
1993
1994
- 1710-1785MHz (uplink)
- 1805-1880MHz (downlink)
GSM demonstrated for the first time in Africa at Telkom '93 in
Cape Town. Roaming agreements between several operators
are established. By December 1993 there were 32 GSM
networks operating in 18 areas.
The first GSM network in Africa was launched in South
Africa. The GSM Phase 2 data/fax bearer services were
launched. By December 1994 there were 69 GSM networks in
operation.
The GSM MoU is formally registered as an Association in
Switzerland with 156 members from 86 areas. The GSM
World Congress was held in Madrid with 1400 participants.
1995
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There were 117 GSM networks operating around the world.
Fax, data and SMS roaming was implemented. The GSM
phase 2 standardisation was completed, including adaptation
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for GSM 1900 (PCS 1900). The first GSM 1900 network is
implemented in the USA. Telecom '95 is held in Geneva
where Nokia demonstrates 33.6Kbits/s multimedia data via
GSM.
1996
By December 1996 there were 120 networks operating. The
8K SIM was launched in addition to Pre-Paid GSM SIM
Cards.
1998
Over 2million GSM 1900 users in the USA and a total of
120million GSM 900/1800/1900 users world-wide.
350
300
Million
250
200
150
100
50
1992
1994
1996
1998
2000
Figure 1.2 The Number of GSM Customers World-wide
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1.4
Open Interfaces of GSM
The purpose behind the GSM specifications is to define several open
interfaces, which then are limiting certain parts of the GSM system.
Because of this interface openness, the operator maintaining the
network may obtain different parts of the network from different GSM
network suppliers. Also, when an interface is open it defines strictly
what is happening through the interface and this in turn strictly defines
what kind of actions/procedures/functions must be implemented
between the interfaces.
Nowadays, GSM specifications define two truly open interfaces. The
first one is between the Mobile Station and the Base Station. This openair interface is appropriately named the “Air interface”. The second
one is between the Mobile Services Switching Centre – MSC (which is
the switching exchange in GSM) and the Base Station Controller
(BSC). This interface is called the “A interface”. These two network
elements will be discussed in greater detail in later chapters. The
system includes more than the two defined interfaces but they are not
totally open as the system specifications had not been completed when
the commercial systems were launched.
When operating analogue mobile networks, experience has shown that
centralised intelligence generated excessive load in the system, thus
decreasing the capacity. For this reason, the GSM specification, in
principle, provides the means to distribute intelligence throughout the
network. Referring to the interfaces, the more complicated the
interfaces in use, the more intelligence is required between the
interfaces in order to implement all the functions required. In a GSM
network, this decentralised intelligence is implemented by dividing the
whole network into three separate subsystems:
•
Network Switching Subsystem (NSS)
•
Base Station Subsystem (BSS)
•
Network Management Subsystem (NMS)
The actual network needed for establishing calls is composed of the
NSS and the BSS. The BSS is responsible for radio path control and
every call is connected through the BSS. The NSS takes care of call
control functions. Calls are always connected by and through the NSS.
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The NMS is the operation and maintenance related part of the network
and it is needed for the control of the whole GSM network. The
network operator observes and maintains network quality and service
offered through the NMS. The three subsystems in a GSM network are
linked by the Air, A and O&M interfaces as shown.
MS
Air
A
BSS
NSS
O&M
NMS
Figure 1.3
The three Subsystems of GSM and their interfaces
The MS (Mobile Station) is a
combination of terminal
equipment and subscriber data.
The terminal equipment as such is
called ME (Mobile Equipment)
and the subscriber's data is stored
in a separate module called SIM
(Subscriber Identity Module).
Therefore, ME + SIM = MS.
Figure 1.4 Inserting a SIM card in a mobile phone
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1.5
GSM Technical Specifications
From the specification point of view, the GSM system is divided into
twelve different classes and these classes together are called GSM
Technical Specifications. Nowadays, technical specifications are
implemented by ETSI (European Telecommunication Standard
Institute) in Subtechnical Committees and are referred to as Special
Mobile Groups (SMG).
ETSI Subtechnical Committees:
SMG1:
SMG2:
SMG3:
SMG4:
SMG5:
SMG6:
SMG7:
SMG8:
SMG9:
SMG10:
SMG11:
SMG12:
Services and Facilities
Radio Aspects
Network Aspects
Data Services
Closed
Operation & Maintenance
ME Testing
BSS Testing
SIM Aspects
Security
Speech
Architecture
GSM Technical Specifications:
01
02
03
04
05
06
07
08
09
[10
11
12
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General Description of a GSM PLMN
Services
Network Functions
MS - BSS Interface
Radio Path
Speech Processing Functions
Terminal Adaptation Functions
BSS - MSC Interface
Network Inter Working
Service Inter Working] - removed
Type Approval Procedures
Operation and Maintenance
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1.6
Introduction to GSM Review
1.6.1
Review Questions
In the following questions, please select one alternative which you
think is the best answer for the particular question. There may not be
a perfect answer, select the one which you think is the most correct.
1.
Why do you think there should be several network operators in
each country?
a) To help more people make money by operating networks
b) To help rapid expansion of GSM
c) To get more GSM subscribers
d) To help allocate the GSM frequency band easily
2.
Which of the following is not a feature of GSM networks alone
but is also a feature of analogue mobile communication
networks?
a) Digital transmission of user data (speech and data) in the air
interface
b) Possibility of full international roaming in any country which
has GSM network
c) Better speech quality
d) A fully digitised switching exchange
3.
Which of the following interfaces in not truly an open interface?
a) Between Network Switching Subsystem (NSS) and Network
Management Subsystem (NMS)
b) Between Network Switching Subsystem (NSS) and Base
Station Subsystem (BSS)
c) Between Network Switching Subsystem (NSS) and Public
Switched Telephone Network (PSTN)
d) Between Base Station Subsystem (BSS) and Mobile Station
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4.
Year
18 (244)
Match the year on the left-hand column with the corresponding
significant GSM event on the middle column.
Event
1982
Allocation of GSM frequencies
1998
Experimental test in Paris
1995
Frequency allocation for GSM 1800
1989
First official GSM call in the world
1991
Initiation of a new system
1992
Final recommendations Phase 1
1987
Phase 2 recommendations frozen
1986
Total GSM subscribers reaches 100 million
© Nokia Telecommunications Oy
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2
Traffic Management
2.1
Module Objectives
At the end of the module the student is able to:
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Name the three subsystems of GSM.
•
Explain the mobility concept (handover, location update, paging).
•
Describe how mobile originated and mobile terminated calls are
handled in GSM.
•
Explain the concept of distributed charging.
•
Explain the concept of security.
•
List and explain the operation of at least four services offered by
GSM networks.
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2.2
Introduction
A connection between two people - a caller and the called person - is
the basic service of all telephone networks. To provide this service, the
network must be able to set up and maintain a call, which involves a
number of tasks: identifying the called person, determining his location,
routing the call to him and ensuring that the connection is sustained as
long as the conversation lasts. After the transaction, the connection is
terminated and (normally) the calling user is charged for the service he
has used.
In a fixed telephone network, providing and managing connections is a
relatively easy process because telephones are connected by wires to
the network and their location is permanent, at least from the networks’
viewpoint. In a mobile network however, the establishment of a call is
a far more complex task as the wireless (radio) connection enables the
users to move at their own free will - providing they stay within the
service area of the network. In practice, the network has to find
solutions to three problems before it can even set up a call:
• Where is the
subscriber
• Who is the
subscriber
• What does the
subscriber want
Information about
the subscriber
Figure 2.1 Information required by a mobile communications network
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In other words, the subscriber has to be located and identified to
provide him with the requested services. Let’s take an example to
demonstrate these processes.
A well-known professor is travelling around the world. He decides to
spend the night in a hotel in, let’s say, Madrid. The first thing he will
do is to contact the reception desk for registration. Basically, the
reception desk is an office that supports registration. The receptionist
records the registration in a database which we call the visitor’s
register.
The receptionist carefully checks the passport of the professor. The
passport is also a database - a small one, though - and the receptionist
analyses the data recorded in it. She finds the basic facts, such as
citizenship, identification and the name of the professor, and also the
name of the authority that has released the document.
Figure 2.2 Registering into a hotel
The professor appears to have his visa expiring soon, so the receptionist
decides to call the office that has released the passport (presumably an
embassy). This is simple as she knows the nationality and identity of
the professor, and the number of his passport. The receptionist talks
with the embassy secretary who recognises the professor instantly and
advises the receptionist that everything is okay. The professor is
admitted into the hotel and the embassy of the professor’s home
country, registers the latest information about his location.
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E m bassy
Figure 2.3
Updating the location data in the home country
In other words, this is a transaction between the two offices and, as a
result, IDENTIFICATION and LOCALISATION of the customer
takes place in both databases. In this example, the embassy maintains a
database, which contains the basic data of all the citizens who are
travelling around the world and a record of their movements.
When the registration is completed, the professor goes to his room. We
can say that he is using a service provided by the hotel. As all the
hotels in the world give this type of service, we can call it a basic
service. In addition to the basic services (e.g. room and towels etc.),
the hotel also provides additional services (e.g. restaurant, sauna,
swimming pool etc.). These can be called supplementary services.
Figure 2.4 Services provided by a hotel
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To sum up the operations of the hotel:
•
It provides SERVICES.
•
It maintains a VISITOR REGISTER.
•
It informs the HOME REGISTER of a visitor.
The purpose of the two registers is to enable the Identification,
Authorisation and Localisation of the customer.
Let’s assume that the professor checks out of Madrid and goes to Paris.
He registers in another hotel and once again the receptionist informs the
embassy in the home country.
The registration in Madrid is cancelled, registration in Paris is made,
and the location data in the Home Register is brought up-to-date. We
have thus made a successful location update.
Let us move on and take a closer look at the mobile network. The story
of the professor visiting hotels bears a striking resemblance to the users
and functions of a mobile GSM network. Within the mobile network
there are subscribers who move around and register into the service
areas of networks in order to use the services provided by them. The
visitor register of the hotel and the permanent register of the embassy
also have their counterparts within the GSM network. There are fixed
databases that maintain basic information about their customers
including data on their current location, and temporary databases
storing information about the users who are currently located in their
service area. Let’s start with the registration process and the various
databases involved in it.
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2.3
Mobility Functions
2.3.1
Registration and Database
A new subscriber has just bought a mobile telephone and he switches it
on for the first time. He can be practically anywhere in the world
because, thanks to a network connection through a radio link, his
telephone does not need wires.
BTS
Figure 2.5 Person about to use a mobile phone
On the other hand, a connection through the mobile network is possible
only if there is a point to point connection between the caller and the
person who is called. Therefore, it is absolutely necessary that the
network knows the subscriber’s location. The network keeps track of
the subscribers’ location with the help of various databases as in the
hotel example.
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2.3.1.1
The Subscriber Identity Module
From the user’s point of view, the first and most important database is
inside the mobile phone: the Subscriber Identity Module (SIM). The
SIM is a small memory device mounted on a card and contains userspecific identification. The SIM card can be taken out of one mobile
equipment and inserted into another. In the GSM network, the SIM
card identifies the user just like a traveller uses a passport to identify
himself.
Figure 2.6 Example of a SIM card
The SIM card contains the identification numbers of the user, a list of
the services that the user has subscribed to and a list of available
networks. In addition, the SIM card contains tools needed for
authentication and ciphering and, depending on the type of the card,
there is also storage space for messages such as phone numbers, etc. A
so-called “Home Operator” issues a SIM card when the user joins the
network by making a service subscription. The Home Operator of the
subscriber can be anywhere in the world, but for practical reasons the
subscriber chooses one of the operators in the country where he spends
most of his time.
Now, the new subscriber switches on his phone in an area where a local
operator provides network service. The area is connected through an air
interface to a database known as a Visitor Location Register (VLR).
The VLR is integrated into a telephone exchange known as a Mobile
Services Switching Centre (MSC).
The home operator of the subscriber also needs to know the location of
the subscriber and so it maintains another register - just as the embassy
did in our example - which is called a Home Location Register
(HLR).
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GSM Network
HLR
VLR
MSC
SIM
Figure 2.7 Databases in a GSM Network
The HLR stores the basic data of the subscriber on a permanent basis.
The only variable data in the HLR is the current location (VLR address)
of the subscriber. However, in the VLR, the subscriber data is stored
temporarily. When the subscriber moves to another VLR area, its data
is erased from the old VLR and stored in the new VLR.
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2.3.2
Location Update
As an owner of a mobile phone, the subscriber does not stay in one
place but keeps moving from one place to another. No matter how often
or how quickly he moves, the network must be able to locate him
continuously in case somebody wants to call him. The transaction that
enables the network to keep track of the subscriber is called a Location
Update and it happens in roughly the same way as in the example of
the two hotels.
The mobile phone constantly receives information sent by the network.
This information includes identification (ID) of the VLR area in which
the mobile is currently located. In order to keep track of its location, the
mobile stores the ID of the area in which it is currently registered.
Every time the network broadcasts the ID of the area, the mobile
compares this information to the area ID stored in its memory. When
the two IDs are no longer the same, the mobile sends the network a
request, i.e. a registration inquiry to the area it has just entered. The
network receives the request and registers the mobile in the new VLR
area. Simultaneously, the subscriber’s HLR is informed about the new
VLR location and the data concerning the subscriber is cleared from the
previous VLR.
HLR
VLR
VLR
MSC
(old)
MSC
(new)
Location
Update
SIM
Mobile moves
Figure 2.8 Elements involved in location update
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The following figure gives a detailed description of the location update
process.
MS
BSS
MSC
VLR
HLR
LOCATION UPDATE REQUEST
REQUEST SUBSCRIBER ID
SEND SUBSCRIBER ID
REQUEST SUBSCRIBER INFO
SEND SUBSCRIBER INFO
AUTHENTICATION
AUTHENTICATION RESPONSE
ALL OK - HLR UPDATE
Figure 2.9 Location update procedures
In this way, the network can keep track of the subscriber all the time,
however, that is only a part of the job! Things become more
complicated when it becomes necessary to set up a call. Let’s start with
a call originating in a fixed telephone network, a Public Switched
Telephone Network.
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2.4
Call Set-Up in a GSM Network
Let’s go through the main call set-up cases. The first is a call
originating from the fixed network. Setting up a call appears to be a
quick and simple operation, but if we study the process more closely,
we discover that it consists of a considerable number of sub operations.
These operations include signalling between switching centres,
identifying and locating the subscriber who is being called, making
routing decisions and traffic connections etc. This section contains a
step by step analysis of setting up a connection between a telephone in
a fixed network and a GSM mobile station (i.e. a mobile phone).
Setting up a connection between two mobile stations is studied later.
1. A subscriber in a fixed network dials the number of a mobile
station. This can be either a national or an international number.
An example of a national number is:
040 2207959
As you can see there is no country code in this number. The following
is an example of an international number:
+358 40 2207959
The dialled number is called an MSISDN (Mobile Subscriber
International ISDN Number) which contains the following elements:
MSISDN = CC + NDC + SN
• CC= Country code (33=France, 358=Finland, etc.)
• NDC= National Destination Code
• SN= Subscriber Number
MSISDN
PSTN
Figure 2.10 PSTN originates the call
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2. The PSTN exchange analyses the dialled number. The result of the
analysis is the routing information required for finding the mobile
network (Public Land Mobile Network, PLMN) in which the
called subscriber has made his subscription. The PSTN identifies
the mobile network on the basis of the NDC, after which it accesses
the mobile network via the nearest Gateway Mobile Services
Switching Centre (GMSC).
HLR
GMSC
PSTN
MSISDN
VLR
GSM
Network
Figure 2.11 Incoming Call from PSTN to GSM network
3. The GMSC analyses the MSISDN in the same way as the PSTN
exchange did. As a result of the analysis, it obtains the HLR
address in which the subscriber is permanently registered. Notice
that the GMSC itself does not have any information about the
location of the called subscriber. The subscriber’s location can only
be determined by the two databases, the HLR and VLR. At this
stage however, the GMSC only knows the HLR address and so it
sends a message (containing the MSISDN) to the HLR. In practice
this message is a request for locating the called subscriber in order
to set up a call. This is called an “HLR Enquiry”.
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4. The HLR analyses the message. It identifies the called subscriber
on the basis of MSISDN and then checks its database to determine
the subscribers location. As you remember, the HLR is informed
every time the subscriber moves from one VLR area to another, i.e.
the HLR knows in which VLR area the subscriber is currently
registered.
It has to be pointed out that the HLR does not handle network traffic at
all. A traffic connection requires two network elements that are able to
provide speech connections. A speech connection is a network service
and it can be handled only by an MSC. Therefore, to enable the traffic
connection, maybe two MSC’s will have to be connected. The first
MSC is the Gateway MSC which is contacted by the PSTN exchange.
The HLR acts as a co-ordinator to set up the connection between the
GMSC and the destination MSC (which could of course be the GMSC
itself).
HLR
HLR
Enquiry
GMSC
MSC
PSTN
MSISDN
VLR
VLR
GSM
Network
Figure 2.12 Routing the call inside the GSM Network
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Let’s have a look at the contents of an HLR database to discover how it
locates the called subscriber. We will use an Italian subscriber as an
example:
HLR
MSISDN:
39 347 220759
IMSI:
222 10 1234567890
VLR address:
xyz
Subscriber Data:
services...
As you can see the first field contains the identity numbers of the
subscriber. The MSISDN has already been explained, but there is also
another identification number involved in the process known as the
International Mobile Subscriber Identity (IMSI). The purpose of
IMSI is to identify the subscriber in the mobile network. The total
length of the IMSI is 15 digits and it consists of the following elements:
IMSI = MCC + MNC + MSIN
• MCC = Mobile Country Code (three digits)
• MNC = Mobile Network Code (two digits)
• MSIN = Mobile Subscriber Identification Number (ten digits)
The IMSI number is used for registering a user in the Public Land
Mobile Network (PLMN). To locate the subscriber and to enable the
traffic connection, the HLR has to associate the MSISDN with the
IMSI of the mobile subscriber. But why do we need the IMSI? Why not
simply use the MSISDN both for network registration and for setting
up a call? The reason for this can be explained with an example: Let’s
suppose that three subscribers from three countries (Finland, Italy and
the USA) are in the same location and their mobile stations try to
register with the same VLR. Let’s also assume that they try to register
using their MSISDN (which is actually not the case):
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•
John, from the USA, MSISDN = + 1 XYZ 1234567
•
Ilkka, from Finland,
MSISDN = + 358 AB 6543210
•
Claudio, from Italy,
MSISDN = + 39 GHI 1256890
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Notice that the length of, for example, the country code is different for
each number. If the MSISDN numbers were used in registering the
subscribers, we would also need a length indicator for each field to
prevent the various parts of the number from getting mixed up with
each other - and that would be too complicated. If the length of the
fields are the same for all countries, no extra information is needed and
the identification process is relatively simple. Another reason for using
the IMSI is that the MSISDN identifies the service used such as speech,
data, fax, etc. Therefore one subscriber may need several MSISDNs
depending on the type of services he uses, whereas he has only one
IMSI.
To get back to the HLR database: one data field is reserved for the
address of the MSC/VLR where the called subscriber is currently
registered. (Normally the VLR and the MSC have the same address.)
This is needed in the next phase of establishing the connection.
5. Now the HLR interrogates the MSC/VLR that is currently serving
the called subscriber.
But why do we need to interrogate instead of connecting right away?
First of all, the current status of the mobile station is stored in the VLR
database and we need to know the status to avoid setting up a call to a
subscriber whose phone is switched off. Secondly, we need to have
some sort of information that enables the GMSC to route the call to the
target MSC, wherever in the world it may be.
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6. In terms of routing the call, the serving MSC/VLR is the destination
of the call. This means that we must direct the call to it by using the
following procedure: after receiving the message from the HLR, the
serving MSC/VLR generates a temporary Mobile Station Roaming
Number (MSRN) and associates it with the IMSI. The roaming
number is used in initiating the connection and it has the following
structure:
MSRN = CC + NDC + SN
•
CC = Country Code (of the visited country)
•
NDC = National Destination Code (of the serving network)
•
SN = Subscriber Number
HLR
Routing
information
request
message
MSC
VLR
MSC
VLR
Figure 2.13 MSRN request from HLR to the second MSC
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If we compare the MSRN and the MSISDN, we notice that they have
the same structure, though they are used for different purposes. The
MSISDN is used to interrogate the HLR, whereas the MSRN is the
response given by the serving MSC/VLR and it is used for routing the
call. The SN field of the MSRN is actually an internal number that is
temporarily associated with the IMSI. The MSRN does not merely
identify the subscriber, it also points to the exchange itself so that all
intermediate exchanges, if there are any, know where the call is to be
routed. Since the roaming number is temporary, it is available for
establishing another traffic connections after the call has been set up. In
essence, the SN field in the MSISDN points to a database entry in the
HLR, and the SN field in the MSRN points to a database entry in the
VLR.
Let’s take a look at an example with real numbers. This time the called
subscriber is roaming in Finland.
VLR
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IMSI:
222 10 1234567890
MSRN:
358 50 456456
Data:
abc..
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7. The MSC/VLR sends the roaming number to the HLR.
The HLR does not analyse it because the MSRN is used for traffic
transactions only and the HLR does not handle traffic, it is only a
database that helps in locating subscribers and co-ordinates call set-up.
Therefore, the HLR simply sends the MSRN forward to the GMSC that
originally initiated the process.
HLR
MSRN No.
to HLR
358 50 456456
MSC
VLR
MSC
VLR
Figure 2.14 The HLR is giving the MSRN to the originating MSC.
8. When the GMSC receives the message containing the MSRN, it
analyses the message. The roaming number identifies the location
of the called subscriber, so the result of this analysis is a routing
process which identifies the destination of the call - the serving
MSC/VLR.
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9. The final phase of the routing process is taken care of by the
serving MSC/VLR. In fact, the serving MSC/VLR also has to
receive the roaming number so that it knows that this is not a new
call, but one that is going to terminate here - i.e. a call to which it
has already allocated an MSRN. By checking the VLR, it
recognises the number and so it is able to trace the called
subscriber.
At this point, we have to summarise what happened behind the scenes
to be able to understand the rest of the process. We will take a closer
look at two basic subsystems GSM network: The Network Switching
Subsystem (NSS) and the Base Station Subsystem (BSS).
A ir
A
M SC
M SC
VLR
HLR
VLR
O&M
Figure 2.15 GSM Subsystems
2.4.1
Network Switching Subsystem (NSS)
The GSM network is divided into three subsystems: Network
Switching Subsystem (NSS), Base Station Subsystem (BSS), and
Network Management Subsystem (NMS). The concept of the NSS is
introduced in this section and the BSS and NMS are explained later.
The elements of Network Switching Subsystem that have been
discussed so far are:
•
MSC
(Mobile Services Switching Centre)
•
VLR
(Visitor Location Register)
•
HLR
(Home Location Register)
The MSC is responsible for controlling calls in the mobile network. It
identifies the origin and destination of a call (either a mobile station or
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a fixed telephone in both cases), as well as the type of a call. An MSC
acting as a bridge between a mobile network and a fixed network is
called a Gateway MSC. An MSC is normally integrated with a VLR,
which maintains information related to the subscribers who are
currently in the service area of the MSC. The VLR carries out location
registrations and updates. The MSC associated with it initiates the
paging process. A VLR database is always temporary (in the sense that
the data is held as long as the subscriber is within its service area),
whereas the HLR maintains a permanent register of the subscribers. In
addition to the fixed data, the HLR also maintains a temporary database
which contains the current location of its customers. This data is
required for routing calls.
In addition, there are two more elements in the NSS: the Authentication
Centre (AC) and the Equipment Identity Register (EIR). They are
usually implemented as part of HLR and they deal with the security
functions that will be discussed later.
To sum up, the main functions of NSS are:
Call Control
This identifies the subscriber, establishes a
call and clears the connection after the
conversation is over.
Charging
This collects the charging information about
a call such as the numbers of the caller and
the called subscriber, the time and type of the
transaction, etc., and transfers it to the
Billing Centre.
Mobility management
This maintains information about the
location of the subscriber.
Signalling with other
networks and the BSS
This applies to interfaces with the BSS and
PSTN.
Subscriber data handling This is the permanent data storage in the
HLR and temporary storage of relevant data
in the VLR.
Locating the subscriber
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This locates a subscriber before establishing
a call.
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2.4.2
Locating the Subscriber
The GMSC/VLR and the MSC/VLR has now been connected via a
traffic and signalling channel and the call set up has almost been
completed. The caller is connected to the PSTN exchange, the PSTN is
connected to the GMSC, the GMSC is connected to the MSC/VLR that
is serving the called subscriber but we have not yet established a
connection to the called subscriber. In order to set up the connection,
we first have to understand how the subscriber is located.
As we do not know the exact location of the subscriber, it seems
inevitable that we have to search for him in the entire VLR service
area. This could be a wide geographical area and so finding the
subscriber requires a lot of work for the MSC/VLR. Things will be
easier to grasp if we go back for a moment and follow the famous
professor in the hotel.
The hotel is an establishment that provides services. If we want to find
our professor, we need to search the entire area of the hotel, which can
be really frustrating. He might be in his room, in the sauna, in the
swimming pool, in the restaurant, in fact practically anywhere. The
only thing we know is that he is in the hotel, because he has not
checked out. In order to simplify the search, we can register his
movements by setting up a registration routine for the various parts of
the hotel. In other words, the hotel service area is divided into location
areas. This simplifies the search for the professor.
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Reception Restaurant
Bar
Pool
Figure 2.16 Location areas inside a hotel
Note that these location areas are only known within the area of the
hotel, i.e. no information is given to the embassy (police department).
Something similar happens in the cellular network. When we want to
find the subscriber, it would be necessary to conduct a search
throughout the entire MSC/VLR area unless this area is divided into
smaller areas. Therefore, the MSC/VLR area is divided into smaller
areas. These are called Location Areas (LA) and they are managed by
the MSC/VLR.
LA 5
LA 1
LA 4
M SC
VLR
LA 3
LA 2
Figure 2.17 Location Areas under one MSC/VLR
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Each MSC/VLR contains several Location Areas. We can define a LA
as an area in which we search for the subscriber in case there is a call
addressed to a mobile station.
Let’s have a look at the data in the VLR and then get back to
establishing the connection with the called subscriber:
VLR
IMSI:
222 10 1234567890
LAC:
262 15 0987
Data:
abc...
MSRN:
358 50 456456
Each LA is identified by a Location Area Identity (LAI). Its structure
is as follows:
LAI = MCC + MNC + LAC
•
MCC= Mobile Country Code (of the visited country)
•
MNC= Mobile Network Code (of the serving PLMN)
•
LAC= Location Area Code
10. Now that we know the LA of the subscriber, we can start searching
for him. To locate the subscriber, a Paging process is initiated in
the Location Area.
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2.4.3
Base Station Subsystem (BSS)
To understand the paging process, we must analyse the functions of the
BSS.
The Base Station Subsystem consists of the following elements:
•
BSC
Base Station Controller
•
BTS
Base Transceiver Station
•
TC
Transcoder
The Base Station Controller (BSC) is the central network element of
the BSS and it controls the radio network. This means that the main
responsibilities of the BSC are: Connection establishment between MS
and NSS, Mobility management, Statistical raw data collection, Air and
A interface signalling support.
The Base Transceiver Station (BTS) is a network element
maintaining the Air interface. It takes care of Air interface signalling,
Air interface ciphering and speech processing. In this context, speech
processing refers to all the functions the BTS performs in order to
guarantee an error-free connection between the MS and the BTS.
The TransCoder (TC) is a BSS element taking care of speech
transcoding, i.e. it is capable of converting speech from one digital
coding format to another and vice versa. We will describe more about
the transcoder functions later.
BSC
TC
BSC
BTS
TC
BTS
BTS
Figure 2.18
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The Base Station Subsystem (BSS)
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The BTS, BSC and TC together form the Base Station Subsystem
(BSS) which is a part of the GSM network taking care of the following
major functions:
Radio Path Control
In the GSM network, the Base Station Subsystem (BSS) is the part of
the network taking care of Radio Resources, i.e. radio channel
allocation and quality of the radio connection. For this purpose, the
GSM Technical Specifications define about 120 different parameters
for each BTS. These parameters define exactly what kind of BTS is in
question and how MSs may "see" the network when moving in this
BTS area. The BTS parameters handle the following major items: what
kind of handovers (when and why), paging organisation, radio power
level control and BTS identification.
BTS and TC Control
Inside the BSS, all the BTSs and TCs are connected to the BSC(s). The
BSC maintains the BTSs. In other words, the BSC is capable of
separating (barring) a BTS from the network and collecting alarm
information. Transcoders are also maintained by the BSC, i.e. the BSC
collects alarms related to the Transcoders.
Synchronisation
The BSS uses hierarchical synchronisation which means that the MSC
synchronises the BSC and the BSC further synchronises the BTSs
associated with that particular BSC. Inside the BSS, synchronisation is
controlled by the BSC. Synchronisation is a critical issue in the GSM
network due to the nature of the information transferred. If the
synchronisation chain is not working correctly, calls may be cut or the
call quality may not be the best possible. Ultimately, it may even be
impossible to establish a call.
Air & A Interface Signalling:
In order to establish a call, the MS must have a connection through the
BSS. This connection requires several signalling protocols that are
explained in the Signalling Chapter.
Connection Establishment between MS and NSS
The BSS is located between two interfaces, the Air and the A interface.
From the call establishment point of view, the MS must have a
connection through these two interfaces before a call can be
established. Generally speaking, this connection may be either a
signalling type of connection or a traffic (speech, data) type of
connection.
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Mobility Management and speech transcoding
BSS Mobility Management mainly covers the different cases of
handovers. These handovers and speech transcoding are explained in
later sections
Collection of Statistical Data
The BSS collects a lot of short-term statistical data that is further sent
to the NMS for post processing purposes. By using the tools located in
the NMS the operator is able to create statistical "views" and thus
observe the network quality.
A Base Station Subsystem is controlled by an MSC. Typically, one
MSC contains several BSSs. A BSS itself may cover a considerably
large geographical area consisting of many cells. (A cell refers to an
area covered by one or more frequency resources). Each cell is
identified by an identification number called Cell Global Identity
(CGI) which comprises the following elements:
CGI = MCC + MNC + LAC + CI
•
MCC Mobile Country Code
•
MNC Mobile Network Code
•
LAC
Location Area Code
•
CI
Cell Identity
Let’s take an example of two adjacent BTSs. One serves an industrial
area and the other a nightlife area.
Figure 2.19 Different types of areas
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It is obvious that traffic is not handled the same way and at the same
time in these BTSs. One has traffic peaks during weekdays and
especially during working hours, and the other during the evenings and
weekends.
Figure 2.20 Traffic channel usage times for different areas
There is one 2Mbit/s PCM line reserved for each BTS to provide the
connection to NSS. But as you can see, the BTS’s are used at different
times and on different days. Why not use the same line for both of the
two BTSs? It can be done, but in this case there has to be a
concentrator between MSC and BTS. The BSC acts as a
concentrator (in addition to being the radio network controller). One
BSC is capable of serving several BTSs.
BSC concentrates
the traffic to the MSC
BSC
BTS
BTS
MSC
Many partially used
2Mbit/s PCM lines
A few efficiently used
2Mbit/s PCM lines
Figure 2.21 BTS-BSC- BTS connections
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Note that there is no relation between a BSC area and Location Area
and the serves as a concentrator in addition to its major role of radio
network control. The purpose of the location area is to facilitate the
paging process (searching for the subscriber), whereas a BSC area is
related to traffic connections and radio resources.
2.4.4
A Mobile Terminated Call and Paging
Let us go back to our professor. We know that he is within the hotel
area. Thanks to the registration system of the hotel, we also know that
he went to the restaurant and registered his presence there. Somebody
calls him and the receptionist answers. The receptionist checks the
registration system of the hotel and discovers that the professor is in the
restaurant. A message about an incoming call is sent to the restaurant
and one of the waiters starts looking for the professor. If the waiter does
not know the right table, he uses the public address system and
"pages" the professor as follows: “There is a telephone call for Mr. So
and So. Could you please come forward?” Once the professor raises his
hand, the search is complete and the call is set up.
Location Area
Paging
Paging
BTS
Mobile responds
to paging
BTS
Paging
BTS
Figure 2.22 The paging process
Again, a similar process is used in the cellular network. Paging is a
signal that is transmitted by all the cells in the Location Area (LA). It
contains the identification of the subscriber. All the mobile stations in
the LA receive the paging signal, but only one of them recognises the
identification and answers to it. As a consequence of this answer, a
point to point connection is established. Now the two subscribers are
connected, and traffic can be carried through the network. Let’s sum up
the entire process:
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ASubscriber
PSTN
GMSC
HLR
MSC/VLR
CALL SETUP (MSISDN)
ANALYSE NUMBER
CALL SETUP (MSISDN)
MSISDN
IMSI
MSRN
MSRN
CALL SETUP (MSRN)
PAGING
Figure 2.23 Simplified steps in setting up a call
1. A subscriber in a fixed network dials a number of a mobile phone.
The dialled number is the MSISDN.
2. The Public Switched Telephone Network (PSTN) exchange
analyses the number and contacts the Gateway Mobile Services
Switching Centre (GMSC).
3. The Gateway MSC analyses the MSISDN and sends a message to
the Home Location Register (HLR).
4. The HLR checks its database to determine the current location of
the called subscriber.
5. The HLR interrogates the MSC/VLR (Visitor Location Register)
that is currently serving the called subscriber.
6. The serving MSC/VLR generates a temporary MSRN (Mobile
Subscriber Roaming Number).
7. MSC/VLR sends MSRN to HLR and the HLR forwards the MSRN
to the GMSC.
8. The GMSC identifies the serving MSC/VLR as the destination for
routing the call.
9. Destination MSC/VLR receives MSRN. It identifies the number
that is called and traces the called subscriber.
10. The destination MSC/VLR initiates a paging process in the
Location Area to locate the called subscriber. The mobile phone of
the called subscriber recognises the paging signal and answers it.
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2.4.5
Mobile Originated Call
We have studied the phases of a PSTN originated call and traced the
movements of the subscriber. We have examined the functions and
architecture of the network elements.
Now it’s time to investigate another case: how is a connection
established when the call is initiated by a mobile subscriber instead of a
fixed one?
The mobile subscriber dials a number. In other words, the subscriber
issues a service request to the network in which he is currently
registered as a visitor. After receiving the request, the network analyses
the data of the calling subscriber in order to do three things:
•
Authorise or deny the use of the network.
•
Activate the requested service.
•
Route the call.
The call may have two types of destinations: a mobile station or a
telephone in a fixed network. If the call is addressed to a telephone in a
fixed telephone network, it is routed to the PSTN, which in turn routes
it to the destination. If the called number is another mobile station in
the same network, the MSC starts the HLR Enquiry procedure which is
processed in the same way as in the example of a PSTN originated call.
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EXC
GMSC
HLR
MSC
VLR
BSS
MS
1. channel assignment
2. security procedures
3. call setup
4. check services etc.
5. all ok
6. call is proceeding
7. traffic channel allocated
8. set up the call
9. call set up complete
10. alert
11. B answers
Figure 2.24 Mobile Originated Call procedure.
Identifying and locating the called subscriber are the two key
preconditions of setting up a point to point connection. The MSISDN
fulfils the purpose of identification, but locating requires a quick and
comprehensive system for keeping track of the subscriber. If the
network does not have up-to-date information about the subscriber’s
current location, setting up a call would mean paging large network
areas in order to find the subscriber and that would be a complex and
time-consuming task. To avoid this, the GSM network monitors and
records the movements of the subscribers all the time. This process is
called Location Update. We have already discussed it briefly, but now
we will analyse it in detail.
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2.5
Location Update
2.5.1
Types of Location Update
In practice, there are three types of location updates:
•
Location Registration (power on)
•
Generic
•
Periodic
Location registration takes place when a mobile
station is turned on. This is also known as IMSI
Attach because as soon as the mobile station is
switched on it informs the Visitor Location Register
(VLR) that it is now back in service and is able to
receive calls. As a result of a successful registration,
the network sends the mobile station two numbers that
are stored in the SIM (Subscriber Identity Module)
card of the mobile station.
These two numbers are the Location Area Identity
(LAI) and the Temporary Mobile Subscriber Identity
(TMSI). The network, via the control channels of the air
interface, sends the LAI. The TMSI is used for security
purposes, so that the IMSI of a subscriber does not have
to be transmitted over the air interface. The TMSI is a temporary identity,
which regularly gets changed.
A Location Area Identity (LAI) is a globally unique number.
A Location Area Code (LAC) is only unique in a particular network.
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MSC
VLR
LA 2
LA 1
Every time the mobile
receives data through
the control channels, it
reads the LAI and
compares it with the
LAI stored in its SIM
card. A generic location
update is performed if
they are different. The
mobile starts a Location
Update process by
accessing the MSC/VLR
that sent the location
data.
A channel request message is sent that contains the subscriber identity
(i.e. IMSI/TMSI) and the LAI stored in the SIM card. When the target
MSC/VLR receives the request, it reads the old LAI which identifies
the MSC/VLR that has served the mobile phone up to this point. A
signalling connection is established between the two MSC/VLRs and
the subscriber’s IMSI is transferred from the old MSC to the new MSC.
Using this IMSI, the new MSC requests the subscriber data from the
HLR and then updates the VLR and HLR after successful
authentication.
Air
A
MSC
MSC
VLR
VLR
O&M
Figure 2.25 Network Elements involved in location update
Periodic location update is carried out when the network does not
receive any location update request from the mobile in a specified time.
Such a situation is created when a mobile is switched on but no traffic
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is carried, in which case the mobile is only reading and measuring the
information sent by the network. If the subscriber is moving within a
single location area, there is no need to send a location update request.
9999
0000
Location Update Request
BTS
BTS
Figure 2.26 Example of Periodic Location Update
A timer controls the periodic updates and the operator of the VLR sets
the timer value. The network broadcasts this timer value so that a
mobile station knows the periodic location update timer values.
Therefore, when the set time is up, the mobile station initiates a
registration process by sending a location update request signal. The
VLR receives the request and confirms the registration of the mobile in
the same location area. If the mobile station does not follow this
procedure, it could be that the batteries of the mobile are exhausted or
the subscriber is in an area where there is no network coverage. In such
a case, the VLR changes the location data of the mobile station to
“unknown”.
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2.5.2
Procedures
MS
BSS
MSC
VLRnew
VLRold
HLR
1. channel assignment
2. location update request
3. request subscriber identity
4. request subscriber identity
5. request subscriber data
7. security procedures
6. request subscriber data
8. update location
9. update HLR
10. update acknowledgement
11. cancel old location
12. location cancelling accepted
Figure 2.27 Location Update procedures
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2.6
Handover
In a mobile communications network, the subscriber can move around.
How can we maintain the connection in such cases? To understand this,
we must study the process of handing over the calls.
Maintaining the traffic connection with a moving subscriber is made
possible with the help of the handover function. The basic concept is
simple: when the subscriber moves from the coverage area of one cell
to another, a new connection with the target cell has to be set up and the
connection with the old cell has to be released. There are two reasons
for performing a handover:
1. Handover due to measurements occurs when the quality or the
strength of the radio signal falls below certain parameters specified
in the BSC. The deterioration of the signal is detected by the
constant signal measurements carried out by both the mobile station
and the BTS. As a consequence, the connection is handed over to a
cell with a stronger signal.
2. Handover due to traffic reasons occurs when the traffic capacity
of a cell has reached its maximum or is approaching it. In such a
case, the mobile stations near the edges of the cell may be handed
over to neighbouring cells with less traffic load.
The decision to perform a handover is always made by the BSC that is
currently serving the subscriber, except for the handover for traffic
reasons. In the latter case the MSC makes the decision. There are four
different types of handover and the best way to analyse them is to
follow the subscriber as he moves:
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Intra cell - Intra BSC handover
The smallest of the handovers is the intra cell handover where the
subscriber is handed over to another traffic channel (generally in
another frequency) within the same cell. In this case the BSC
controlling the cell makes the decision to perform handover.
Air
A
hann
Old C
TC
BSC
BTS
el
n
Chan
New
el
Figure 2.28 Intra Cell - Intra BSC Handover
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Inter cell - Intra BSC handover
The subscriber moves from cell 1 to cell 2. In this case the handover
process is controlled by BSC. The traffic connection with cell 1 is
released when the connection with cell 2 is set up successfully.
Air
A
BTS
BSC
TC
BTS
Old Cell / BTS
New Cell / BTS
Figure 2.29 Inter Cell - Intra BSC handover
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Inter cell - Inter BSC handover
The subscriber moves from cell 2 to cell 3, which is served by another
BSC. In this case the handover process is carried out by the MSC, but,
the decision to make the handover is still done by the first BSC. The
connection with the first BSC (and BTS) is released when the
connection with the new BSC (and BTS) is set up successfully.
New Cell / BTS
Air
BSC
BTS
A
TC
MSC
BTS
BSC
VLR
TC
Old Cell / BTS
Figure 2.30 Inter Cell - Inter BSC Handover
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Inter MSC handover
The subscriber moves from a cell controlled by one MSC/VLR to a cell
in the domain of another MSC/VLR. This case is a bit more
complicated. Considering that the first MSC/VLR is connected to the
GMSC via a link that passes through PSTN lines, it is evident that the
second MSC/VLR can not take over the first one just like that.
The MSC/VLR currently serving the subscriber (also known as the
anchor MSC), contacts the target MSC/VLR and the traffic connection
is transferred to the target MSC/VLR. As both MSCs are part of the
same network, the connection is established smoothly. It is important to
notice, however, that the target MSC and the source MSC are two
telephone exchanges. The call can be transferred between two
exchanges only if there is a telephone number identifying the target
MSC.
New Cell / BTS
Air
BSC
BTS
BTS
BSC
TC
A
TC
MSC
MSC
VLR
VLR
Old Cell / BTS
Figure 2.31 Inter Cell - Inter MSC Handover
Such a situation makes it necessary to generate a new number, the
Handover Number (HON). The generation and function of the HON
are explained in the following text.
The anchor MSC/VLR receives the handover information from the
BSS. It recognises that the destination is within the domain of another
MSC and sends a Handover Request to the target MSC via the
signalling network. The target MSC answers by generating a HON and
sends it to the anchor MSC/VLR, which performs a digit analysis in
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order to obtain the necessary routing information. This information
allows the serving MSC/VLR to connect the target MSC/VLR. When
the two MSCs are connected, the call is transferred to a new route.
In practice, the handover number is similar to the roaming number.
Moreover, the roaming number and the handover number have a
similar purpose, that is connecting two MSCs. The structure of the
handover number is shown below:
HON = CC + NDC + SN
•
CC= Country Code
•
NDC= National Destination Code (of the serving network)
•
SN= Subscriber Number
The call will not last forever and the connection has to be released
sooner or later. To understand the process of releasing the connection,
we must consider a few things such as: Who pays for the call, which
exchange takes care of the charging operation and where is the
subscriber data stored. This will be discussed in the next section but
before that, let us sum up the stages of Inter MSC handover.
MS
BSSold
MSCold
MSCnew
BSSnew
MS (after HO)
1. measurement reports
2. handover required
3. request HON
4. request for radio resources
5. radio resources reserved
6. provide HON and target cell info
7. set up speech connection (HON)
8. handover command
9. handover complete
10. handover complete
11. connect
12. release old connections
Figure 2.32 Inter MSC handover procedure
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2.7
Charging
2.7.1
What to Charge?
Charging in GSM networks follows similar principles to that used in
fixed telephone networks. In addition to a standard fee, subscribers
have to pay for the calls they make and the services they use. However,
there are a few differences in how the costs are calculated and who is
liable to pay them.
Note that the information presented in this chapter outlines only the
basic features of charging. The actual charging practices vary
considerably from one network operator to another.
2.7.2
Subscription Charge
When a person joins a GSM network, he receives a personal SIM card
from the network operator and his basic information (phone number,
type of ordered services, etc.) is recorded in the network databases such
as the HLR. To cover the costs of these operations, network operators
often charge the subscriber an initial subscription charge.
2.7.3
Renting of Service
After the subscription has been made and the subscriber has become a
customer of the particular network, he is usually charged for the
availability of the network services and the right to use them. This is a
regular fee which is charged irrespective of whether the subscriber
makes any calls or not. This kind of charge is also known as renting
the service of the network.
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• Installation fee
• Renting of the service
• Use of the network
Figure 2.33
2.7.4
Different ways of charging a subscriber
Charge for use of the network
The third charging type is applied for the use of network on a call by
call basis There are many factors that affect how the subscriber is
charged for making (and, sometimes, receiving) a call. The following is
a list of parameters that can be used as a basis for charging the
subscribers.
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•
Type of the service, e.g. speech, short message service.
•
Duration of the call.
•
Time of day the call was made, e.g. working hours, evening.
•
Destination of the call.
•
Origin of the call, e.g., a certain cell.
•
Use of networks, e.g. the PSTN.
•
Use of supplementary services such as call forwarding and call
barring.
•
Use of radio resources.
•
International Roaming leg (explained later)
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Call forwarding and roaming leg are factors that not only affect the
amount of charge, but also bring up the issue of who is liable to pay.
This is a major difference when compared to fixed telephone networks,
which is why we have to take a closer look at it in the following
chapter.
2.7.5
Whom to Charge?
Where is the
Called Party?
Bill to subscriber
Billing
BSC
BC
Transfer of
Charging data
MSC
MSC
PLMN 2
PSTN
HLR
Charging
BC
Path of
the call
PLMN 1
Calling Party
Figure 2.34 PSTN - GSM Call Path
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Let’s take a closer look at the number dialled by the calling
subscriber. The number includes the national destination code, which
identifies the called subscriber’s home network. If the called subscriber
is registered in a location area belonging to his home network, the
connection is established as explained in the previous chapter and the
calling subscriber pays for the call.
If however, the called subscriber is outside the service area of his home
network (in a foreign country, for instance) and is connected to another
network, then the call has to be routed to him using the services of one
or more foreign networks. In such a case, we talk about international
roaming leg, which refers to the connection between the home network
and subscriber via a foreign network. In such a case, the charge for the
call will be shared according to the following principle:
•
The calling subscriber pays for the connection to the number he
dialled (MSISDN).
•
The called subscriber pays for the international roaming leg.
The same principle is applied when the mobile subscriber has
forwarded incoming calls to another number. The calling subscriber is
only responsible for the costs incurred by calling to the mobile station,
and the mobile user pays for the forwarded call. This may seem strange
and complicated, but the reason is quite clear: the calling subscriber
does not necessarily know the location of the called subscriber or the
services and connections that are required to access him. The calling
subscriber only knows that he is dialling a number in a certain mobile
network, and therefore he can only be charged for the services that he is
aware of. The called subscriber knows - at least he should know whether he is using the services of a foreign network or some
chargeable supplementary services of his home network, and therefore
he is liable to pay for them.
Collect call is the third case in which the called subscriber pays for the
call. A collect call in a GSM network is similar to a collect call in a
fixed network: first the called subscriber has to accept the call, after
which he is responsible for all the costs.
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2.7.6
Charging Procedure in GSM
In the fixed network, the charging is normally determined by collecting
metering pulses, by which the exchange can calculate the price of the
call. This method is called time charging. The calling subscriber
(subscriber A) is normally the one who pays for the call. In the mobile
network the called subscriber is normally also charged for the so called
roaming leg, because A does not necessarily know, where B is located.
In the GSM network there are so many different ways to define the
charging that it is sensible to create a record in the MSC or/and HLR
about every event which can be a basis for charging. These events can
be the defined call cases or other possible chargeable events, such as
location updates. The record containing the information about one
chargeable event is called the charging record. These records are
stored primarily as charging files in the MSC or HLR and then
transferred to a separate billing centre. The serving operator controls
the entire charging process. The process begins when a call is set up
and at the same time, a charging record is opened in the serving
MSC/VLR. In general the first and the last MSC involved in a call set
up, collect the charging record.
GMSC
BSC
PSTN
BTS
HLR
Charging Record
Figure 2.35 GMSC is responsible for creating charging records
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As the call continues, the subscriber moves in the service area of the
operator and enters the service area of another MSC/VLR and thus an
inter-MSC handover is performed. The charging record is not
transferred to the new MSC during the handover. Instead, the first MSC
keeps record of the call as long as it lasts.
HLR
GMSC
BSC
BTS
PSTN
MSC
BSC
Figure 2.36 Elements involved in call handling
When the call has been released, the charging record is closed. When a
sufficient number of charging records have been accumulated they are
sent to a billing centre in one bulk via an X.25 or Ethernet connection.
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HLR
GMSC
MSC
PSTN
BSC
BTS
X.25 or
Ethernet
Billing Centre (BC)
Figure 2.37
Transfer of charging data to Billing Centre
As the actual charging is affected by a variety of factors, the charging
record contains all the events that can be used as a basis for
determining the charge.
Information for one customer is collected from many MSCs and billing
centres that the mobile has visited and is presented as one bill.
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2.7.6.1
Distributed Charging
In order to produce bills for each subscriber, Billing Centres should
collect detailed charging data from all the MSCs within the PLMN.
With International Roaming, this operation should be extended
covering all the PLMNs where a Roaming Contract is signed. Charging
information must be collected from the Billing Centres (BC) of all the
networks that subscribers have been visited and passed to the Billing
Centre of the home network.
Home Billing Centre
Figure 2.38 Distributed Charging
When two GSM operators sign a “roaming contract”, they agree how
often they will transfer charging data between each other. The home
billing centre analyses the charging information collected from all the
networks where a roaming contract exists, and produces a bill for each
customer.
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2.8
Network Architecture
After examining the course of a call from beginning to end it is useful
to summarise the entire process by putting the pieces together and
studying the network as a whole. Note that the network picture below
contains equipment from a typical GSM network, some of which is not
included in the GSM specifications.
Base Station Subsystem
Mobile Stations
Base Station
Controller
Base
Transceiver
Stations
Network Management Subsystem
Database Server
Data
Workstations
Communication
Network
Communications
Server
Network
Planning
System
Network
Measurement
System
Network Switching Subsystem
Home Location Register/
Authentication Centre/
Equipment Identity
Register
TCP/IP
Data Communications
Server
Digital Cross
Connect
Transcoder
Submultiplexer
Mobile Switching Centre/
Visitor Location Register
PSTN/ISDN
Voice
mail
Short Message
Service Centre
A-Interface
Air Interface
X.25 Interface
IN Service Control Point
Abis Interface
Figure 2.39 GSM Network Architecture
If we take a look at the entire network, we can see that some Network
Elements have not been discussed yet. These are:
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•
TransCoder (TC) - This belongs to the BSS.
•
Equipment Identity Register (EIR) – This belongs to the NSS.
•
Authentication Centre (AC) – This belongs to the NSS.
•
The complete Network Management Subsystem (NMS)
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2.8.1
NSS - Network Switching Subsystem
The Network Elements we have seen so far are:
•
The Mobile Services Switching Centre (MSC)/Visitor Location
Register (VLR) one or all of which are also a Gateway MSC.
•
The Home Location Register (HLR)
Also part of the Network Switching Subsystem are the Authentication
Centre (AC) and Equipment Identity Register (EIR).
The Authentication Centre (AC) and Equipment Identity Register (EIR)
are used to provide security. The subscriber and the mobile station have
to be identified and authorised before accessing the network. These
functions will be discussed later.
2.8.2
BSS - Base Station Subsystem
The Network Elements we have seen so far are:
•
Base Station Controller (BSC)
•
Base Transceiver Station (BTS)
Also part of the Base Station Subsystem is the Transcoder.
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Transcoder
In the case of the air interface, the media carrying the traffic is a radio
frequency, but, as we saw in the example of a PSTN originated call, the
traffic signal is also carried through fixed networks. To enable the
efficient transmission of the digital speech information over the radio
Air Interface the digital speech signal is compressed.
For transmission over the air interface, the speech signal is compressed
by the mobile station to 13Kbits/s (Full Rate) or 6.5Kbits/s (Half
Rate). This compression algorithm is known as "Regular Pulse
Excitation with Long Term Prediction" (RPE-LTP). However, the
standard bit rate for speech in the PSTN is 64Kbits/s Therefore, a
converter has to be provided in the network to change the bit rate from
one to another. This is called the TransCoder (TC). If the TC is
located as close as possible to the MSC with standard PCM lines
connecting the network elements, we can, in theory, multiplex four
traffic channels in one PCM channel. This increases the efficiency of
the PCM lines. But when connecting to the MSC, the multiplexed lines
have to be de-multiplexed. In this case the unit is called Transcoder
and Submultiplexer (TCSM).
BSC
Transcoder and
Submultiplexer (TCSM)
MSC
TC
TC
SM2M
TC
TC
A ter Interface
A ter’ Interface
A Interface
Figure 2.40 Location of Transcoder and Submultiplexer
In the Nokia solution, the submultiplexing and the transcoding function
is combined in one equipment which is called a TCSM2E (European
version) or TCSM2A (American version).
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2.8.3
Network Management Subsystem
The Network Management Subsystem (NMS) is the third subsystem
of the GSM network in addition to the Network Switching Subsystem
(NSS) and Base Station Subsystem (BSS) which we have already
discussed. The purpose of the NMS is to monitor various functions and
elements of the network. These tasks are carried out by the NMS/2000
which consists of a number of Work Stations, Servers and a Router
which connects to a Data Communications Network (DCN).
Unix
Workstations
HLR/AC/EIR
MSC/VLR
Database and
Communications
Servers
Router
NMS/2000
TCSM
BSC
GSM Network
Data
Communications
Network (DCN)
Figure 2.41 The NMS and the GSM Network
The operator workstations are connected to the database and
communication servers via a Local Area Network (LAN). The database
server stores the management information about the network. The
communications server takes care of the data communications between
the NMS and the equipment in the GSM network known as “Network
Elements”. These communications are carried over a Data
Communications Network (DCN) which connects to the NMS via a
router. The DCN is normally implemented using an X.25 Packet
Switching Network.
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The functions of the NMS can be divided into three categories:
•
Fault Management
•
Configuration Management
•
Performance Management
These functions cover the whole of the GSM network elements from
the level of individual BTSs, up to MSCs and HLRs.
Fault Management
The purpose of Fault Management is to ensure the smooth operation of
the network and rapid correction of any kind of problems that are
detected. Fault management provides the network operator with
information about the current status of alarm events and maintains a
history database of alarms.
The alarms are stored in the NMS database and this database can be
searched according to criteria specified by the network operator.
Figure 2.42 Fault Management
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Configuration Management
The purpose of Configuration Management is to maintain up to date
information about the operation and configuration status of network
elements. Specific configuration functions include the management of
the radio network, software and hardware management of the network
elements, time synchronisation and security operations.
Figure 2.43 Configuration management
Performance Management
In performance management, the NMS collects measurement data from
individual network elements and stores it in a database. On the basis of
these data, the network operator is able to compare the actual
performance of the network with the planned performance and detect
both good and bad performance areas within the network.
Figure 2.44 Performance management
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2.8.4
Authentication Principle
Authentication is a procedure used in checking the validity and
integrity of subscriber data. With the help of the authentication
procedure the operator prevents the use of false SIM modules in the
network. The authentication procedure is based on an identity key, Ki,
that is issued to each subscriber when his data are established in the
HLR. The authentication procedure verifies that the Ki is exactly the
same on the subscriber side as on the network side.
Air
A
SIM
card
AC
* I MSI
* Ki
MSC
VLR
* I MSI
* Ki
Figure 2.45 Authentication
Authentication is performed by the VLR at the beginning of every call
establishment, location update and call termination (at the called
subscriber side). In order to perform the authentication, the VLR needs
the basic authentication information. If the mobile station was asked to
broadcast its Ki, this would undermine the principle of authentication,
because identification data would be sent across the air. The trick is to
compare the Ki stored in the mobile with the one stored in the network
without actually having to transmit it over the radio air interface. The
Ki is processed by a random number with a “one way” algorithm called
A3 and the result of this processing is sent to the network. Due to the
type of the algorithm A3, it is easy to get the result on the basis of Ki
and a random number, but it is virtually impossible to get the Ki on the
basis of the result and random number (hence the name “one way”
algorithm).
Since the security issue concerns confidentiality as well, the network
uses more than one algorithm. These are introduced in the following
sections.
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2.8.5
Security Algorithms
The GSM system uses three algorithms for the purposes of
authentication and ciphering. These algorithms are A3, A5 and A8. A3
is used in authentication, A8 is used in generating a ciphering key and
A5 is used in ciphering.
Air
A
AC
A3
BTS
BSC
TC
MSC
ME + SIM
A5
A3
A8
VLR
A5
A8
Figure 2.46 Security Algorithms
Algorithms A3 and A8 are located in the SIM module and in the
Authentication Centre (AC). A5 is located in the mobile station and in
the BTS.
Before an operator starts to use the security functions, the mobile
subscriber is created in the Authentication Centre. The following
information is required in creating the subscriber:
•
IMSI of the Subscriber
•
Ki of the subscriber
•
Algorithm Version Used
The same information is also stored in the Mobile Subscriber's SIM.
The basic principle of GSM security functions is to compare the data
stored by the network to the data stored in the subscriber’s SIM.
The IMSI number is the unique identification of the mobile subscriber.
Ki is an authentication key with a length of 32 hexadecimal digits.
The algorithms A3 and A8 use these digits as a basic value in
authentication.
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The Authentication Centre generates information that can be used for
all the security purposes during one transaction. This information is
called an Authentication Triplet.
The authentication triplet consists of three numbers:
•
RAND
•
SRES
•
K c.
RAND is a Random number, SRES (Signed Response) is a result that
the algorithm A3 produces on the basis of certain source information
and Kc is a ciphering key that A8 generates on the basis of certain
source information.
Random Number
Generator
AC
A3
RAND
SRES
Ki
A8
Kc
Authentication Triplet
VLR
RAND
SRES
Kc
Authentication Triplet
Figure 2.47 Authentication Triplet
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All the values included in the authentication triplet depend on each
other i.e. a certain RAND inserted to the algorithms with a certain Ki
always produces a certain SRES and a certain Kc.
When the VLR has this kind of three-value combination and the Mobile
Subscriber authentication procedure is initiated, the VLR sends the
random number RAND through the BSS to the SIM in the mobile
station. As the SIM has (or it should have) exactly the same algorithms
as used in triplet generation on the network side, the RAND number
that the SIM receives and inserts to the algorithm should produce
exactly the same SRES value as the one generated on the network side.
Authentication Triplet
VLR
RAND
SRES
Kc
Comparison
BSC
BTS
Kc
MS
RAND
SIM
Ki
A3
SRES
A8
Kc
Figure 2.48 Authentication Procedure
If the SRES value in the authentication triplet is the same as the SRES
calculated and sent by the mobile station, the authentication procedure
is successful.
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2.8.6
Ciphering/Speech Encryption
Ciphering is used across the Air interface to provide speech and
signalling encryption. When the authentication procedure has been
completed successfully, the BTS and the mobile station are ready to
start the ciphering procedure for further signalling and speech/data
transmission.
The speech of the user and the ciphering key, Kc, are processed by the
ciphering algorithm (A5) which produces the coded speech signal.
SPEECH/DATA
BTS
A5
Kc
TDMA
A5
ENCRYPTED
SPEECH/DATA
A5
Kc
TDMA
A5
SPEECH/DATA
Figure 2.49 Speech Encryption
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2.8.7
IMEI Checking
An option exists in GSM where the network may check the validity of
the mobile station hardware. The mobile station is requested to provide
the International Mobile Equipment Identity (IMEI) number. This
number consists of type approval code, final assembly code and serial
number of the mobile station. The network stores the IMEI numbers in
the Equipment Identity Register (EIR).
2.8.8
User Confidentiality
User confidentiality in general refers to methods of ensuring that
nobody calls at the expense of another person. From the end user's
point of view, the mobile station secures itself against misuse by asking
for the Personal Identification Number (PIN) when the station is
turned on. If the PIN entered by the subscriber is correct, the phone is
unlocked and it is ready for use. User identity is kept confidential by
using Temporary Mobile Subscriber Identity (TMSI) numbers.
After a successful first time location update, a mobile subscriber is
allocated a Temporary Mobile Subscriber Identity (TMSI). The next
time a transaction between the GSM network and the MS is initiated,
the subscriber is identified by the use of the TMSI. This is carried out
in the case of both mobile originating transactions (such as channel
request) in addition to network originating transactions (such as
paging). TMSI is reallocated after every successful authentication
verification. The format of a TMSI is operator dependent. It is a 32 bit
binary number and these TMSI numbers are reallocated in a ciphered
format. All relevant signalling information (IMEI, IMSI and directory
numbers) is ciphered before transferring them over the radio Air
Interface.
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2.9
Services
2.9.1
What Are Services?
In the broadest sense of the concept, any subscriber action that uses the
facilities provided and supported by the GSM system, can be
categorised as a service. Therefore, a person who has access to a GSM
mobile phone and wishes to make a call, is trying to access the speech
service provided by the system.
2.9.2
Classification Of Services
GSM is a multiservice system that allows various types of
communication that can be distinguished by the nature of the
transmitted information. Generally, based on the nature of the
transmitted information, services can be grouped as speech services,
where the transmitted data is speech and data services which covers
the rest of the information types such as text, facsimile, etc.
Services
Basic
Services
Teleservices
Supplementary
Services
Bearer
Services
However, if a person registers as a GSM subscriber and buys a mobile
station, he takes it for granted that at least the speech service is
guaranteed (after all, that is the reason why he bought the phone in the
first place).
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This raises another distinction in services:
•
Basic Services which are individual functions and may be
automatically available and included in the basic rights of the
subscriber as soon as he registers.
•
Supplementary Services which are extra services that are not
included as basic features, but are associated with the basic
services by enhancing and/or adding extra features to the basic
services.
As an example, take a person who has a subscription for two basic
services, Speech and Group 3 fax. In association with the fax service
the subscriber may request a supplementary service of Call Forwarding
On Mobile Subscriber Not Reachable, so that his data calls will be
forwarded to another destination.
When a user subscribes for more than one basic service, he will have a
different MSISDN for every basic service to which he subscribes. In
the example above, the calling party has to dial a different number
depending on whether he wants to talk or send a fax.
When these services are provided as a basic or supplementary service,
it is not only important to know what is transmitted. How the
transmission is carried out plays a major role, too. In addition, there is
the question of whether sufficient resources are available to support the
service from end to end. It may be that the service is available at one
end, but the target user may not have access to it and hence the use of
this service is not possible. Basically, there are three essential points
that need to be fulfilled before a service becomes available.
1. Whether the subscriber has access to the service or not?
2. Whether the GSM network from where the user is getting the
service has the necessary resources or not?
3. Whether the equipment owned by the user is capable of supporting
the service or not?
The second and third points raise an interesting scenario: using a
service that is provided by a network which is independent of the end
user equipment. An example of this is the use of facsimile in PSTN. It
utilises the basic network resources, but the transmitted information is
data which appears to be speech as far as the network is concerned.
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This brings us to the standard classification of services as described in
the GSM 900 and GSM 1800 Technical Specifications. The first
group is Teleservices, which provide the full communication capacity
by means of terminals and network functions as well as those provided
by dedicated centres. The other group is Bearer Services, which
provide the capability of transmitting signals between a GSM network
access point and an appropriate access point in the terminating network.
The latter allows the use of the network resources only.
2.9.3
Teleservices
The various types of teleservices provided by GSM network can be
summarised as shown in the following table.
Service Description
GSM Spec.
Code
Characteristics
Speech (Telephony)
T11
The most important service for
mobile systems, normal speech
service, including emergency calls.
Speech, Emergency
calls
T12
Emergency calls are possible
automatically.
Short message
Service (Mobile
Terminated)
T21
For the reception of Short
messages.
Short Message
Service (Mobile
Originated)
T22
For sending a short message to
another GSM subscriber.
Short Message
Service
T23
For the reception of broadcasted
short messages.
Group 3 Facsimile
transmission (with
alternate speech)
T61
Presently not supported by
NOKIA.
Automatic Group 3
Facsimile
transmission
T62
For sending and receiving
facsimile messages.
(Cell broadcast)
Table 2.1 Teleservices
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2.9.3.1
Speech (Telephony) and Emergency Calls
These are the most common teleservices used in the GSM network.
Speech is also the basic service that each subscriber is guaranteed to.
Telephony is a teleservice offering normal, traditional voice calls. The
normal security procedures apply to all such calls except in the case of
emergency calls which are processed regardless of possible security
violations (up to the point that emergency calls can be made with a
normal GSM mobile equipment that is not equipped with a SIM card).
It is worth remembering that 112, the standard emergency number in
most fixed networks is also used in GSM networks, even if 112 is not
necessarily the standard emergency number in every country.
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2.9.3.2
Short Message Services: Mobile Originated, Mobile
Terminated and Cell Broadcast
The Short Message Service (SMS) is a service enabling the mobile
subscriber to receive and/or send short (max. 160 characters) messages
in text format. These messages can be received at any time (also during
a conversation).
This service requires a dedicated equipment called Short Message
Service Centre (SMSC) which may be located in the Network
Switching Subsystem or outside the GSM network, but it always has
signalling connections to MSC. The SMSC acts as a temporary storing
and forwarding centre if the Mobile Station is unreachable.
Air
A
MSC
VLR
Short Message Service
Mobile Terminated (MT)
Short Message Service
Mobile Originated (MO)
SMS
Alfa s kop m 346
Alfas k op m3 46
Alfa skop m34 6
Figure 2.50 Short Message Service, MO and MT
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In the case of SMS-MO (Mobile Originated) the message sent by the
mobile is stored in the SMSC, while in the case of SMS-MT (Mobile
Terminated) the message stored in the SMSC is transmitted to the
target mobile station. In a cell broadcast SMS, the information is sent to
all the stations within a certain predefined geographical area. The
services of SMSC are not required in cell broadcasting, as the BSC is
equipped with the necessary SMSC functions. The maximum length of
a cell broadcast SMS is 93 characters.
Air
A
BTS
BTS
BSC
BTS
BTS
Short Message Service
Cell Broadcast
BTS
O&M
Alfas k op m34 6
Alfa s k op m 34 6
Alfa s k op m3 46
Figure 2.51 Short Message Service, Cell Broadcast
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2.9.3.3
Facsimile Transmission (T61 and T62)
Facsimile transmission is a teleservice that sets requirements for
terminal equipment and their adaptation. There is one predefined case
in which the Mobile Station needs to be interfaced with a computer
equipped with a fax modem. However, because it is used for data
transmission, there has to be a provision for the bearer service in order
to define the characteristics of the bearer such as data transmission rate
and Air Interface error correction protocol.
In the case of T61 Facsimile transmission, the receiver is either not
aware that the incoming call is addressed to the fax and so he has to
establish the nature of the call by talking with the calling party first, or
the receiver knows that it is a facsimile call but still wants to talk with
the calling party. In both cases, the nature of the transmitted
information is data (group 3 facsimile) and speech alternately (during
the same call). As shown in the table, this service is not currently
supported by Nokia.
The T62 automatic facsimile is an automatic fax service where the
receiver has a different MSISDN for the fax service and all calls to this
number are purely data transmission calls.
Air
A
HLR
Transparent /
Non Transparent
BTS
BSC
TC
MSC
AC
EIR
VLR
IWF
Modems/
Rate Adaptation
Figure 2.52
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Facsimile Transmission (T61/Transparent, T62/Non
Transparent)
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2.9.4
Bearer Services
Bearer Services come into the picture when data transmission services
are needed and there are a number of different types of data services
available. The distinctions between these data services are based on the
users (which can be connected to the PSTN, ISDN or a PSPDN
network) and the mode of transmission (packet or circuit switched,
whether end-to-end digital or not, synchronous or asynchronous).
Bearer Services (BS) supported by the GSM system only provide the
capability of transmitting signals between the originating and
terminating access points. There are no recommendations on the end
user terminals. At the moment, the bearer services are divided into 10
categories, each of which describes the characteristics of the bearer.
Four of the most essential categories are listed in the table below.
–
Circuit Mode unstructured with Unrestricted Digital Capability
Transparent.
–
Circuit Mode unstructured with Unrestricted Digital Capability Non
Transparent.
– PAD Service.
–
Packet Service.
The first two types of bearer services are used for data communication
in a similar fashion as in the PSTN and are essentially used for data
communication between GSM networks and PSTN. Since the PSTN
network is designed for voice communication with a bandwidth of
3.1Khz, digital data has to be modulated with an audio frequency
carrier in order to enable transmission via the PSTN. This practice is
still in extensive use throughout the world. Thus it is necessary for the
GSM users to be able to use this function while communicating with
the PSTN.
However, this is not easily realised in GSM networks because of the
radio characteristics of the Air Interface. This interface is based on a
special speech coding algorithm that ensures the best quality with the
lowest possible bit rate e.g. 13Kbits/s which makes it incompatible for
modem signals. Thus a GSM user will never need a modem for data
communication. The connection between the mobile station and the
GSM network is fully digital. Within the GSM network, the digital
signal passes through a suitable modem to make the signal compatible
for the addressed equipment in the PSTN. Alternatively, modem signals
coming from the PSTN will be demodulated into digital signals in the
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GSM network and the unrestricted digital data will be forwarded to the
mobile user. The modems are located in the MSC.
Air
A
HLR
Transparent /
Non Transparent
BTS
BSC
TC
MSC
AC
EIR
VLR
IWF
Synchronous /
Asynchronous
Modems /
Rate Adaptation
Figure 2.53 Data Services
Thus it becomes necessary to characterise the bearer (specifying the
data rates and type of modem to be used). The terms transparent and
non-transparent identify whether a second layer of error correction
protocol is employed in the air interface or not. A non-transparent
service employs re-transmission in case of errors whereas a transparent
service does not employ it.
The last two types of data services refer to accessing a Packet
Switched Public Data Network (PSPDN). These are “general
purpose” data networks that use packet transmission techniques mostly
between computers, as opposed to the conventional circuit switched
techniques. Information is sent in packets along whichever route is
available at the time. At the receiving end, these packets are
reassembled in a sequential order and the original information is
recreated.
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A PSPDN can be accessed in a number of ways and some of the most
common solutions are:
•
A direct X.25 connection from the PSPDN to the user.
•
Through PSTN or ISDN using a Packet Assembler/Dissembler
(PAD) service in the network. The user is connected to the
intermediate network (either PSTN or ISDN) and the PAD of this
network will assemble/disassemble the user’s data to/from
packets sent/received by/from the PSPDN.
•
Through ISDN, which has the capability of sending and receiving
data packets without the PAD service.
Thus a GSM user may use any of the methods available to him as
shown in the figure below.
GSM
PSTN/ISDN
1
PAD
PAD
2
PAD
PAD
GSM
PSPDN
PAD
PAD
3
GSM
PAD
PAD
Packet Assembler/Disassembler
Packet Interfaces
Non Packet Interfaces
1 Normal data communication access. PAD at PSPDN
2 PAD service from GSM. PAD at GSM
3 Packet Service from GSM. PAD at user terminal
Figure 2.54 Packet Data Network Access methods
At the moment it is possible to achieve 14.4Kbits/s data connections
thanks to HSCSD (High Speed Circuit Switched Data). During 1999 it
will be possible to use more than one traffic channel in the air interface
to achieve up to 57.6Kbits/s data rates. We will discuss these new
developments under the chapter “Next Step”.
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2.9.5
Supplementary Services
Supplementary services enhance or supplement the basic
telecommunication services. The same supplementary services may or
may not be employed by a number of different basic services such as
basic telephony or T62 automatic facsimile service. The following list
covers most of the common services, as well as the essential
supplementary services.
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
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Advice of charge - AOC
Alternate Line Service (ALS) – personal or business
Barring of all incoming calls - BAIC
Barring of all Incoming calls when roaming outside the HPLMN
Barring of Incoming Calls when abroad
Barring of outgoing calls - BOC
Barring of outgoing International Calls - BOIC
Barring of outgoing international calls excluding those directed to the
HPLMN country
Call forwarding on mobile subscriber busy - CFB
Call forwarding on no answer - CFNA
Call forwarding unconditional - CFU
Call Hold
Call Waiting - CW
Calling line identification presentation - CLIP
Calling line identification restriction – permanent or per-call - CLIR
Centrex Services
Closed user group - CUG
Conference call - CONF
Explicit Call Transfer
Operator Determined Barring (ODB)
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2.10
Summary of the Learning Points
The GSM network is divided into three subsystems:
Network Switching Subsystem (NSS) - Contains the elements Mobile
Services Switching Centre (MSC), Home Location Register (HLR),
Visitor Location Register (VLR), Authentication Centre (AC),
Equipment Identity Register (EIR). Its functions are:
•
Call control. A mobile terminated call requires HLR enquiry to
locate the called subscriber.
•
Mobility Management.
– The HLR always knows in which MSC/VLR area a particular
subscriber is located.
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–
An MSC/VLR knows in which Location Area a subscriber is
located. This is enabled by a Location Update of which there
are three types: Power On, Generic and Periodic.
–
Mobility Management also helps in maintaining ongoing calls
for a moving subscriber by a procedure known as Handover.
There are four types of Handovers: Intra Cell, Inter Cell-Intra
BSC, Inter Cell - Inter BSC and Inter MSC.
–
Subscriber Data handling. A subscriber’s data is located in
three places: the HLR, VLR and SIM card.
•
Security Issues. Subscriber verification is performed in the VLR
by an authentication process. Speech encryption is carried out
between BTS and Mobile Station.
•
Various types of numbers are used in the GSM network for
different functions. The most important ones are: IMSI,
MSISDN, MSRN, LAI, LAC, CGI, TMSI and HON.
•
Charging. The MSC is responsible for collecting charging
information. It is sent to the Billing Centre which creates bills for
the subscriber.
•
Signalling towards Base Station Subsystem and other networks.
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•
The services offered by the GSM network are classified as
–
–
Teleservices, and
–
Bearer Services
A teleservice or a bearer service is also a basic service which
has an enhanced performance by the use of a number of
supplementary services
Base Station Subsystem (BSS). The BSS consists of the following
network elements: Base Station Controller (BSC), Base Transceiver
Station (BTS) and Transcoder (TC). Its main functions are:
•
Radio Network control and management.
The BSS assigns, monitors and releases traffic and control
connections on the radio interface. If necessary it performs
handovers within one cell, between two cells under the same or
different BSC and between cells connected to different MSCs.
•
Speech transcoding
The Transcoder is responsible for decreasing (towards the MS)
respectively increasing (towards the MSC) the data rate for
speech only (never for data or signalling) according to the
transmission restrictions on the air interface.
•
Air interface signalling and data processing
•
Signalling towards the NSS and air interface
Network Management Subsystem (NMS). The main functions of the
NMS are:
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•
Fault Management to detect and cancel faults as soon as possible
to keep the network running correctly.
•
Configuration Management to have centralised authority for
managing hardware and software changes in addition to security
operations.
•
Performance Management on the one hand to avoid traffic
overload and on the other hand to use the installed hardware in
the most economic way.
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2.11
Traffic Management Review
2.11.1
Review Questions
In the following questions, please select one alternative which you
think is the best answer for the particular question. There may not be
a perfect answer, select the one which you think is the most correct.
1. Which of the following does not contain subscriber data?
a) Home Location Register (HLR)
b) Visitor Location Register (VLR)
c) Mobile Services Switching Centre (MSC)
d) Subscriber Identity Module (SIM)
2. Location Update procedure is initiated by
a) Mobile Station (MS)
b) Mobile Services Switching Centre (MSC)
c) Base Station Controller (BSC)
d) Home Location Register (HLR)
3. The format of International Mobile Subscriber Identity (IMSI) is
a) CC + NDC + SN
b) MCC + MNC + MSIN
c) MCC + MNC + LAC
d) Operator Specific 32 bit binary number
4. The three subsystems of GSM/DCS are
a) NMS, PSTN, MS
b) NMS, BSS, MS
c) NSS, BSS, MS
d) NSS, BSS, NMS
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5.
Which of the following cases will result in an HLR Enquiry?
a) PSTN originated PSTN terminated call
b) Mobile originated PSTN terminated call
c) PSTN originated mobile terminated call
d) None of the above
6. Which of the following is not a task of the Network Switching
Subsystem (NSS)?
a) Identifying the calling subscriber
b) Starting the location update procedure
c) Sending charging data to the billing centre
d) Paging a subscriber for mobile terminated calls
7. A Location Area
a) is the geographical area under one Base Station Controller
(BSC)
b) is equal to one MSC area
c) is equal to one cell
d) is identified by a unique Location Area Identity
8. Which of the following combination best describes the Base Station
Subsystem?
a) Base Station Controller, Transcoder, Base Transceiver Station
b) Mobile Station, Base Station Controller, Base Transceiver
Station
c) Transcoder, Submultiplexer, Base Transceiver Station
d) Base Station Controller, Base Transceiver Station, Mobile
Equipment
9. The Base Station Subsystem (BSS)
a) is responsible for radio network control
b) is located between Air and A interfaces
c) gets its synchronisation signal from the MSC
d) All of the above
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10. Initiation of the paging process for a mobile station is done by
a) MSC over a location area
b) all Base Transceiver Stations in a location area
c) a Base Station Controller over one BSC area
d) a Base Transceiver Station in one cell
11. In a mobile terminated call a traffic channel from the NSS in the
terminating side to the called mobile station is reserved when
a) the user of the Mobile Station answers the ringing of the phone
b) B subscriber accepts the international roaming leg charging
c) MSRN analysis is done and the location area of the subscriber
is known
d) the mobile station answers the paging signal
12. Why is it necessary to check the calling subscriber’s data before
allowing the call to proceed in the originating NSS side?
a) To make sure that he has the correct IMSI
b) To make sure that he is provisioned the requested service
c) To make sure that he has paid the bill
d) To make sure that the NSS knows whom to charge for the call
13. If in a GSM/DCS network the periodic location update timer is set
as 10 hours, then a periodic location update is done
a) 10 hours after the last periodic location update
b) 10 hours after the last power on location update
c) 10 hours after the last generic location update
d) 10 hours after the last transaction of any kind with the NSS
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14. In an inter VLR location update, the new VLR asks the old VLR for
some information about the Mobile Subscriber. The old VLR
responds to this query by providing which of the following
information?
a) IMSI
b) IMSI, TMSI and Subscribed services
c) IMSI and last Location update time
d) IMSI and HLR address in case of a roaming subscriber
15. If a handover occurs during a call
a) the new frequency resource always belongs to a cell other than
the current one
b) a handover number is always required
c) the initial speech path is not disconnected until a successful
message comes from the mobile station on the new channel
d) the subscriber has to pay extra for the additional network
operation required to maintain his call
16. A PSTN originated mobile terminated call has just had an inter
MSC handover from GMSC A to MSC B. It has now become
necessary to make another inter MSC handover to MSC C. After
successful handover for the second time, the new call path will be:
a) PSTN - GMSC A - MSC C
b) PSTN - GMSC A - MSC B - MSC C
c) PSTN - MSC C
d) PSTN - GMSC A - MSC B - GMSC A -MSC C
17. Which of the following is not a possible way to charge the
subscriber?
a) Subscription charge
b) Purchase of a Mobile Equipment
c) Purchase of a SIM card
d) Air time on a per call basis
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18. In which of the following situation the actual receiver of the call
will also be responsible for paying part of the call charge?
a) Mobile A to mobile B of the same network but B roaming in
another country
b) Mobile A to Mobile B of the same network, but B has
unconditional call forwarding to mobile C of a different
network
c) PSTN to mobile A who is wandering around in his own
network
d) Mobile A to Mobile B of the same network, but B has
unconditional call forwarding to PSTN C of a different network
19. Which network element creates bills for the subscriber?
a) HLR with information from MSC
b) MSC with information from Billing Centre
c) Billing Centre with information from MSC
d) Billing Centre with information from Transcoder
20. Authentication means
a) verifying the mobile phone user
b) verifying the mobile equipment
c) verifying the correct algorithm
d) verifying the SIM card
21. The contents of the authentication triplet are
a) SRES, RAND, A3
b) SRES, RAND, Kc
c) A3, A5, A8
d) RAND, A3, A8
22. What is the result when you use Ki and RAND as inputs through
A8?
a) coded speech
b) Signed response
c) Authentication triplet
d) Kc
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23. Encryption of user data is done between which network elements?
a) Base Transceiver Station and Mobile Station
b) Base Station Controller and Mobile Station
c) Mobile Services Switching Centre and Mobile Station
d) Mobile Station to Mobile Station - end to end
24. What is standard classification of services as mentioned in the
specifications?
a) Basic services and Supplementary services
b) Speech service and data services
c) Teleservices and Bearer services
d) All of the above
25. Which of the following can not be a Basic Service of a mobile
subscriber?
a) Emergency Calls
b) Unrestricted digital data at 9600 bps
c) Call forwarding unconditional
d) Short Message Service Mobile Originated Point to Point
26. Mobile Subscriber A has CLIP active, mobile subscriber B has
CLIR active. A calls B, then
a) B can see A’s MSISDN
b) B cannot see A’s MSISDN
c) MSC prevents A’s MSISDN from going to B
d) None of the above.
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3
Signalling
3.1
Module Objectives
At the end of the module the student is able to:
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Explain the needs of signalling
•
Describe the C7 protocol stack and their functions in telephone
exchanges and how they are different for GSM compared to
PSTN.
•
Identify the protocol stacks implemented in each GSM network
element BSC, MSC and HLR
•
Explain the specific needs for the GSM network and the
additional protocol layers.
•
Explain the SS7 requirements for individual GSM element
(MSC/HLR/BSC).
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3.2
Introduction
Signalling in telecommunication networks has come a long way since
the early days when a lady operator used to sit at the central exchange.
Telecommunication networks were relatively simple and the general
procedure of setting up a call would go something like this:
You would pick up the “handset” of your telephone, electrical current
would flow to the exchange and a light would start blinking
accompanied by a sound. This would let the lady know that you are
requiring service. She would plug in one connector to your terminal
and the other to her “headphone” and inquire about whom you wanted
to talk to. After listening to your answer, she would then try to connect
you to the person you want to talk with.
Then she would pull out the connector from your terminal and connect
it to your intended party. He would then hear his phone ringing. After
he answers, the lady will connect you to him. While you are talking,
she will supervise the call, and once the conversation is over (which
will be indicated by another light), she will “pull out the plugs.” That
would be a typical scenario at a telephone exchange during the first half
of this century.
%#!&?:^*
(%&¤#”/=
Figure 3.1
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Signalling in the old days.
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3.2.1
Standard Messages
Let us once again go over the actions and see what exactly is
happening.
1.
When you lift the receiver a light at the exchange starts to glow
which is an Indication that you need service.
2.
The lady asks you who you want to talk with and you tell her the
person’s name. An indication that service will be provided with a
further request for address, which is provided.
3.
She makes the connection to the called party and the phone starts
ringing. Connection of the speech path is completed and the
called party is alerted.
4.
The called party answers, and you are connected. Answer and
conversation.
5.
The lady at the exchange monitors your call during the
conversation. Supervision.
6.
When you hang up, a light glows to indicate this and she pulls out
the plugs. An indication of disconnection and clearing of the
call.
Calling Party
Exchange
Called Party
Request for Service
Request Address
Provide Address
Process Information
and make connection
Alert Called party
Called Party Answer
Conversation
Disconnection
Figure 3.2 Signalling Operations
This example highlights two important aspects. The conversation,
which is the primary task, and what went on behind the scene to make
that conversation successful. This has been the essence of Signalling in
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telecommunications for a long time. The functions of signalling are as
follows:
•
To set up a call
•
To supervise the call
•
To clear a call.
There is a uniform set of standard messages in the signalling repertoire.
Over the years, there has been a vast improvement in signalling
technology and the lady operator has long since been replaced by
automatic digital switching exchanges. They are still doing exactly the
same job as the lady, but faster and more reliably over a global
network.
3.2.2
Implementation and Evolution
As mentioned in the previous section, signalling in telecommunication
systems is basically a set of messages used for setting up, supervising
and clearing the call.
Many different factors have led to a variety of signalling systems being
developed in telecommunications networks.
Different signalling standards were developed in different parts of the
world. They were all doing the same task, but in a different way. This
would obviously mean that when a call originates in one network with
one type of signalling implementation and terminates in another
network with another type of signalling system, some compromise, or
adaptation would have to be used. Due to these kind of differences the
then international governing body for telecommunications, CCITT
(now ITU), recommended the Channel Associated Signalling System
(CAS) as the standard.
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Drawbacks of the CAS System
As a signalling system for setting up calls CAS was a very good system
that performed quite well. A large number of telephone exchanges in
the world are still using this system but its implementation is such, that
it is only suitable for cases where traffic is low. Another problem with
CAS is that it is not possible to send signalling messages in the absence
of a call. This causes bottlenecks and wastes bandwidth.
Common Channel Signalling (CCS)
The CCITT (now the ITU) came up with a new recommendation for a
signalling system which was the Common Channel Signalling
System Number 7. One of the main advantages of this system was that
signalling did not have to go along the same path as the speech. It is
abbreviated as CCS7, CCS#7, SS7 or simply C7 but they all refer to the
same system.
SS7 was developed in the mid to late ‘80s and is a Common Channel
Signalling system (CCS) with a signalling path bandwidth of 64Kbits/s.
It is modular in design although the modules are not as clearly defined
as is the case with the OSI 7-layer model, which it pre-dates.
Let us take a closer look at this system in the following sections.
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3.3
Common Channel Signalling System No.7
Originally, the Common Channel Signalling System No. 7 (hereafter
referred to as SS7) consisted of two parts. The first part was responsible
for transferring the message within a signalling network. The second
part was the user of these messages.
As an analogy we can compare it to two managers with their own
message runners. One manager writes a message, puts it in the
envelope and gives it to the messenger. The messenger in turn looks at
the address on the envelope, and gives it to the messenger of the other
manager. The messenger of the receiving manager, looks at the address
and gives it to his manager, who will then read and act as necessary.
Figure 3.3
Message bearers taking the message to their managers
Thus the initial phase of SS7 consisted of two parts
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1.
Message Transfer Part - MTP (responsible for transferring
messages)
2.
Telephone User Part - TUP (user of messages)
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3.3.1
Message Transfer Part (MTP)
We have so far established that signalling is used for setting up calls,
and that there are standard sets of messages which are send back and
forth to help facilitate this. The part which is responsible for taking
these messages from one network element to other network element is
known as the Message Transfer Part (MTP). The entire SS7 is built
on the foundation of this MTP which consists of three sublayers as
shown.
Message
Transfer
Part (MTP)
Layer 3
Signalling Message Handling
Layer 2
Data Link Control
Layer 1
Physical Connections
Figure 3.4 Message Transfer Part Layers
The lowest level, MTP layer 1 (Physical Connections), defines the
physical and electrical characteristics. MTP layer 2 (Data link
control) helps in error free transmission of the signalling messages
between adjacent elements. MTP layer 3 (Network layer) is
responsible for taking the message from any element in a signalling
network to any other element within the same network.
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3.3.2
Telephone User Part (TUP)
The previous section explained the MTP. But who is the user who
receives, sends and acts on these messages? The answer is the
Telephone User Part (TUP). Those standard sets of messages that
were mentioned previously are the standard TUP messages which help
to set up the call, to supervise and clear it.
For many the SS7 in the fixed telephone network consisted of only two
parts, the MTP and TUP. The CCITT (now the ITU) allowed for small
variations in messages within one country alone. These variations were
minor and very similar to the TUP, but they were called the National
User Part (NUP).
TUP
NUP
Call Control
Messages
TUP
NUP
ISUP
ISUP
MTP
Layer 3
Transport of Signalling
Messages within one network
Layer 3
Layer 2
Data Link Control
Layer 2
Layer 1
Physical Connections
Layer 1
Figure 3.5 Protocol stack of MTP and TUP/NUP/ISUP
With the introduction of the Integrated Services Digital Network
(ISDN), which has a broader capability than the PSTN, some extra sets
of messages were required. These became known as the ISDN User
Part (ISUP). Whether it’s TUP, NUP or ISUP they are all doing the
same job in helping to set up a call.
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3.3.3
Signalling Connection and Control Part (SCCP)
The structure of SS7 with TUP/NUP/ISUP on top of MTP was quite
satisfactory for call handling. However, with the passage of time and
the development of newer and advanced technology, signalling
requirements also started to become more stringent and demanding.
It was realised that the TUP/MTP combination alone was not sufficient
when "virtual connections" became necessary. MTP guarantees the
transfer of messages from any "signalling point" in the signalling
network to any other "signalling point", safely and reliably. However,
each message could reach the destination signalling point by using
different paths. This may cause situations where the order of messages
that are received, are different from the original sequence. When this
order is important, there is need for establishing a "virtual connection".
Virtual Connections use a "Connection Oriented" protocol that will
provide sequence numbers to enable the messages to be placed in the
correct order at the distant end.
Another instance of when the TUP/MTP structure is inefficient, is
when a signalling message has to be sent across multiple networks in
the absence of a call. MTP is capable of routing a message within one
network only. The case of setting up a call across multiple networks is
not the same as signalling across the same network. The signalling
goes leg by leg according to the call. But in the absence of a call, MTP
cannot route a signalling message across multiple networks.
Virtual
Connection using
“Connection Oriented”
SCCP
A
MTP
Signalling
Point
Signalling
Point
Signalling
Point
MTP
MTP
B
Destination
Signalling
Point
Figure 3.6 Virtual Connections
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The solution to these two problems was the creation of another protocol
layer on top of MTP which was called the Signalling Connection and
Control Part (SCCP). SCCP takes care of virtual connections and
connectionless signalling. Note that the tasks of TUP and SCCP are
different, and thus they are parallel to each other, but both use the
services of MTP.
TUP
NUP
ISUP
SCCP
MTP
Figure 3.7 Location of SCCP
As far as the fixed telephone network (the Public Switched Telephone
Network, PSTN) is concerned, this is all there is to SS7 and these
protocol layers serve the purpose very well. At the moment there is no
other protocol in SS7 for PSTN exchanges.
3.3.4
Summary
MTP is the message transfer part. It is responsible for transferring
messages from one network element to another within the same
network. It consists of three sublayers.
TUP is the user part of the messages transferred by MTP. These
messages deal with setting up, supervising and clearing the call
connections. It has two variations: NUP and ISUP.
SCCP is the signalling connection and control part. Its main function is
to provide virtual connections and connectionless signalling.
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3.4
Other Applications of SS7 in GSM Networks
In GSM networks, signalling is not as simple as in the PSTN. There are
extra signalling requirements in GSM due to the different architecture
of the network which requires a large amount of non-call-related
signalling. In the first instance the subscriber is mobile, unlike the
PSTN telephone which is always in one place. Therefore, a continuous
tracking of the mobile station is required which results in what is
known as the Location Update procedure. This procedure is an example
of non-call-related signalling, where the mobile phone and the network
are communicating but no call is taking place. This requires additional
sets of standard messages to fulfil the signalling requirements of GSM
networks.
These additional protocol layers are
1. Base Station Subsystem Application Part (BSSAP)
2. Mobile Application Part (MAP)
3. Transaction Capabilities Application Part (TCAP)
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3.4.1
Base Station Subsystem Application Part (BSSAP)
The first of these additional protocol layers, which are specific to GSM
networks, is the Base Station Subsystem Application Part (BSSAP).
This layer is used when an MSC communicates with the BSC and the
mobile station. Since the mobile station and MSC have to communicate
via the BSC, there must be a virtual connection, therefore the service of
SCCP is also needed.
The authentication verification procedure and assigning a new TMSI all
take place with the standard sets of messages of BSSAP.
Communication between MSC and BSC also uses the BSSAP protocol
layer. Therefore, BSSAP serves two purposes:
•
MSC-BSC signalling
•
MSC-MS signalling.
BSSAP
TUP
NUP
SCCP
ISUP
MTP
Figure 3.8 Location of BSSAP in SS7
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3.4.2
Mobile Application Part
The example of a location update procedure mentioned previously is
not confined only to the MSC-BSC section, it spans multiple PLMNs.
In case of a first time location update by an international roaming
subscriber (where he is not in his home network), the VLR has to get
the data from the subscriber’s HLR via the gateway MSC of the
subscriber’s home network.
While a mobile terminated call is being handled, the MSRN has to be
requested from the HLR without routing the call to HLR. Therefore, for
these cases another protocol layer was added to the SS7 called Mobile
Application Part (MAP). MAP is used for signalling communication
between NSS elements.
NOTE: The MSC-MSC communication using MAP is used only in
case of non-call-related signalling. For routing a call from one MSC
to another MSC, TUP or ISUP is still used.
3.4.3
Transaction Capabilities Application Part (TCAP)
In MAP signalling, one MSC sends a message to an HLR, and that
message requests (or invokes) a certain result. The HLR sends the
result back, which may be the final result or some other messages
might also follow (or it might not be the last result). These invocations
and results that are sent back and forth between multiple elements using
MAP need some sort of secretary to manage the transactions. This
secretary is called the Transaction Capabilities Application Part
(TCAP). This completes the SS7 protocol stack in the GSM network
and their functions.
The SS7 picture is now complete.
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MAP
BSSAP
TCAP
SCCP
TUP
NUP
ISUP
MTP
Figure 3.9 MAP and TCAP
3.4.4
Summary
Protocol
Name
Function
MTP
Message Transfer Part
Responsible for transferring an SS7 message
from one network element to another within the
same signalling network
TUP
Telephone User Part
NUP
National User Part
ISUP
ISDN User part
User parts of MTP. They send, receive, analyse
and act on the messages delivered by MTP. All
of these are Call Control Messages that help
in setting up, supervising and clearing a call.
SCCP
Signalling Connection
and Control Part
Protocol layer responsible for making virtual
connections and making connectionless
signalling across multiple signalling networks.
BSSAP
Base Station Subsystem
Application Part
Protocol layer responsible for communicating
GSM specific messages between MSC and
BSC, and MSC and MS.
MAP
Mobile Application Part
A GSM specific protocol for non-call- related
applications between NSS elements
TCAP
Transaction Capabilities
Application Part
Protocol layer responsible for providing service
to MAP by handling the MAP transaction
messages between multiple elements.
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3.5
SS7 Layers in GSM Elements
In this section, the SS7 requirements for individual GSM elements will
be shown. The previous sections explained why SS7 was needed in
GSM and what are the protocol layers that are used. It is useful to note
that not all the GSM elements have all the protocols in the SS7 stack.
For example, a BSC would never need TUP because call control is not
the task of the BSC.
3.5.1
Protocol Stack in MSC
Since the MTP is the foundation on which SS7 is built, this will be
required in every element which is capable of processing SS7. The
MSC is the element in GSM networks which is responsible for call
control, therefore, TUP/ISUP sits on top of MTP for that purpose. The
MSC/VLR is also responsible for location updates and communication
with the BSC and the HLR. For this reason it also needs to have
BSSAP and MAP which sit on top of SCCP. The MSC also has TCAP
to provide services for MAP. It can be seen therefore, the MSC/VLR
has all the SS7 protocol stacks implemented in it.
MAP
BSSAP
TCAP
SCCP
TUP
NUP
ISUP
MTP
Figure 3.10
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Protocol stack in MSC
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3.5.2
Protocol Stack in HLR
The HLR is not responsible for call control, therefore, TUP/ISUP is not
necessary. In addition, the HLR does not communicate directly with the
BSC; therefore, BSSAP is also not needed, which leaves MTP, SCCP,
TCAP and MAP as the signalling protocols in the HLR.
MAP
TCAP
SCCP
MTP
Figure 3.11 Protocol stack in HLR
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3.5.3
Protocol Stack in BSC
The BSC only needs BSSAP, but since BSSAP needs the services of
the SCCP which in turn needs the MTP, the BSC contains MTP, SCCP
and BSSAP.
BSSAP
SCCP
MTP
Figure 3.12 Protocol stack in BSC
MSC
MAP
TCAP
BSSAP
TUP
NUP
ISUP
SCCP
MTP
PSTN
Exchange
SCCP
MTP
MAP
BSSAP
BSC
SCCP
MTP
TUP
NUP
ISUP
TCAP
HLR
SCCP
MTP
Figure 3.13 SS7 Protocols in various network elements.
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3.5.4
Other Signalling Protocols in GSM
As we have already seen, the GSM core network elements use SS#7
(Signalling System No. 7) to pass signalling messages between them.
Mobility and Connection Management
Radio Resource Management
BSC
BSC
LAPDm
BTS
BTS
MSC
MSC
SS#7
LAPD
PSTN / HLRs /
other MSCs
SS#7
Figure 3.14 Signalling in GSM
Between the BSC and the BTS, a signalling protocol is used known as
LAP-D (Link Access Procedure for the ISDN "D" channel). This is the
same protocol that is used in ISDN networks between the customer and
the network.
Between the mobile station and the BTS, the same signalling protocol
is used with small modifications to cope with the characteristics of the
radio transmission medium. This protocol is known as LAP-Dm where
the "m" denotes modified.
The LAP-D message structure is similar to SS#7 but it does not support
networking capabilities, therefore, it is used for point to point
connections.
Protocols for Radio Resource (RR) management are passed using
LAP-Dm and LAP-D. Other protocols for Mobility Management
(MM) and Connection Management (CM) are passed between the
Mobile Station and the MSC.
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3.5.5
Summary
The following table highlights the function of the SS7 protocol in every
GSM network element capable of processing SS7.
MSC
BSC
HLR
Transfer of SS7
messages between
different network
elements
Transfer of SS7
messages between
different network
elements
Transfer of SS7
messages between
different network
elements
Setting up, supervising
and clearing call
connections
Unavailable
Unavailable
Connection-less
signalling and virtual
connections
Virtual Connection
between MSC and
MS
BSSAP
GSM signalling with
BSC and MS
GSM Signalling
with MSC
Unavailable
MAP
GSM Specific
signalling with HLR
and other MSC
Unavailable
GSM Specific
signalling with MSCs
and other HLRs
TCAP
Service Provider to
MAP
Unavailable
Service Provider to
MAP
MTP
TUP/IS
UP
SCCP
Connection-less
signalling
A Virtual Connection uses packet type switching principles and the
connection only exists when packets or messages are being transferred.
In the simplest form of packet switching each packet is regarded as a
complete transaction in itself. This is known as “Connectionless” mode
as there is no sense of a connection being set up before communication
begins and the network treats each packet independently. Some
applications, however, involve the transfer of a sequence of packets, for
which the “Connection-oriented” approach is more appropriate. In this
case, a virtual connection is established by an initial exchange "set-up"
packets between the communicating terminals. During the data transfer,
each packet associated with a connection is passed over the same route
through the network.
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3.6
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Summary of the Learning Points
•
Signalling is the transfer of information between subscriber
interface points and the network and between different network
elements to help establish a call.
•
Signalling Information is interchanged as standard sets of
messages which was developed and standardised in to the present
SS7 system.
•
GSM networks need non-call related signalling which is possible
with SS7.
•
The SS7 used in PSTN networks is not sufficient to fulfil the
signalling requirements of GSM networks, thus new protocols
specific to GSM were developed.
•
MTP is the basis of SS7, and it is responsible for transferring of
signalling messages from one element to another within the same
signalling network.
•
TUP/ISUP are the user parts of MTP which handle call control.
•
SCCP is needed for virtual connections and connectionless
signalling.
•
BSSAP is used for signalling between MSC-BSC and MSC-MS.
•
MAP is needed for signalling between MSC-HLR, MSC-VLR,
HLR-VLR (and MSC-MSC in the case of non-call related
signalling).
•
Link Access Protocol in D channel (LAP-D) provides a point-topoint signalling capability. It is used between the MS and the
BTS and also between BTS and BSC.
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3.7
Signalling Review
3.7.1
Review Questions
In the following questions, please select one alternative which you
think is the best answer for the particular question. There may not be
a perfect answer, select the one which you think is the most correct.
1. Which of the following is not a signalling function?
a) To analyse the dialled digits
b) To digitise users speech before transmission
c) To make speech path connections
d) To inform the user of the progress of call
2. Which of the following is not a need for signalling?
a) The need to supervise a call
b) The need to make circuit reservations
c) The need to clear connections after call is over
d) The need to transfer charging information
3. Which of the following was a drawback of the CAS signalling?
a) It supported only call related signalling
b) It required to have one signalling channel for every PCM line
c) It was not possible to have many different signalling messages
d) All of the above
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4. Which of the following is an advantage of SS7?
a) It can send call set-up messages
b) One signalling channel can support approximately ten thousand
traffic channels
c) It can support non call related signalling
d) All of the above
5. Which of the following signalling requirements is specific to GSM
networks only?
a) The ability to reserve circuits on the outgoing direction
b) The ability of one signalling channel to handle calls in other
physically different cables
c) The ability to transport service dependent messages across
switching exchanges
d) The ability to perform non call related signalling procedures
6. Which of the following combination of SS7 protocols is not present
in PSTN exchanges?
a) MTP, SCCP
b) MTP, ISUP
c) MTP, TUP
d) MTP, SCCP, TCAP, MAP
7. Which of the following combination of SS7 protocols are specific
to GSM networks only?
a) MAP, BSSAP
b) MAP, BSSAP, TUP
c) MAP, BSSAP, ISUP, SCCP
d) MAP, BSSAP, TCAP, SCCP, MTP
8. Which of following combination best contains the network
elements in GSM network which do not have SS7?
a) MSC, HLR
b) BSC, BTS
c) MSC, OMC
d) BTS, TC
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9. Which of the following pictures is correct?
a)
MAP
ISUP
TCAP
BSSAP
SCCP
MTP
b)
MAP
BSSAP
SCCP
TCAP
TUP
NUP
ISUP
MTP
c)
MAP
BSSAP
TCAP
SCCP
TUP
NUP
ISUP
MTP
d)
BSSAP
MAP
TCAP
TUP
NUP
ISUP
SCCP
MTP
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4
Transmission
4.1
Module Objectives
At the end of the module the student will able to:
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•
Differentiate between physical and logical channels.
•
List and describe the twelve different types of logical channels
and their functions.
•
Describe how the air interface properties affect the transmission
of speech between the mobile station and the network and briefly
explain the GSM solutions to these problems.
•
Describe the main function of transcoder.
•
List the three Base Station Controller (BSC)/Base Transceiver
Station (BTS) connections.
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4.2
Introduction to Radio and Terrestrial
Transmission
In a mobile communications network, part of the transmission
connection uses a radio link and another part uses 2Mbit/s PCM
links. Radio transmission is used between the Mobile Station and the
Base Transceiver Station and the information must to be adapted to be
carried over 2Mbit/s PCM transmission through the remainder of the
network.
The radio link is the most vulnerable part of the connection and a great
deal of work is needed to ensure its high quality and reliable operation.
This will be analysed later in this chapter.
The frequency ranges of GSM 900 and GSM 1800 are indicated
below:
890MHz
GSM-900
915MHz
Uplink
1710MHz
GSM-1800
935MHz
960MHz
Downlink
1785MHz 1805MHz
Uplink
1880MHz
Downlink
Figure 4.1 Frequency Allocations for GSM
Note that the uplink refers to a signal flow from Mobile Station (MS)
to Base Transceiver Station (BTS) and the downlink refers to a
signal flow from Base Transceiver Station (BTS) to Mobile Station
(MS). The simultaneous use of separate uplink and downlink
frequencies enables communication in both the transmit (TX) and the
receive (RX) directions. The radio carrier frequencies are arranged in
pairs and the difference between these two frequencies (uplinkdownlink) is called the Duplex Frequency.
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The frequency ranges are divided into carrier frequencies spaced at
200kHz. As an example, the following table shows the distribution of
frequencies in GSM 900:
Channel
Uplink signal (MHz)
Downlink signal (MHz)
1
890.1 – 890.3
(890.2 -centre freq.)
935.1 – 935.3
(935.2 -centre freq.)
2
890.4 (centre freq.)
935.4 (centre freq.)
3
890.6 (centre freq.)
935.6 (centre freq.)
…
...
...
124
914.8 (centre freq.)
959.8 (centre freq.)
In GSM 900 the duplex frequency (the difference between uplink and
downlink frequencies) is 45 MHz. In GSM 1800 it is 95 MHz. The
lowest and highest channels are not used to avoid interference with
services using neighbouring frequencies, both in GSM 900 and GSM
1800.
The total number of carriers in GSM 900 is 124, whereas in GSM 1800
the number of carriers is 374.
The devices in the Base Transceiver Station (BTS) that transmit and
receive the radio signals in each of the GSM channels (uplink and
downlink together) are known as Transceivers (TRX).
The radio transmission in GSM networks is based on digital
technology. Digital transmission in GSM is implemented using two
methods known as Frequency Division Multiple Access (FDMA) and
Time Division Multiple Access (TDMA).
Frequency Division Multiple Access (FDMA) refers to the fact that
each Base Transceiver Station is allocated different radio frequency
channels. Mobile phones in adjacent cells (or in the same cell) can
operate at the same time but are separated according to frequency. The
FDMA method is employed by using multiple carrier frequencies, 124
in GSM 900 and 374 in GSM 1800.
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Time Division Multiple Access (TDMA), as the name suggests, is a
method of sharing a resource (in this case a radio frequency) between
multiple users, by allocating a specific time (known as a time slot) for
each user. This is in contrast to the analogue mobile systems where one
radio frequency is used by a single user for the duration of the
conversation. In Time Division Multiple Access (TDMA) systems each
user either receives or transmits bursts of information only in the
allocated time slot. These time slots are allocated for speech only when
a user has set up the call however, some timeslots are used to provide
signalling and location updates etc. between calls.
The figure below illustrates the TDMA principle.
TSL 7
TSL 6
TimeSLot 0
TSL 5
TSL 4
TSL 3
TSL 2
TSL 1
BTS
BTS
Figure 4.2 Time Division Multiple Access principle
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GSM uses digital techniques where the speech and control information
are represented by 0s and 1s. How is it possible to transmit digital
information over an analogue radio interface?
The digital values 0 and 1 are used to change one of the characteristics
of an analogue radio signal in a predetermined way. By altering the
characteristic of a radio signal for every bit in the digital signal, we can
"translate" an analogue signal into a bit stream in the frequency
domain. This technique is called modulation. Analogue signals have
three basic properties: Amplitude, Frequency, Phase. Therefore, there
are basically three types of modulation process in common use:•
Amplitude Modulation
•
Frequency Modulation
•
Phase Modulation
Digital Signal
0
1
0
Frequency Modulation
Amplitude Modulation
Figure 4.3 Examples of Frequency and Amplitude Modulation
GSM uses a phase modulation technique over the air interface known
as Gaussian Minimum Shift Keying (GMSK). In order to understand
how it works, let’s take a simple example.
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At the GSM air interface, the bit rate is approximately 270Kbits/s.
(This will be explained later) At this bit rate, the duration of one bit is
3.69 µs, i.e. the value of the bit requires 3.69 µs of transmission time.
GMSK changes the phase of the analogue radio signal depending on
whether the bit to be transmitted is a 0 or a 1.
Digital Signal
0
0
1
1
Phase Modulation
3.69µs
0
0
0
90
0
90
0
0
Figure 4.4 Example of Phase Modulation
The radio air interface has to cope with many problems such as variable
signal strength due to the presence of obstacles along the way, radio
frequencies reflecting from buildings, mountains etc. with different
relative time delays and interference from other radio sources.
With such levels of interference, complex equalisation techniques are
required with GMSK.
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4.3
Transmission Through the Air Interface
To enable us to understand the principles of the air interface, let us
imagine that an army has to be moved from one place to another, and a
convoy of vehicles is set aside to do the job. The army consists of
soldiers and officers.
Each vehicle has 8 seats and therefore only 8 people can be carried in
each vehicle.
8 seats in each vehicle
Figure 4.5
A small logistical problem
Obviously, the only solution is to divide the army into groups of eight
people. One officer and seven soldiers are allocated to each vehicle.
The officer sits in the front seat and seven soldiers sit in the others.
There are different types of people in the army, soldiers and officers.
These could be referred to as "Logical" differences as they are all
human beings but their functions are different. In addition, there can be
many different ranks of officers, each one with different
responsibilities.
To move them from one place to another, a "Physical" connection is
employed i.e. the vehicles and the seats.
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4.3.1
Physical and Logical Channels
Time Division Multiple Access (TDMA) divides one radio frequency
channel into consecutive periods of time, each one called a "TDMA
Frame". The TDMA frame can be compared to the vehicle in our
example.
Each TDMA Frame contains eight shorter periods of time known as
"Timeslots". These timeslots can be compared to the seats in the
vehicle. The TDMA timeslots are called "Physical Channels" as they
are used to physically move information from one place to another.
The radio carrier signal between the Mobile Station and the BTS is
divided into a continuous stream of timeslots which in turn are
transmitted in a continuous stream of TDMA frames - like a long line
of vehicles with eight seats in each.
If the time slots of the TDMA frame represent the physical channels,
what about the contents? The contents of the physical channels - i.e. the
soldiers and officers travelling in the eight seats of the vehicle,
according to their roles, are called "logical channels". In the example
of the army, the soldiers are one type of logical channel and the officers
are other types of logical channels and they exercise some kind of
control depending on their responsibilities.
In GSM the logical channels can be divided into two types:
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Dedicated Channels
•
Common Channels
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Let’s look at things in a more practical way: a subscriber switches on
his mobile phone and receives a call. This simple act of switching on
the phone involves the following steps:
1. The mobile scans all the radio frequencies and measures them.
2. It selects the frequency with the best quality and tunes to it.
3. With the help of a synchronisation signal in a TDMA frame, the
mobile synchronises itself to the network.
TDMA
TDMA Frame
Frame
BTS
Sync.
Sync.
Information
Information
BTS
Figure 4.6 Tuning into the Network
The synchronisation information required by this process is broadcast
by the network and analysed by the mobile.
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Registration and authentication are the next steps and these consist of
the following operations:
1. A point to point connection must be set up. The mobile station
makes a request for a channel to establish the connection.
2. The network acknowledges the request and allocates a channel. The
mobile receives and reads this information.
3. The mobile then moves to the allocated (dedicated) channel for
further transactions with the network. The next steps are registration
and authentication.
TDMA
TDMA Frame
Frame
Request
Request
Channel
Channel
allocation
allocation
BTS
Traffic
Traffic
Figure 4.7
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Initiation of a Call
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Once the subscriber is registered in the network and the authentication
is successful, calls can be set-up. In the case of a mobile terminated
call, the subscriber has to be paged. This process is described below:
1. The network sends a Paging message to all the Base Transceiver
Stations (BTS) within the Location Area (LA) where the
subscriber is registered.
2. The mobile station answers the paging message by sending a
service/channel request.
3. The network acknowledges this request and again the authentication
is needed. A dedicated signalling channel is assigned in order to
transmit the data related to the call.
4. A Traffic Channel is assigned for the conversation.
During the conversation, the mobile measures the signal strength of
adjacent carriers and sends measurement reports to the Base Station
Controller (BSC). A channel must be dedicated also for this function.
TDMA
TDMA Frame
Frame
Answer
Answer
BTS
BTS
BTS
BTS
Paging
Paging
Traffic
Traffic
Figure 4.8 Call completion from called side
This is a simplified description of the process, but it conveys the idea
that there are many functions involved in the air interface to enable a
mobile user to have conversation. Each one of these functions requires
a separate "logical channel" as the data contents are different. Some of
them are Uplink, others are Downlink and some are Bi-directional.
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4.3.2
Logical channels
There are twelve different types of Logical Channels which are mapped
into Physical Channels in the radio path. Logical channels comprise of
Common Channels and Dedicated Channels. Common Channels are
those which are used for broadcasting different information to mobile
stations and setting up of signalling channels between the MSC/VLR
and the mobile station.
LOGICAL
LOGICAL
CHANNELS
CHANNELS
COMMON
COMMON
CHANNELS
CHANNELS
BROADCAST
BROADCAST
CHANNELS
CHANNELS
FCCH
FCCH
SCH
SCH
COMMON
COMMON
CONTROL
CONTROL
CHANNELS
CHANNELS
RACH
RACH
TRAFFIC
TRAFFIC
CHANNELS
CHANNELS
DEDICATED
DEDICATED
CONTROL
CONTROL
CHANNELS
CHANNELS
SDCCH
SDCCH
BCCH
BCCH
PCH
PCH
Figure 4.9
DEDICATED
DEDICATED
CHANNELS
CHANNELS
AGCH
AGCH
SACCH
SACCH
FACCH
FACCH
TCH/F
TCH/F
TCH/H
TCH/H
TCH/EFR
TCH/EFR
Logical Channels
Over the radio path, different type of signalling channels are used to
facilitate the discussions between the mobile station and the BTS, BSC
and MSC/VLR. All these signalling channels are called Dedicated
Control Channels.
Traffic channels are also Dedicated Channels as each channel is
dedicated to only one user to carry speech or data.
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26 Frame - Multiframe
SACCH
Unused
12
0 1 2 3
24 25
TDMA Frame
0 1
2 3 4
5 6 7
Dedicated Channels
51 Frame - Multiframe
0
1
2
49 50
3
TDMA Frame
0 1
2 3 4
5 6 7
Common Channels
Figure 4.10 TDMA frames with Common and Dedicated Channels
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4.3.2.1
Broadcast Channels
Base Stations can use several TRXs but there is always only one TRX
which can carry Common Channels. Broadcast channels are downlink
point to multipoint channels. They contain general information about
the network and the broadcasting cell. There are three types of
broadcast channels:
1.
Frequency Correction Channel (FCCH)
FCCH bursts consist of all "0"s which are transmitted as a pure sine
wave. This acts like a flag for the mobile stations which enables them
to find the TRX among several TRXs, which contains the Broadcast
transmission. The MS scans for this signal after it has been switched on
since it has no information as to which frequency to use.
2.
Synchronisation Channel (SCH)
The SCH contains the Base Station Identity Code (BSIC) and a reduced
TDMA frame number. The BSIC is needed to identify that the
frequency strength being measured by the mobile station is coming
from a particular base station. In some cases, a distant base station
broadcasting the same frequency can also be detected by the mobile
station. The TDMA frame number is required for speech encryption.
3.
Broadcast Control Channel (BCCH)
The BCCH contains detailed network and cell specific information
such as:
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Frequencies used in the particular cell and neighbouring cells.
•
Frequency hopping sequence. This is designed to reduce the
negative effects of the air interface, which sometimes results in
the loss of information transmitted, the mobile station may
transmit information on different frequencies within one cell. The
order in which the mobile station should change the frequencies
is called the "frequency hopping sequence". (However,
implementing Frequency Hopping in a cell is optional.)
•
Channel combination. As we mentioned previously, there are a
total of twelve logical channels. All the logical channels except
Traffic Channels are mapped into Timeslot 0 or Timeslot 1 of the
broadcasting TRX. Channel combination informs the mobile
station about the mapping method used in the particular cell.
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4.3.2.2
•
Paging groups. Normally in one cell there is more than one
paging channel (describer later). To prevent a mobile from
listening to all the paging channels for a paging message, the
paging channels are divided in such a way that only a group of
mobile stations listen to a particular paging channel. These are
referred to as paging groups.
•
Information on surrounding cells. A mobile station has to know
what are the cells surrounding the present cell and what
frequencies are being broadcast on them. This is necessary if, for
example, the user initiates a conversation in the current cell, and
then decides to move on. The mobile station has to measure the
signal strength and quality of the surrounding cells and report this
information to the base station controller.
Common Control Channels
Common Control Channels comprise the second set of logical
channels. They are used to set up a point to point connection. There
are three types of common control channels:
1.
Paging Channel (PCH)
The PCH is a downlink channel which is broadcast by all the BTSs of a
Location Area in the case of a mobile terminated call.
2.
Random Access Channel (RACH)
The RACH is the only uplink and the first point to point channel in the
common control channels. It is used by the mobile station in order to
initiate a transaction, or as a response to a PCH.
3.
Access Grant Channel (AGCH)
The AGCH is the answer to the RACH. It is used to assign a mobile a
Stand-alone Dedicated Control Channel (SDCCH). It is a downlink,
point to point channel.
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4.3.2.3
Dedicated Control Channels
Dedicated Control Channels compose the third group of channels. Once
again, there are three dedicated channels. They are used for call set-up,
sending measurement reports and handover. They are all bidirectional and point to point channels. There are three dedicated
control channels:
1.
Stand Alone Dedicated Control Channel (SDCCH)
The SDCCH is used for system signalling: call set-up, authentication,
location update, assignment of traffic channels and transmission of
short messages.
2.
Slow Associated Control Channel (SACCH)
An SACCH is associated with each SDCCH and Traffic Channel
(TCH). It transmits measurement reports and is also used for power
control, time alignment and in some cases to transmit short messages.
3.
Fast Associated Control Channel (FACCH)
The FACCH is used when a handover is required. It is mapped onto a
TCH, and it replaces 20 ms of speech and therefore it is said to work in
"stealing" mode.
4.3.2.4
Traffic Channels (TCH)
Traffic Channels are logical channels that transfer user speech or data,
which can be either in the form of Half rate traffic (6.5Kbits/s) or Full
rate traffic (13Kbits/s). Another form of traffic channel is the
Enhanced Full Rate (EFR) Traffic Channel. The speech coding in
EFR is still done at 13Kbits/s, but the coding mechanism is different
than that used for normal full rate traffic. EFR coding gives better
speech quality at the same bit rate than normal full rate. Traffic
channels can transmit both speech and data and are bi-directional
channels.
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4.3.3
Time Slots And Bursts
We have already seen that the technique used in air interface is Time
Division Multiple Access (TDMA) where one frequency is shared by,
at the most, eight users. Consider the example of a 2Mbit/s PCM signal
which can carry 30 speech channels with each channel occupying
64Kbits/s. The speech signals from the mobile stations must be placed
into a 2Mbit/s signal that connects the BTS and the BSC.
It is very important that all the mobile stations in the same cell send the
digital information at the correct time to enable the BTS to place this
information into the correct position in the 2Mbit/s signal.
How do we manage the timing between multiple mobile stations in one
cell? The aim is that each mobile sends its information at a precise
time, so that when the information arrives at the Base Transceiver
Station, it fits into the allocated time slot in the 2Mbit/s signal. Each
Mobile Station must send a burst (a burst occupies one TDMA
timeslot) of data at a different time to all the other Mobile Stations in
the same cell. The mobile then falls silent for the next seven timeslots
and then again sends the next burst and so on.
It can be seen that the mobile station is sending information
periodically. All the mobile stations send their information like this. If
we go back to the analogy of the army, the road is the radio carrier
frequency, the vehicle is the TDMA frame and the seats in each
vehicle are the TDMA timeslots.
TDMA
TDMA Time
Time Slot
Slot
...
TDMA
TDMA Frame
Frame
...
BTS
BTS
Bursts
Bursts from
from Mobile
Mobile Stations
Stations
2Mbit/s
2Mbit/s to
to BSC
BSC
Figure 4.11 TDMA Bursts and Timeslots
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In the air interface a TDMA timeslot is a time interval of approximately
576.9 µs which corresponds to the duration of 156.25 bit times. All
bursts occupy this period of time, but the actual arrangement of bits in
the burst will depend on the burst type. Two examples of burst types
are :
•
Normal Burst is used to send the traffic channels, stand alone
dedicated channels, broadcast control channel, paging channel,
access grant channel, slow and fast associated control channels.
•
Access Burst which is used to send information on the Random
Access Channel (RACH). This burst contains the lowest number
of bits. The purpose of this “extra free space” is to measure the
distance between the Mobile Station to Base Transceiver Station
at the beginning of a connection. This process determines a
parameter called "timing advance" which ensures that the bursts
from different mobile stations arrive at the correct time, even if
the distances between the various MSs and the BTS are different.
This process is carried out in connection with the first access
request and after a handover. In GSM a maximum theoretical
distance of about 35 km is allowed between the base transceiver
station and mobile station.
Guard Time
(8.25 Bits)
Normal
Bursts
148 Bits
148 Bits
148 Bits
576.9 micro secs
(156.25 bit times)
Access
Bursts
88 Bits
88 Bits
88 Bits
Guard Time
(68.25 Bits)
Figure 4.12 Normal Bursts and Access Bursts
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4.4
Problems and Solutions of the Air Interface
It has already been pointed out that the radio air interface link is the
most vulnerable part of the GSM connection. In this section we will
briefly discuss some of the problems that occur in air interface and
some solutions. There are three major sources of problems in the air
interface, which can lead to loss of data. These are:
4.4.1
•
Multi path propagation
•
Shadowing
•
Propagation delay
Multipath propagation
Whenever a mobile station is in contact with the GSM network, it is
quite rare that there is a direct "line of sight" transmission between the
mobile station and the base transceiver station. In the majority of cases,
the signals arriving at the mobile station have been reflected from
various surfaces. Thus a mobile station (and the base transceiver
station) receives the same signal more than once. Depending on the
distance that the reflected signals have travelled, they may affect the
same information bit or corrupt successive bits. In the worst case an
entire burst might get lost.
Depending on whether the reflected signal comes from near or far, the
effect is slightly different. A reflected signal that has travelled some
distance causes "inter symbol interference" whereas near reflections
cause "frequency dips". There are a number of solutions that have been
designed to overcome these problems:
140 (244)
•
Viterbi Equalisation
•
Channel Coding
•
Interleaving
•
Frequency Hopping
•
Antenna Diversity
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Approx.
17cm
RX Sensitivity
BTS
BTS
Inter Symbol Interference
Fading Dips
Fading dips caused by
multipath propagation
Figure 4.13 The effects of Multipath Propagation
Viterbi Equalization
This is generally applicable for signals that have been reflected from far
away objects. When either the base transceiver station or mobile station
transmits user information, the information contained in the burst is not
all user data. There are 26 bits which are designated for a "training
sequence" included in each TDMA burst transmitted. Both the mobile
station and base transceiver station know these bits and by analysing
the effect the radio propagation on these training bits, the air interface
is mathematically modelled as a filter. Using this mathematical model,
the transmitted bits are estimated based on the received bits. The
mathematical algorithm used for this purpose is called "Viterbi
equalisation".
Channel Coding
Channel coding (and the following solutions) is normally used for
overcoming the problem caused by fading dips. In channel coding, the
user data is coded using standard algorithms. This coding is not for
encryption but for error detection and correction purposes and requires
extra information to be added to the user data. In the case of speech, the
amount of bits is increased from 260 per 20 ms to 456 bits per 20 ms.
This gives the possibility to regenerate up to 12.5% of data loss.
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Interleaving
Interleaving is the spreading of the coded speech into many bursts. By
spreading the information onto many bursts, we will be able to recover
the data even if one burst is lost. (Ciphering is also carried out for
security reasons).
Speech
Digitising and
Source Coding
13kbit/s
Channel
Coding
22.8kbit/s
Interleaving
and Ciphering
Air
Interface
22.8kbit/s
GMSK
Modulation
33.8kbit/s
TDMA Burst
Formatting
Figure 4.14 Speech processing in the mobile station
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Frequency Hopping
With Frequency Hopping, the frequency on which the information is
transmitted is changed for every burst. Frequency hopping generally
does not significantly improve the performance if there are less than
four frequencies in the cell.
F1
F2
F3
F4
Time
Figure 4.15 Example of Frequency Hopping
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Antenna Receiver Diversity
In this case two physically separated antennas receive and process the
same signal. This helps to eliminate fading dips. If a fading dip occurs
at the position of one antenna, the other antenna will still be able to
receive the signal. Since the distance between two antennas is a few
metres, it can only be implemented at the Base Transceiver Station.
Approx. 6m (GSM-900)
Approx. 3m (GSM-1800)
Received Signal
Antennas
RX
RX
Signal
Processing
Figure 4.16 Antenna Receiver Diversity
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4.4.2
Shadowing
Hills, buildings and other obstacles between antennas cause
shadowing (also called Log Normal Fading). Instead of reflecting the
signal these obstacles attenuate the signal.
BTS
Figure 4.17 Shadowing effect.
Shadowing is generally a problem in the uplink direction, because a
Base Transceiver Station transmits information at a much higher power
compared to that from the mobile station. The solution adopted to
overcome this problem is known as adaptive power control. Based on
quality and strength of the received signal, the base station informs the
mobile station to increase or decrease the power as required. This
information is sent in the Slow Associated Control Channel
(SACCH).
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4.4.3
Propagation Delay
As you remember, information is sent in bursts from the mobile station
to the Base Transceiver Station (BTS). These bursts have to arrive at
the base transceiver station such that they have to map exactly into their
allocated time slots. However, the further away the mobile station is
from the BTS then the longer it will take for the radio signal to travel
over the air interface. This means that if the mobile station or base
station transmits a burst only when the time slot appears, then when the
burst arrives at the other end, it will cross onto the time domain of the
next timeslot, thereby corrupting data from both sources.
The solution used to overcome this problem is called "adaptive frame
alignment". The Base Transceiver Station measures the time delay
from the received signal compared to the delay that would come from a
mobile station that was transmitting at zero distance from the Base
Transceiver Station. Based on this delay value, the Base Transceiver
Station informs the mobile station to either advance or retard the time
alignment by sending the burst slightly before the actual time slot. The
base station also adopts this time alignment in the down link direction.
allocated time slot
allocated time slot
BTS
Effect Due to Propagation Delay
BTS
Solution Using Adaptive Frame Alignment
Figure 4.18 Propagation delay problem and solution
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4.5
Terrestrial Transmission
So far, we have concentrated solely on the radio link between Base
Transceiver Station (BTS) and the mobile station. Now we are going to
follow the signal to the next phase and take a look at the transmission
between the other network elements, in particular from Base
Transceiver Station to Base Station Controller (BSC) and up to Mobile
Services Switching Centre (MSC).
4.5.1
Base Transceiver Station
A base transceiver station is a physical site from where the radio
transmission in both the downlink and uplink direction takes place. The
radio resources are the frequencies allocated to the Base Station. The
particular hardware element inside the Base Transceiver Station (BTS)
responsible for transmitting and receiving these radio frequencies is
appropriately named "Transceiver (TRX)". A Base Station site might
have any number of TRXs from one to twelve. These TRXs are then
configured into one, two or three cells. If a BTS is configured as one
cell it is called an "Omnidirectional BTS" and if it is configured as
either two or three cells it is called a "Sectorised BTS". In an
omnidirectional BTS the maximum number of TRXs is ten, and in a
sectorised BTS the maximum number of TRXs is four per sector.
Omnidirectional BTS
f1,f2, f3
f1, f2
f1
f2
2 sectorised BTS
f5, f6
f3, f4
3 sectorised BTS
Figure 4.19 Examples of BTS configurations
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4.5.2
Transmission between BSC and BTS
There are three alternative methods to provide the connections between
a BSC and several BTSs. The method used will depend on a number of
factors such as the distance between the Base Station Controller (BSC)
and Base Transceiver Station (BTS), the number of TRXs used at a
particular BTS site, the signalling channel rate between Base Station
Controller (BSC) and Base Transceiver Station (BTS). There are three
options available: point-to-point connection, multidrop chain and
multidrop loop.
BSC
Point to Point Connection
BTS
BTS
BTS
BTS
BTS
BTS
BTS
BTS
BTS
BTS
Multi drop Chain
Multi drop loop
Figure 4.20 BTS - BSC connections
Point-to-point connection indicates that the Base Station Controller
(BSC) is connected directly to every BTS with a 2Mbit/s PCM line.
This is a simple and effective method particularly in cases when the
distance between BSC and BTS is short. However, if the BSC -BTS
distance is a few kilometres whereas the distance between a group of
BTS’s is much shorter, it does not make sense to draw a point-to-point
connection to every BTS. One PCM line has ample capacity to transfer
data to several BTSs simultaneously. Therefore, it is possible to draw
just one BSC - BTS connection and link the BTSs as a chain. This
technique is called "multidrop chain". The BSC sends all the data in
one 2Mbit/s PCM line and each BTS in turn analyses the signal,
collects the data from the correct timeslots assigned for itself and
passes the signal to the next BTS.
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But there is one problem with a multidrop chain. Consider what would
happen if there is a malfunction somewhere along the line and the chain
breaks. More BTSs are isolated and, if the BSC is not informed, it will
continue to send data. The solution to this problem is called
"multidrop loop" and instead of a chain we connect the BTSs in the
form of a loop. Previously a dynamic node was needed to split the
signal into the two directions around the loop, but later versions of BTS
are capable of carrying out this function. The flow of the signal is
similar to the signal flow in multidrop chain, except that a BTS will
change the “listening” direction if the signal from one side fails. This
ensures that the BTSs always receive information from the BSC even if
the connection is cut off at some point in the loop.
4.5.3
The Concept of Multiplexing
According to GSM 900 and GSM 1800 specification, the bit rate in the
air interface is 13 Kbits/s and the bit rate at the Mobile Services
Switching Centre (MSC) and PSTN interface is 64 Kbits/s. This means
that the bit rate has to be converted at some point after the signal has
been received by the BTS and before it is sent to other networks. But
the specifications do not put a constraint as to where exactly the
conversion should take place. This brings up some interesting
scenarios.
The actual hardware which does the conversion from 13 Kbits/s to 64
Kbits/s and vice versa is called a transcoder. In theory this piece of
equipment belongs to the Base Transceiver Station. However, by
putting the transcoder at a different place we can take some advantages
in reducing the transmission costs.
If the transcoder is placed at the BTS site (in the BSC interface), then
the user data rate from BTS to Base Station Controller (BSC) would be
64 Kbits/s. The transmission for this would be similar to standard PCM
line transmission with 30 channels per PCM cable. The same would
also apply between BSC and MSC.
If we put the transcoder somewhere else, say just after MSC, then also
we can not get significant advantage. This is because although after
transcoding the bit rate reduces to 13 kbit/s we still have to use the
PCM structure to send the traffic channels, with 8 bits per time slot.
However since after transcoding we have a bit rate of 13 Kbits/s and an
additional 3 Kbits/s (making 16 Kbits/s) only two bits per time slot will
be used. The other 6 bits are effectively wasted. The next figure shows
these two types of connections.
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Independent from its actual position, the transcoder belongs to the BSS
even if it is placed next to the MSC. (When the TC is placed away from
the BTS it is called a Remote TC according to the GSM
recommendations).
BSC
MSC
64 kbps
TC
BTS
64 kbps
13 kbps
Transcoder is at BTS site
MSC
BSC
TC
BTS
13 kbps
64 kbps
16 (13+3) kbps
16 (13+3) kbps
Transcoder is at MSC site
Figure 4.21 Implementation of Transcoder at different sites.
But the real advantage comes if we use the second configuration shown
in the figure with another piece of hardware called submultiplexer. We
saw that from the MSC data comes out at 64Kbits/s rate and from the
Transcoder it comes out at 16Kbits/s. Each PCM channel (time slot)
has 2 bits of information. It appears that we are able to put in data from
other 3 PCM lines also here by multiplexing. However there are other
issues as well such as Common Channel Signalling information, OMC
data and some other network information which can not be transcoded.
Thus we are able to multiplex 3 PCM lines and send 90 channels in one
PCM line from MSC (transcoder) towards the BSC. The BSC is able to
switch 2 bits per time slot (or 1 bit) to the correct direction. The next
figure shows the configuration.
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TCSM (Transcoder/Submultiplexer)
MSC
TC
TC
TC
BSC
BTS
S
M
U
X
64 kbps
13 kbps
16 kbps (90 Channels)
16 (13+3) kbps
16 kbps
Figure 4.22 Transcoder and submultiplexer
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4.6
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Summary of the Learning Points
•
GSM networks use Time Division Multiple Access (TDMA)
technology in the air interface. By this method, one frequency
resource can be shared by maximum eight mobile users.
•
There are eight physical channels per frequency in the air
interface.
•
Logical channels are classified according to the type of
information contained within each channel.
•
There are eleven logical channels. Two of which are half and full
rate traffic channels. The remaining nine are various control
channels used to transfer information related to call set up.
•
Information is sent from the mobile station to the Base
Transceiver Station (BTS) in intermittent bursts.
•
There are four primary types of bursts. Normal Burst, Access
burst, synchronisation burst and frequency correction burst.
•
There are primarily three sources of problems in the air interface.
There is multipath propagation, shadowing and propagation
delay. The methods adopted to overcome these problems are
Viterbi Equalisation, Channel Coding, Frequency Hopping,
interleaving, Antenna Receiver Diversity, Adaptive Power
Control and Adaptive Frame Alignment.
•
A Base Transceiver Station (BTS) site can be configure as an
omni-directional BTS or a sectorised BTS.
•
There are three different methods of connecting a Base
Transceiver Station (BTS) to the Base Station Controller (BSC):
Point to Point, Multi Drop Chain and Multi Drop Loop.
•
Transcoder is placed at the Mobile Services Switching Centre
(MSC). A transcoder used in conjunction with a submultiplexer
makes it possible to multiplex traffic channels from three PCM
lines, thereby reducing transmission costs.
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4.7
Transmission Review
4.7.1
Review Questions
In the following questions, please select one alternative which you
think is the best answer for the particular question. There may not be
a perfect answer, select the one which you think is the most correct.
1. Duplex frequency means
a) The difference between the uplink and downlink frequency pair
b) The uplink and downlink frequency pair
c) Twice the uplink or downlink frequency band
d) GSM 900 and GSM 1800 frequency bands
2. The modulation scheme used in GSM is
a) Frequency modulation
b) Amplitude modulation
c) Phase modulation
d) None of the above
3. Which of the following are Dedicated Channels?
a) FCCH, SCH, AGCH
b) SDCCH, TCH, SACCH
c) RACH, FACCH, TCH
d) BCCH, SDCCH, SACCH
4. The function of AGCH is to
a) inform the mobile station of the frequency hopping sequence
b) provide the mobile station the handover information
c) inform the mobile station of a dedicated signalling channel
d) transmit adaptive frame alignment information to the mobile
station
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5. Short message service is transmitted in
a) SDCCH
b) SACCH
c) Both of them
d) Neither of them
6. Information about frequency hopping sequence is in
a) BCCH
b) FCCH
c) RACH
d) AGCH
7. Inter symbol interference is caused by
a) Fading dips
b) Viterbi equaliser
c) Reflection
d) Interleaving
8. Frequency Hopping
a) Eliminates the problem of Fading dips
b) Eliminates the problem of inter symbol interference
c) is part of channel coding
d) spreads the problem of fading dips to many mobile stations
9. Speech transcoding from 13 to 64 Kbits/s and vice versa is done by
a transcoder between which two points?
a) BTS and BSC at BTS site
b) BTS and BSC at BSC site
c) BSC and MSC at MSC site
d) All above are possible
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5
Network Planning
5.1
Module Objectives
At the end of the module the student can:
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list the main steps of the radio network planning process
•
define the main radio network parameters
•
explain how the frequencies are reused
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5.2
Introduction
The geographical distribution of the subscribers poses a difficult
problem for GSM networks. Without wire-connected telephones the
subscribers can be virtually everywhere, but still the network must be
able to provide a connection in spite of their movements. A good
geographical coverage is the basis for providing network services.
Careful network planning is thus a primary aspect of implementing
GSM networks. Several requirements must be taken into consideration
already in the early stages of the planning process:
•
Costs of building the network
•
Capacity of the network
•
Coverage
•
Maximum congestion allowed (grade of service)
•
Quality of calls
•
Further development of the network.
Various factors affecting the demand for network services must also be
considered. These are mostly related to the inhabitants of the area, such
as distribution of the population and vehicles, income of the population
and statistics on telephone usage.
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The main steps of a Network Planning process are as follows:
•
Collection of all relevant information such as topographical maps
and statistical books
•
Network Dimensioning based on coverage and capacity
requirements
•
Selection of Base Station sites
•
Survey of intended sites
•
Use of computer aided design system for coverage prediction,
interference analysis and frequency planning
Figure 5.1
Simulated Cellular Radio Network Planning.
With these data it is possible to prepare a preliminary plan of site
distribution, which enables the coverage prediction. During this phase
the network is also dimensioned.
The survey of the installation sites refers to evaluating the intended
location of each Base Transceiver Station (BTS), as well as its
surroundings, possible structural and geographical obstacles and
existing radio equipment. This determines whether the location is
suitable for the installation.
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The type and location of the BTS depends on the characteristics of the
surroundings. In sparsely populated areas we use powerful BTS’s
which are usually mounted on high ground to provide maximum
unobstructed coverage to all directions. This type of BTS is called
Omnidirectional BTS. The maximum theoretical distance from an
omnidirectional BTS to the edge of the cell is 35 kilometres and the
number of available frequencies depends on the traffic volume. In
urban areas, where traffic volume is higher, the size of a cell is much
smaller and the distance between BTS’s is shorter. The standard type of
BTS is also different: the cell is divided into three sectors that have a
few frequencies each. This is called Sectorised BTS. Another type of
sectorised BTS is used to provide coverage for highways. Instead of
three-sector BTS’s, a string of two-sector BTS’s is installed along the
road.
Omnidirectional BTS
f1,f2, f3
f1, f2
f1
f2
2 sectorised BTS
f5, f6
f3, f4
3 sectorised BTS
Figure 5.2 BTS configurations.
After all the installation sites have been surveyed, a detailed network
plan can be made. This includes the design of a transmission network
which is usually supplied by existing operators (leased PCM lines), or
by microwave links.
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After the installation work has been completed, the radio environment
has to be measured and tested to ensure its proper operation and
coverage before putting it into use. This is carried out in the
surroundings of each individual site using portable test transmitters that
are normally installed in a vehicle.
Nokia offers an integrated Radio Network Planning solution which
brings flexibility for continuous improvement and adaptation to
demand, more reliable and well managed mass parameterization and
more streamlined operations.
Network
Measurement
System
BTS
BTS
NMS/X
Network
Planning
System
NPS/X
BTS
BSC
Planning
Tools
NMS/2000
NPS/i
NPS/ct
Database
Database
File transfer
Figure 5.3 Nokia’s integrated Network Planning solution
Parameters generated by the NPS tools can be converted to format
which can be understood by the NMS/2000. Then, these parameters can
be transferred to NMS/2000 to be used to configure/re-configure the
base stations in question.
After the configuration/re-configuration of base stations, drive survey is
done with Nokia’s NMS/X tool.
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Figure 5.4 Toolkit Drive survey with NMS/X
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5.3
Radio Network
5.3.1
Dimensioning Cells
A cell is the basic ‘construction block’ of a GSM network. One cell is
the geographical area covered by one BTS. The actual size of a cell
depends on several factors: the environment, number of users, etc. Cells
are grouped under Base Station Controllers (BSC).
Dimensioning a cell means finding answers to two fundamental
questions: How many traffic channels (TCH) does the cell need to
handle and how many traffic channels are necessary? To solve these
problems, i.e. to determine the traffic capacity, we have to calculate the
number of Erlangs. Erlang is the measuring unit of network traffic.
One Erlang equals the continuous use of a mobile device for one hour.
The traffic is calculated using a simple formula:
x Erlangs =
(calls per hour) × (average conversation time)
3600 Seconds
Amount of traffic is independent of the observation duration. For
example, it is possible to make the observation for only 15 minutes and
then, in the formula above calls per 15 minutes is taken and it is divided
to 900 seconds.
The more traffic on available resources, the more chance that there will
be congestion on these resources. Network planners carefully analyse
the traffic volume on installed traffic channel capacity and according to
quality limits in the network, decide if there is need to install more
capacity.
Let’s take an example: if there are 400 calls per hour and the average
conversation time is 100 seconds, the traffic capacity is approximately
11 Erlangs. After obtaining this value, we must take a look at to the
Erlang Table.
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Chs
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Table 5.1
1%
0.01
0.15
0.46
0.87
1.36
1.91
2.50
3.13
3.78
4.46
5.16
5.88
6.61
7.35
8.11
8.88
9.65
10.40
11.20
12.00
2%
0.02
0.22
0.60
1.09
1.66
2.28
2.94
3.63
4.34
5.08
5.84
6.61
7.40
8.20
9.01
9.83
10.70
11.50
12.30
13.20
3%
0.03
0.28
0.72
1.26
1.88
2.54
3.25
3.99
4.75
5.53
6.33
7.14
7.97
8.80
9.65
10.50
11.40
12.20
13.10
14.00
5%
0.05
0.38
0.90
1.52
2.22
2.96
3.75
4.54
5.37
6.22
7.08
7.95
8.83
9.73
10.60
11.50
12.50
13.40
14.30
15.20
Chs
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
1%
12.80
13.70
14.50
15.30
16.10
17.00
17.80
18.60
19.50
20.30
21.20
22.00
22.90
23.80
24.60
25.50
26.40
27.30
28.10
29.00
2%
14.00
14.90
15.80
16.60
17.50
18.40
19.30
20.20
21.00
21.90
22.80
23.70
24.60
25.50
26.40
27.30
28.30
29.20
30.10
31.00
3%
14.90
15.80
16.70
17.60
18.50
19.40
20.30
21.20
22.10
23.10
24.00
24.90
25.80
26.80
27.70
28.60
29.60
30.50
31.50
32.40
5%
16.20
17.10
18.10
19.00
20.00
20.90
21.90
22.90
23.80
24.80
25.80
26.70
27.70
28.70
29.70
30.70
31.60
32.60
33.60
34.60
Erlang B table
As you can see, the table contains also the grade of service (GOS)
figure which is the maximum congestion allowed. Supposing that GOS
is 5 % - which means that during a certain observation period (usually 1
hour) 5 out of 100 calls fail due to lack of resources - the required
number of channels is 16. Since each carrier supports eight channels,
we can make a rough estimation that this cell must be equipped with
two carriers, i.e. two Transceivers or TRX’s.
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5.3.2
Frequency Reuse
Now we have to resolve another problem. There is a limited number of
frequencies available to each Base Station Subsystem and they must be
distributed between the cells to ensure a balanced coverage throughout
the BSS. Let’s take an exercise to illustrate the situation.
You are the network planner and the number of frequencies assigned to
this project is 9. Your task is to distribute the frequencies in the
network that is shown in the following figure with one frequency per
cell.
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Figure 5.5 Frequency planning exercise
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As you can see, the frequencies have to be reused. If you do not
distribute the frequencies properly throughout the network the result
will be a high level of interference caused by overlapping frequencies.
To avoid this, the GSM network includes a specification of the
Frequency reuse patterns, one of which is presented in figure below.
6
7
3
4
•
8
6
3
4
•
•
5
1
9
7
6
•
4
5
•
3
4
6
3
1
•
2
7
8
9
7
•
8
1
5
9
2
6
7
3
4
•
•
•
6
4
5
•
5
2
3
1
1
9
7
8
•
8
•
•
2
•
2
•
8
1
5
9
2
9
Figure 5.6 Frequency reuse pattern example
The next step involves the dimensioning of the Location Areas. This is
carried out according to the traffic characteristics of each area. The
final phase is the dimensioning of the Fixed Network on the basis of
the traffic requirements and dimensioning of the entire radio network.
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5.4
Summary of the Learning Points
•
Two of the most important factors in planning a GSM network
are the coverage and capacity of the network.
•
The demographic data of the intended coverage area plays a
major role in network planning.
•
Frequencies have to be reused in GSM network according to
certain predefined methods.
•
Nokia offers an integrated solution to network planning with
Network Planning System (NPS) and Network Measurement
System (NMS) tools.
5.5
Network Planning Review
5.5.1
Review Questions
In the following questions, please select one alternative which you
think is the best answer for the particular question. There may not be
a perfect answer, select the one which you think is the most correct.
1. Which of the following is not a factor in Network Planning?
a) Intended coverage area
b) Intended grade of service
c) Bit rate of the SS7 signalling links
d) cost of the network elements
2. Radio network Planning process starts with
a) Selection of Base station Sites
b) Survey of intended sites
c) Collection of all relevant information
d) Working out the frequency plan
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3. Frequency reuse is done in GSM Networks, because
a) A financially viable GSM network is not possible with
available frequencies without reusing them
b) The spacing of 200 kHz between carriers instead of 25 kHz
(like in analogue networks) reduces the number of frequencies
c) it helps in increasing the number of subscribers
d) None of above are quite correct
4. In a certain PLMN, an average subscriber makes 5 calls during
office hours (8 AM - 6 PM). It is known that in a certain cell area,
there are going be 1000 subscribers, at any given hour, during these
office hours. Assuming that a subscriber’s conversation lasts for
100 seconds, how many TRXs are needed in this cell to provide a
grade of service of 2%?
a) 2
b) 3
c) 4
d) There is not enough information given for an exact answer
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6
Nokia Implementation
6.1
Module Objectives
At the end of the module the student is able to:
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Differentiate between the generic GSM network architecture and
Nokia implementation of it.
•
Describe the DX 200 platform’s modularity, distributed
processing and network element architecture.
•
Describe the NMS functions and architecture.
•
List the different types of Nokia BTSs.
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6.2
Introduction
The preceding chapters have discussed the GSM network and it’s
various concepts in considerable detail. These concepts and ideas are
generic in nature and are based on the standard specifications of the
GSM defined by ETSI (European Telecommunication Standard
Institute). All the GSM networks operate in the same way as explained
in the preceding chapters. However, the implementation of the network
may differ between different organisations. The end result will of
course be the same as specified in the standards, but the equipment and
the implementation of it can vary. In this chapter we will see how
Nokia has implemented the full GSM network with its various network
elements.
6.3
Network Architecture
Consider the generic GSM network architecture shown below, which
has all the elements discussed so far. In this diagram all the network
elements and the interfaces are shown. Then compare this with the
Nokia Implementation diagram of the same GSM network on figure 6.2
and note the differences.
BSS - Base Station
Subsystem
OMC
VLR
BTS
BSC
HLR
MSC
IWF
BTS
BTS
EIR
BSC
NSS - Network
Subsystem
PSTN
ISDN
PSPDN
BTS
BTS
NMS - Network
Management
System
SC
MS
Air
Abis
AC
A
Transcoder
Figure 6.1 Generic GSM Network Architecture
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BSS - Base Station
Subsystem
DX 200
MSC/VLR
BTS
BTS
DX 200
TCSM
BTS
NSS - Network
Subsystem
BTS
BTS
Air
DX 200
HLR/AC/EIR
PSTN
ISDN
PSPDN
DX 200
BSC
Abis
A
NSS Site
NMS / 2000
Network
Management
System
SMSC
Figure 6.2 Nokia Implementation of GSM Network.
The differences are quite obvious. The main highlight seems to be a
certain piece of equipment called DX 200 which has been implemented
in BSC, MSC, TC, VLR, HLR, AC and EIR. We will discuss the DX
200 shortly, but let us first examine some other differences.
Integration of Elements
The picture shows that different GSM network elements are integrated
into one DX 200 system. The first of these is the DX 200 MSC/VLR in
which the MSC and the VLR are integrated. In the standard GSM
specifications the MSC and VLR are two different network entities
with functions of their own. However the amount of signalling between
MSC and VLR is quite heavy and their tasks are so related with each
other that it makes good sense to integrate them together.
The second integration is the DX 200 HLR/AC/EIR. All these three
elements are basically databases which hold various types type of
information. The HLR keeps the subscribers’ subscription data, AC
holds the authentication data and the EIR, equipment data. In Nokia
implementation these three elements are integrated and implemented as
parts of the DX 200 HLR.
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6.4
DX 200 Platform
In the previous section we saw that the BSC, MSC/VLR, HLR/AC/EIR
and TC are designed on the DX platform and this section will explain
and show what it is.
In the early days of the telecommunication history, switching
exchanges were huge, cumbersome and not so effective, requiring
manual switching. As technology evolved, switches also started
becoming automatic and more efficient. The cross bar exchange was
considered one of the major developments in telecommunication,
however, the development of the transistor after the Second World War
has revolutionised telecommunications and other industries.
The computer industry changed dramatically and microprocessors
started to evolve and develop. To make use of these developments,
exchanges became heavily computerised. The various tasks required in
a telephone exchange could be executed using the power of
microprocessors.
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Signalling towards Subscribers
Collecting dialled numbers
Collecting Charging data
Hunting for a free circuit
Making Speech path connections
Signalling towards other exchanges
Analysing and subscriber data
Supervising the processes running
Collecting statistical data
Figure 6.3 Centralised central processing Unit
We can see that there are different tasks for a CPU (Central Processing
Unit) of a switching exchange. For one processor to carry out all the
functions, a very powerful processor would be required. However, what
if these tasks were to be distributed among many different
computers? Would we still need very powerful processors? The
answer is no. We could use commercially available processors and
write software for it to execute. This is the principle on which Nokia
has built switching equipment. All the various tasks of the exchange are
distributed to different functional units which have a CPU of their own.
Since these functional units have to communicate with each other, a
message bus is used for communication. The next figure shows the
distributed architecture of Nokia’s DX platform.
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Exchange
Signalling
towards
Subscribers
Signalling
towards
other exchanges
Computer Units
Message bus
Collecting
dialled
numbers
Collecting
Charging
data
Making Speech
path
connections
Collecting
statistical data
Supervising the
processes
running
Hunting for a
free circuit
Figure 6.4 Distributed structure of Nokia’s DX platform
This kind of architecture has a number of advantages which make this a
very reliable platform to implement high fault tolerant systems.
Distributed Processing is the first one. Since the tasks are distributed
among different computer units, it makes it easier and faster to track
down and solve problems. Modularity is another advantage as only
those units which are necessary need to be installed in an exchange. If
the units are required at a later date they can be installed later. One does
not have to buy everything to have just a fraction of its resources.
Alternatively, if a single unit performing certain task is not enough,
then the same type of unit can be “added-on.” Reliability is another
significant advantage. A functional unit, depending on its vulnerability
to disturbances is either duplicated, or a spare unit for a number of units
is installed. These are called the 2N redundancy and N+1
redundancy. In the former, there is always a unit in “hot stand-by”
mode for each working one, in the latter there is one spare unit for N
number of units. Based on this DX platform, Nokia has built different
types of switching exchanges. DX210 and DX220 are the fixed
telephone network (PSTN) switches. The DX 200 is the platform used
for GSM elements BSC, MSC/VLR, HLR/AC/EIR
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6.5
DX 200 MSC/VLR Architecture
The DX 200 MSC/VLR consists of a number of functional units each
with its own processor and back up facility carrying out a number of
tasks. These functional units have independent tasks but communicate
when and as necessary using a common message bus. The MSC is
integrated with the VLR and communication between the two entities is
entirely internal signalling. The MSC also has gateway functionality
that is to say that it has interfaces to other networks outside the GSM
PLMN.
CDSU
GSW
CLS
Group Switch
ET
BSS
ECU
Base Station
Subsystem
ET
PSTN
Public Switched
Telephone Network
TGFP
CNFC
ET
HLR
Home Location
Register
VANG
X.25 to
AdC (Administrative Centre)
2n
PAU
CASU
LSU
IWCU
BSU
CCSU
M
BDCU
STU
CHU
MB Message Bus
2n
CCMU
CMU
2n
CM
VLRU
OMU
I/O
Figure 6.5 Block diagram of the DX 200 MSC/VLR
The maximum number of visitor subscribers roaming under a DX 200
MSC is 150,000 and in the DX 200 MSC-i it is 400,000.
DX 200 MSC has full SSP functionality (refer to chapter “Intelligent
Networks”).
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6.5.1
Functional Units in MSC/VLR
The DX 200 MSC/VLR is the biggest element (with respect to number
of functional units) in the GSM network. It is not the intention of this
section to give a complete explanation of all the units in this section,
but a simple overview of them only to give an understanding. For the
purpose of ease of understanding the units have been classified
according their functions.
Signalling Units
There are six different types of signalling units. These are CCSU
(Common Channel Signalling Unit) which handles trunk signalling
(SS7) towards HLR, other MSC’s and PSTN exchanges and is
responsible for call control for Trunk originated calls. The other one is
BSU (Base Station Signalling Unit) which takes care of SS7
signalling towards BSC and call control for mobile originated calls.
The CCMU (Common Channel Signalling Management Unit)
handles the centralised functions of the SS7 signalling system. It is only
needed in big exchanges. In smaller exchanges the CCMU's function
are taken over by the CM and the STU. For MSC’s which are
connected to networks which still use R2 signalling there is CASU
(Channel Associated Signalling Unit) which performs R2 signalling.
PAU (Primary Access Unit) handles DPNSS signalling towards
PABXs and LSU (Line Signalling Unit) controls the announcement
machine (ANM). One special unit is the IWCU (Inter Working
Control Unit) which controls Compact Data Services Unit (CDSU),
Echo Cancellers (EC) and any other interworking functional units.
Switching related units
In switching related units of MSC, GSW (Group Switch) is the
switching matrix. The basic function of the MSC is switching of
telephone calls. This is implemented by a group switch where each
input being capable of being switched to any output. M (Marker)
controls and supervises the GSW. There are three additional units
which work together with GSW. The first of these is the DTMFG (Dual
Tone Multi-Frequency Generator) for generating DTMF signals when
required, such as when user is trying to transmit DTMF signals to an
equipment at the other end. The next one is TG (Tone Generator) which
is responsible for generating various types of tones such as dial tone,
busy tone, information tone etc. The TG and DTMFG functions are
combined in the TGFP unit in recent MSCs. The third is the CNFC
(Conference Circuit), used for enabling multi-party conference.
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Database and statistics related units
One of the most important unit in an MSC is the VLRU (Visitor
Location Register Unit). This is the Nokia implementation of VLR as
a functional unit of the MSC; thus it performs the VLR functions.
CMU (Cellular Management Unit) controls and supervises the
cellular network and handovers. Exchange specific statistical data is
collected by STU (Statistical Unit). For a switch with relatively low
traffic STU can also collect charging data, but for switches which
handle high traffic CHU (Charging Unit) is needed to collect charging
data.
External interface and data related units
The unit with which a person can do the normal operation and
maintenance tasks is the OMU (Operation and Maintenance Unit).
This is the link between the user and the MSC. It monitors the
exchange continuously and starts recovery procedures if errors occur.
The BDCU (Basic Data Communications Unit) contains all
communication links to O&M network (terminals for X.25 packet
network and/or for time slots of PCM link) and to the Billing Centre.
For subscribers’ data (data calls, fax etc.) there are Data Service Pools.
They contain modems for data communication services to the PSTN.
ECU (Echo Canceller Unit) is needed in interworking to PSTN. It
cancels the echo generated in the 2 wire subscriber cable in the PSTN.
The ET (Exchange Terminal) is the unit which handles the external 2
Mb PCM circuits. There is no duplication of ETs since their reliability
is much greater than that of the actual link, therefore in the event of
failure, redundancy is taken care of by reorganising of the signalling
and traffic.
Other Units
CM (Central Memory) is one of the most important units. It is RAM
of the exchange, which holds the system software and also keeps a
copy of all exchange specific software data. CLSU (Clock and
Synchronisation Unit) is responsible for generating the
synchronisation signals for different units as well as for other elements
such as BSC, HLR. VANG (Verbal ANnouncement Generator) is
used for playing recorded announcements. MB (Message Bus) is the
parallel, duplicated message bus for sending and receiving DX
messages between different functional units.
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6.6
DX 200 HLR/AC/EIR
Once the architecture of DX 200 MSC/VLR is understood, it becomes
a matter of relative ease to understand all the other DX 200 elements,
because of the same architecture and the presence of similar units in all
the elements. HLR is no exception. Figure 6.6 shows the architecture
DX 200 HLR and a brief explanation of HLR specific units follows
thereafter.
CLS
GSW
Group Switch
ET
MSC
Mobile-services
X.25 to
AdC
M
CCSU
BDCU
MB Message Bus
2n
EIRU
CM
HLRU
2n
ACU
STU
OMU
I/O
Figure 6.6 Block Diagram of the DX 200 HLR
The maximum number of created subscribers in a DX 200 HLR is
300,000 and in a DX 200 HLR-i it is 1,200,000.
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6.6.1
Functional Units in HLR
It can be seen that there is only one signalling related unit in HLR
compared to the many in MSC. This is because user traffic does not
come to HLR. The rest of the units and their functions are exactly same
as in MSC. However we see that there are three extra units in HLR
which were not in MSC. These are the database related units. HLRU
(Home Location Register Unit) is responsible for subscriber data
management and mobility management. Authentication Unit (ACU)
is responsible for the management of authentication data. It generates
authentication triplets and sends them to the VLR and EIRU
(Equipment Identification Register Unit) handles the equipment
identity and its checking.
6.7
DX 200 BSC
Nokia’s DX 200 Base Station Controller (BSC) is also based on the
same switching platform on which MSC and HLR are designed.
However there is one major difference in the hardware of one particular
unit. This is the group switch. The group switch used in MSC is
capable of switching one PCM channel (8 bits) at one time only. That is
a PCM channel as a whole (8 bits) can be switched from any cable to
any channel. The group switch used in BSC, GSWB is capable of
switching 1 bit from inside a PCM channel and switch to any other
position in any other channel in any other PCM cable. This makes it
very convenient to switch transcoded speech which is at the rate of 13
Kbits/s. This will also enable switching for half rate speech when it is
implemented.
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GSWB
Group Switch
MSC (TC)
M obile Serv ices Switching
(Transcoder)
ET
ET
BTS
Base Transceiv er
Station
CLS
BCSU
MCMU
MB
MB
Message
MessageBus
Bus
Bus
MB
Message
X.25
OMU
I/O
Figure 6.7 DX 200 BSC (second generation) architecture
The capacity of a DX 200 BSC is a maximum of 128 BTSs or 256
TRXs.
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6.7.1
Functional Units in DX 200 BSC
Apart from the operating difference of the group switch, as mentioned,
other units work in the same manner as in MSC and HLR. Not all units
which were present in MSC are present in BSC due to the functional
difference of the network element. The function of the group switch
here is also much reduced than in MSC. For a smaller group switch it is
not necessary to have a marker dedicated to control the group switch.
Thus in BSC the marker and the cellular management unit are
combined to make one MCMU (Marker and Cellular Management
Unit). The Marker part controls and supervises the GSW. Cellular
Management Unit part is responsible of the cells and radio channels. It
manages the configuration of the cellular network. There is also only
one signalling unit. It is the BCSU (Base Station Controller
Signalling Unit) which handles SS7 signalling between MSC and BSC
and LAP-D signalling between BSC and BTS
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6.8
Nokia NMS/2000
The basics of Network Management Subsystem were already
discussed in the Traffic Administration chapter. Here we see the actual
implementation of it. The raw data from the GSM network is
transferred to the NMS/2000 via a router and a Data Communications
Network (DCN).
Workstations
LAN
Servers
Router
Data
Communications
Network
Figure 6.8 Nokia NMS/2000 Architecture
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6.8.1
Functional Units in NMS/2000
The standard Nokia NMS/2000 consists of servers and operator
positions which can be either application workstations or X terminals.
These components are connected to a Local Area network (LAN).
Servers are also provided and consist of a communications server, a
database server and a standby server or combinations of these. A router
is provided to allow communication to the various elements in the
GSM network which is connected to a Data Communication network
(DCN).
NMS/2000
BTS
Data Communications
Network
DN2
BTS
BSC
HLR
AC
EIR
MSC
VLR
BTS
BTS
SMSC
Figure 6.9 NMS/2000 connections to GSM Network Elements.
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6.9
Nokia BTS
Base Transceiver Station, BTS, is a part of the Base Station subsystem, BSS. The BTS provides the radio part for the BSS and is
located between the BSC and the Mobile Stations, MS. The Nokia BTS
is not based on the DX switching platform. It has it’s own platform.
BTS handles the signalling as well as traffic in the air interface. It also
detects the MS.
Frequency Hopping is another function which is implemented using the
Frequency Hopping Unit between the Frame Units and Carrier Units.
Hopping can be cyclic or pseudo-random as defined in the GSM
specifications. The BSC provides the Frequency Hopping parameters
for the BTS that operates according to the parameters. Frequency
hopping is optional. BTS is also responsible for power control in down
link direction. LAP-D signalling is used between the BSC and BTS and
LAP-Dm between MS and BTS.
Note that BTS is not based on the DX switching Platform.
6.9.1
Nokia BTS Families
The Base Stations manufactured by Nokia have gone through multiple
generations. Each successive generation of BTSs have come up with
new features. The following sections briefly give overview of different
generations of BTSs.
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6.9.1.1
Nokia 2nd generation BTS.
The second generation BTSs had advanced system features with wide
range of versions. They include flexible cell extension solutions, and an
optimised transmission solutions. features also included Remote
configuration management, Remote fault management, Downloadable
SW from the OMC, Local MMI for monitoring and control, Nonvolatile flash memory for SW storage Figure 6.11 shows some versions
of the second generation BTS.
INDOOR
Sectorized
Mini
OUTDOOR
Streetlevel
Rooftop
Figure 6.10 Nokia 2nd generation BTS
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6.9.1.2
Nokia Talk Family BTSs
The Nokia Talk family of BTSs has more versions than the second
generation ones. Nokia City Talk and Intratalk both have similar
features and can be a one or two cabinet version. With the two cabinet
version, it can have a maximum sectorised capacity of 4+4+4. When
used in Omnidirectional mode, up to 6 TRXs can be used. With a
height of 136 cm, the City talk is also quite compact. The Intratalk is
slightly bigger with 160 cm height.
The Flexitalk is a small and compact which finds most use while being
wall mounted indoors and in shopping malls. It is also quite useful in
special sites such as underground. Its capacity is 1 or 2 TRX in omni
directional configuration. With all its dimensions around 50 cm, it can
fit virtually anywhere. The Flexitalk+ is similar to flexitalk in
capacities and configurations, but is slightly bigger at 1m height. It is
also suitable for rooftop mountings. Figure 6.12 shows the Nokia Talk
Family of BTSs.
Nokia Intratalk
6 TRX
Nokia Citytalk
6 TRX
Nokia Flexitalk
2 TRX
Nokia Extratalk,
Site Support System
Nokia Flexitalk+
2 TRX
Figure 6.11 Nokia Talk Family of BTSs.
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6.9.1.3
Nokia PrimeSite
The PrimeSite has a high level of hardware integration. It has a
compact card-and-chassis construction. With a weight of less than 25
kg it is light and small in size. It has only one basic configuration.
Large operational temperature range of -40...+50 degrees Celsius
makes it suitable for various climates as well. The PrimeSite has only
one BTS version with one TRX, suitable for outdoor and indoor
applications. Although it includes only one TRX it can be configured to
give sectored results as well, as the example diagram shows. Figure
6.13 shows the PrimeSite size in comparison to a mobile station. Figure
6.14 shows some typical PrimeSite Applications and figure 6.15 shows
a 2+2+2 sectorised example using PrimeSite.
Figure 6.12 Nokia PrimeSite BTS
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Figure 6.13 Nokia PrimeSite Applications
1 BTS 1 BTS
1 TRX 1 TRX
A-bis
and
Clock
Signal
1 BTS 1 BTS
1 TRX 1 TRX
1 BTS 1 BTS
1 TRX 1 TRX
one BTS is
Clock Master
A-bis
BSC
Figure 6.14 A 2+2+2 sectorisation using Nokia PrimeSite
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6.9.1.4
Nokia MetroSite
Nokia MetroSite is a base station and transmission equipment
combined solution. It is designed for micro-cellular applications.
Figure 6.15 Nokia MetroSite
A MetroSite base station is small (49 litres) and light–weighted (22 kg
with 1 TRX and 35 kg with 4 TRXs installed) with a modular
mechanics. It supports GSM 900, 1800 and 1900 frequency bands. It
accommodates upto 4 TRX:s where a dual-band configuration with
GSM 900 and 1800 frequencies can be implemented. It can be used
both for indoors and outdoors purposes.
Simplicity of installation and integration brings shorter integration
times. Having the possibility of different build-in transmission units,
different transmission solutions can be easily applied.
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6.10
Summary of the Learning Points
•
Nokia’s implementation of the generic GSM network architecture
is based on the DX 200 Switching platform.
•
The DX 200 switching platforms based on the principles of
distributed processing and modularity.
•
The DX 200 based network elements from Nokia are:
•
•
DX 200 Mobile Services Switching Centre (MSC) which
includes the GSM network elements MSC and Visitor
Location Register (VLR).
•
DX 200 Home Location Register (HLR) which includes
the GSM network elements HLR, Authentication Centre
(AC) and Equipment Identity Register (EIR).
•
DX 200 Base Station Controller (BSC).
Nokia implementation of the Network Management Subsystem
(NMS) consists of two parts.
•
•
OMC Local Area Network consisting of Work Stations
where various OMC applications are running.
Nokia Base Transceivers Stations (BTS) is not based on the DX
200 platform.
•
There have been four different generations of BTSs from
Nokia.
•
188 (244)
Each generation consists of different families of BTSs
suitable for different environments including indoors
and outdoors.
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6.11
Nokia Implementation Review
6.11.1
Review Questions
In the following questions, please select one alternative which you
think is the best answer for the particular question. There may not be
a perfect answer, select the one which you think is the most correct.
1. In Nokia’s implementation of the GSM network
a) The transcoder is included as part of MSC
b) The EIR is included as part of MSC
c) The VLR is included as part of MSC
d) The HLR is included as part of MSC
2. The following subsystem is entirely built on the Nokia DX 200
switching platform:
a) Network Switching Subsystem (NSS)
b) Network Management Subsystem (NMS)
c) Base Station Subsystem (BSS)
d) None of the above
3. Authentication Centre is implemented as part of
a) Billing Centre
b) Visitor Location Register
c) Short Message Service Centre
d) Home Location Register
4. Which of the following is a feature of the DX 200 Platform?
a) Distributed Processing
b) Ability to start with minimum number of units and “add on”
more later if and when required
c) 2N and N+1 redundancies
d) All of the above
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5. Distributed processing in the DX 200 platform means
a) Sharing of tasks between different network elements like MSC,
HLR and BSC
b) Sharing of tasks between different functional units within one
network element such as BSC
c) Using parallel processing techniques within one computer unit
d) Using DX 200 platform
6. Which of the following is a task of NMS/2000?
a) Controlling the inter MSC handover
b) Routing of user’s calls
c) Counting the number of inter MSC handovers performed
d) Taking automatic recovery action after a fault occurs in MSC
7. How many bits of PCM time slot can the new GSWB in BSC
switch at one time?
a) 1
b) 2
c) 4
d) 8
8. How many maximum frequencies can there be if an Intratalk BTS
is configured as an omnidirectional BTS?
a) 12
b) 6
c) 4
d) 2
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9. How many TRX does a Prime Site have?
a) one
b) two
c) one or two
d) two or more
10. How many TRX can a MetroSite base station have?
a) one
b) two
c) three
d) four
e) any of the above
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7
Next Step
7.1
Module Objectives
At the end of the module the student will be able to:
192 (244)
•
Explain the principles of High Speed Circuit switched Data
(HSCSD).
•
Identify the facilities of the General Radio Packet Service
(GPRS).
•
Describe the capabilities of Enhanced Data rates over GSM
Evolution (EDGE).
•
Identify the facilities provided by the Wireless Application
Protocol (WAP).
•
Identify the 3rd Generation mobile systems and their facilities.
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7.2
Introduction
New demands will be made in the future on mobile cellular systems as
individuals and businesses change the way they work.
Access to the Internet will become more important and executives will
want to access corporate databases from virtually anywhere. New
services will be required in addition to speech and data, therefore
network operators will offer video and other multimedia applications.
Advanced mobile handsets will be required to handle large amounts of
high-speed data in what is known as the 3rd Generation Mobile
systems.
GSM networks will need to evolve to a point where these 3rd
Generation technologies can be introduced more cost effectively. This
GSM evolution will require network operators to have a wellidentified, step-by-step approach to meeting future service and capacity
requirements.
The European 3rd Generation system is known as UMTS (Universal
Mobile Telecommunications System) and ETSI is promoting a
smooth evolution from the present day GSM networks. The radio “air
interface” will be based on W-CDMA (Wideband- Code Division
Multiple Access) using different frequency bands for the uplink and
downlink.
The ITU call the 3rd generation mobile system - IMT-2000
(International Mobile Telecommunications 2000). IMT-2000 refers not
only to the approximate year when it is expected to be launched but
also the frequency band in the region of 2000MHz.
IMT-2000 will provide a seamless, global communications service
through small, lightweight terminals. The 1992 World Administrative
Radio Conference (WARC) allocated the radio frequencies between
1885MHz and 2200MHz to be reserved for the IMT-2000 on a global
basis.
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GSM systems will evolve towards the UMTS by progressively
introducing new techniques to provide higher bandwidth. These steps
are as follows:
•
High Speed Circuit Switched Data (HSCSD)
•
General Packet Radio Services (GPRS)
•
Enhanced Data rates for GSM Evolution (EDGE)
3rd Generation
UMTS
HSCSD
GPRS
EDGE
Fig.7.1 Evolution with GSM
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7.2.1
High-Speed Circuit Switched Data (HSCSD)
Traditionally TDMA timeslot provided a bit rate of 9.6 Kbits/s;
however a new modified air interface brings the speed upto 14.4
Kbits/s.
It is possible to increase the speed in data calls further by using
multiple timeslots (Multi timeslot usage can be done either with 9.6
Kbits/s or 14.4 Kbits/s timeslots). With HSCSD, a combination of upto
four TDMA timeslots could be used to provide data transfer rate at 57.6
Kbits/s. During 1999, Nokia will offer data rates of upto 28.8 Kbits/s
by using two timeslot and upto 57.6 Kbits/s with four timeslots. Using
multiple timeslots probably costs more to the mobile subscribers for
every minute; however, for example the time required to download a
mail or a file will be significantly less.
One Time Slot
...
...
BTS
BTS
Multiple
Multiple Bursts
Bursts from
from
each
each Mobile
Mobile Station
Station
Figure 7.2
Example of Multiple Bursts for HSCSD
In Nokia’s HSCSD solution, there is no need to make hardware
changes in the network (if MSCs have CDSU).
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7.2.2
General Packet Radio Service (GPRS)
The Internet has become part of everyday life, GPRS gives a direct link
between the worlds of the Internet and mobile communications
GPRS is different from existing GSM data services. Firstly it allows
users to have the same experience as if they were connected to their
office LAN. The mobile user doesn't have to connect to the network
each time he wants to transfer data, he can stay connected all day.
Secondly GPRS allows users to be charged for the actual amount of
data they transfer. This makes a whole new area of mobile data
applications possible.
With the higher transmission speeds provided by GPRS, end users will
find that file downloads are faster, applications that were previously not
possible now become possible and the overall attractiveness of the data
services will increase.
For example a user browsing WWW pages will be able to download
pages faster and also won't have to pay for the time in between each
page download when he's reading the last page.
GPRS brings cost efficient use of existing resources to the operators.
By allowing faster or slower data speeds dynamically according to the
amount of radio resources will help to increase the average usage level
of radio resources.
load
extra capacity can
be given to packet
data users
load
GPRS data traffic
Max capacity
Average
used
capacity
level
time of day
Fig.7.3
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time of day
GPRS data traffic increases the efficiency of resource usage
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In order to offer package switched data service, there should be some
modifications done in the GSM network architecture. Data packages
are handled with the help of two new network elements:
•
SGSN (Serving GPRS Support Node)
•
GGSN (Gateway GPRS Support Node)
PSTN
Mobile Operator
A
HLR
BS
BS
BS
SCP
SCP
BS
BS
Mobile
Operator B
BSC
Gb
A
A
ISUP
MAP
CAP
SGSN
CAP
MAP
ISUP
MSC
BS
A
MSC
A
BSC
Gb
BSC
A
BSC
Gb
BS
HLR
Gn
GGSN
GGSN
Gn
SGSN
Gb
BS
BS
BS
Gi
Gi
IP network
Fig. 7.4 GSM Network with GPRS Service
The Serving GPRS Support Node (SGSN) is a router that maintains the
location information of the mobile station and the Gateway GPRS
Support Node (GGSN) enables the data packets to be passed on to
other packet switching networks.
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7.2.3
Enhanced Data rates over GSM Evolution (EDGE)
EDGE will provide a bridge from GSM into the 3rd Generation mobile
networks. It will use an advanced GSM modulation technique to
provide data speeds of 384Kbits/s but still using the existing 200KHz
GSM channel. (384Kbits/s is the H0 channel in ISDN which permits the
transmission of 6 x 64Kbits/s signals).
The extra capacity is achieved by increasing the data capacity of a
single GSM timeslot from 9.6Kbits/s to 48Kbits/s and possibly up to
nearly 70Kbits/s under good radio conditions. These timeslots can be
used flexibly to allow several simultaneous services: for example, a
voice call on one timeslot, Internet browsing on two others, and video
conferencing on the fourth.
EDGE will provide the present-day GSM network with the ability to
handle wireless multimedia services such as Internet/intranet,
videoconferencing, and fast electronic mail transfer. One of the
attractions of EDGE technology is that it requires minor changes to
network hardware and software, and can be introduced into an existing
network using the current frequency bands.
EDGE uses the same TDMA frame structure and channel bandwidth of
200KHz as current GSM networks. Existing cell plans can remain
intact which means that EDGE can be introduced gradually into a
network, starting with high-capacity areas such as dense city areas,
airports, etc.
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7.2.4
The Wireless Application Protocol (WAP)
The purpose of the Wireless Application Protocol is to bring Internet
content and advanced services to digital cellular phones and other
wireless terminals. The aim is to create a global wireless protocol
specification to work across differing wireless network technologies
such as GSM-900, GSM-1800, GSM-1900, CDMA and 3rd Generation
systems.
The Wireless Application Protocol is under development and will
include specifications for the transport and session layers of the OSI
model as well as security features. The mobile terminals will use WML
(Wireless Mark-up Language) which is a browsing language similar to
HTML used on the Internet. WML is a tag-based display language
providing navigational support, data input, hyperlinks in addition to
text and image presentation.
A common standard means the potential for realising economies of
scale, encouraging cellular phone manufacturers to invest in developing
compatible products, and cellular network carriers to develop new
differentiated service offerings as a way attracting new subscribers.
Consumers benefit through more and varied choice in advanced mobile
communications applications and services.
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7.3
3rd Generation Mobile Systems
The ITU has been developing the 3rd Generation mobile systems since
1985 and called it FPLMTS (Future Public Land Mobile
Telecommunications System) but in 1996 it was re-named it as IMT2000 (International Mobile Telecommunications 2000).
A summary of the main objectives for the IMT-2000 air interface are
shown below:
•
Full coverage and mobility for 144Kbits/s, preferably 384Kbits/s.
•
Limited coverage and mobility for 2Mbit/s.
•
Efficient use of radio spectrum compared with existing systems.
•
Flexible architecture to allow introduction of new services.
Compromises must be made on the speed of data transmission
compared with the distance and mobility. This is indicated in the
diagram below.
User Bit Rate
2Mbps
IMT-2000
384Kbps
GSM-EDGE
144Kbps
Evolved 2nd Generation Systems (GSM-HSCSD, GPRS,
10Kbps
2nd Generation Systems (GSM, IS-95, IS-136,
Short Distance / Low Mobility
Wide Area / High Mobility
Figure 7.5 Bit Rate versus Mobility
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7.3.1
Frequency Allocation for 3rd Generation Systems
The World Administrative Radio Conference (WARC'92) identified
230MHz of bandwidth for the IMT-2000. However, this will be
allocated in different ways in different regions and countries.
PCS
Europe
UMTS
(FDD)
Japan
USA
IMT-2000
Mobile
Satellite
IMT-2000
Mobile
Satellite
ITU
Mobile
Satellite
2150
UMTS (TDD)
Mobile
Satellite
2100
Mobile
Satellite
PCS
unlicensed
PCS
IMT-200
2050
IMT-2000
UMTS (TDD)
(TDD)
UMTS (TDD)
UMTS
(FDD)
PHS
GSM
1800
DECT
IMT-2000
2000
Mobile
Satellite
1950
Mobile
Satellite
1900
Mobile
Satellite
1850
Figure 7.6 IMT-2000 frequency allocations
In the USA, part of the IMT-2000 frequency allocation is already used
for PCS systems (GSM-1900). In Europe and Japan, the frequency
allocation is almost identical except that the PHS spectrum partially
overlaps the UMTS TDD spectrum.
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7.3.2
The Universal Mobile Telephone System (UMTS)
The success of GSM is having a strong impact on the standardisation of
the 3rd Generation systems.
Within ETSI, the technical committee SMG has the responsibility of
standardising the 3rd Generation system in Europe which is known as
the Universal Mobile Telephone System (UMTS).
W-CDMA (Wideband-Code Division Multiple Access) will be
employed on the air interface mainly for wide area applications and
will use paired frequency bands, one for the uplink and one for the
downlink. This is commonly referred to as Frequency Division Duplex
(FDD).
UMTS will also employ TD-CDMA (Time Division-Code Division
Multiple Access) for low mobility indoor applications using Time
Division Duplex (TDD) similar to cordless technologies. Together,
these two elements of the air interface (FDD and TDD) are known as
UTRA (UMTS Terrestrial Radio Access).
UMTS has the objective of providing low cost mobile terminals which
ensure compatibility with GSM and will facilitate FDD/TDD dual
mode operation.
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7.3.3
Code Division Multiple Access (CDMA)
Code Division Multiple Access is a technique that allows many
different mobile telephones to use the same frequency at the same time
but with each phone assigned a unique code sequence known as a
"spreading code".
CDMA is a form of "spread spectrum" where the information is spread
across the available bandwidth of the radio channel.
The spreading code is used to encode an information bearing digital
signal. The receiver uses the same code to decode the signal and
recover the information data. As the bandwidth of the code signal is
chosen to be much larger than the bandwidth of the information signal,
the encoding process enlarges (spreads) the spectrum of the signal. This
spectral spreading of the transmitted signal gives CDMA its multiple
access capability.
CDMA is "direct sequence" spread spectrum technique as each
information bit is replaced by a sequence of shorter "code bits" or
"symbols" called "chips".
The basic operation of CDMA is as follows:
If the digital information signal was a “1” then the spreading code
would be transmitted normally but if the digital signal was a “0” then
the spreading code would be transmitted inverted. The resulting signal,
phase modulates an analogue radio frequency but because the bit rate of
the transmitted coded signal is very high, the bandwidth of the radio
signal is spread right the designated radio spectrum. The signal
transmitted has been “squashed” so that it is broad in frequency terms
but flat in power terms. This is “spread spectrum” and the signal could
be so low in power density that it cannot be detected above the normal
radio noise level.
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Symbol
Spreading
Symbol
Spectrum
+1
Data
-1
Chip
Chip
Code
(pseudo
noise)
+1
Data x
Code
+1
-1
-1
Despreading
+1
Code
-1
+1
Data
-1
Figure 7.7 Principle of CDMA
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7.3.4
W-CDMA (Wideband CDMA)
For the 3rd generation mobile systems, a high bit rate is required for
multi-media data. Therefore, the spreading code must be of a higher bit
rate. IS-95 CDMA uses a bandwidth of 1.25MHz but the W-CDMA
systems for UMTS will occupy a bandwidth of approximately 5MHz.
In the W-CDMA system the spreading codes are used to spread out the
data signal to cover the whole wideband spectrum which is allocated
for the data transfer.
The ETSI proposals for W-CDMA uses direct spread with a chip rate
of 4.096Mchips/s. The bandwidth of 5MHz was chosen for various
reasons:
Firstly, the data rates of 144Kbits/s and 384Kbits/s are achievable
within this bandwidth and can provide reasonable capacity 2Mbit/s
peak rate under limited conditions.
Secondly, the large 5MHz bandwidth can resolve more multipaths than
narrower bandwidths. This will increase diversity and improve
performance. Wider bandwidths of 10, 15 and 20MHz may be
proposed in the future to support high data rates more effectively.
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7.3.5
3G Network Architecture
3G networks consists of Radio Access Network (RAN), Core Network
and NMS. In the network solution, there can be GSM and 3G
interworking possibility. Note that RAN represents BSS and Core
Network represents NSS of a GSM network.
Packet Subsystem
GSM
mobile
Co-sited GSM + WCDMA
Base Station Subsystem
SGSN
Base Station
Controller (GSM)
SIM Card
GSM / UMTS
mobile
GSM Base Station
Internet
Internet
(TCP/IP)
(TCP/IP)
Network Subsystem
(GSM )
GGSN
Mobile Switching Centre
Home Location Register
(GSM)
BSC
MSC
UMTS
mobile
RNC
HLR
3G-IWU
UMTS (WCDMA)
Base Station
Radio Network
Controller (WCDMA)
Landline
Landline NW
NW
(PSTN/ISDN)
(PSTN/ISDN)
IN Service Control Point
A mobile station can have possibility of handover between a GSM and
WCDMA networks.
A pure WCDMA network would look like as below:
Internet
Internet
(TCP/IP)
(TCP/IP)
WCDMA Network Subsystem
SIM Card
GSM / UMTS
mobile
WCDMA Mobile Switching Centre
Home Location Register
WCDMA Packet Network
RNC
UMTS (WCDMA)
Base Station
3G-SGSN
WMSC
UMTS
mobile
HLR
RNC
UMTS (WCDMA)
Base Station
Radio Network
Controller (WCDMA)
Landline
Landline NW
NW
(PSTN/ISDN)
(PSTN/ISDN)
IN Service Control Point
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7.3.6
3G Mobile Data Terminals
High bit rate capabilities will allow 3rd Generation mobile terminals to
provide sophisticated data services and multimedia facilities.
Figure 7.7 Example of possible 3rd Generation Data Terminal
Figure 7.8 Evolved SMS: Electronic Postcard
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Data terminals produced for the 3rd Generation systems will have
electronic imaging, electronic postcards and general multimedia
support. Voice calls will be combined with real-time images to provide
mass market video telephony.
The arrival of Personal Mobile Multimedia (PMM) will have a
significant impact on business and private individuals alike. Full scale
web browsing will be available commonly from anywhere.
There will also be a variety of information and entertainment packages
in addition to location based interactive services. Multiple 3rd
Generation standards will be handled by multi-mode terminals that can
communicate with different systems.
7.4
208 (244)
Summary of Learning Points
•
ITU calls the 3rd Generation Mobile systems IMT-2000
(International Mobile Telecommunications 2000), Europe calls it
UMTS (Universal Mobile Telecommunications System). UMTS
will use W-CDMA (Wide band – Code Division Multiple
Access) technique in the radio path.
•
GSM is evolving with HSCSD (High Speed Circuit Switched
Data), GPRS (General Packet Radio Service) and EDGE
(Enhanced Data rates over GSM Evolution) to provide much
higher data rates. This is opening a way of smooth transition to
3rd generation mobile services.
•
Nokia provides flexible total solutions to HSCSD, GPRS and
WAP (Wireless Application Protocol).
•
3G (3rd Generation) Mobile Data Terminals will be furnished
with multimedia facilities.
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7.5
Next Step Review
7.5.1
Review Questions
In the following questions, please select one alternative which you
think is the best answer for the particular question. There may not be
a perfect answer, select the one which you think is the most correct.
1. At most, how many timeslot can be used with HSCSD?
a) 1
b) 4
c) 6
d) 8C
2. What is the benefit of GPRS?
a) One does not pay for thinking when having internet
connection.
b) Faster data rates
c) Operator utilises the resources
d) All of the above
e) None of the above
3. Which is not part of 3G network?
a) Radio Access Network
b) Core Network
c) Base Station Subsystem
d) Network Management System
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8
Intelligent Network
8.1
Module Objectives
At the end of the module the student is able to:
8.2
•
explain the need for IN
•
draw the IN architecture
•
explain the Nokia implementation of IN
•
describe how services are implemented using the IN architecture
Introduction
The public switched telecommunications network (PSTN) is a network
which provides telephone and data services to individuals and
businesses world-wide. The services provided by the PSTN are tending
to become more sophisticated and driven by customer requirements.
This is particularly evident in the last five years after the introduction of
competition into many networks. Also, new technology is tending to
emerge at faster rates. Once a technology has been deployed within a
network, it is generally uneconomic and impractical to immediately
replace it with the next generation. These trends have resulted in PSTN
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operating companies studying ways to make their networks more
flexible in the sense that new emerging technology and services can be
more rapidly and gracefully introduced into their network.
Then, let we try to image what really means, in the future, the
introduction of IN. When we make a telephone call today, we want to
either reach a particular person or perform a particular function,
involving for example a bank, an insurance company or a pizzeria.
When we make the call, we need to know the exact physical location
we are calling so we reach the right telephone. In future, we want to be
able to call a person or perform a function by using a unique number,
no matter where that person or that function is physically located. We
will just call the unique number and the network, not we, will know
where to route the call.
The present and future needs of customers and operating companies are
predicted to be:
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ability to adapt to rapidly changing technical, regulatory, and
marketing environments;
•
stimulating use (and hence revenue) of the network;
•
ability to service niche markets and individual customers;
•
improved customer control of their service, and
•
universal accessibility to services;
•
improved network operation, administration and management,
then
•
easier and more flexible service implementation;
•
ability to deliver sophisticated services,
•
lower cost,
•
greater multivendor participation.
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8.2.1
History
During the 1970s AT&T implemented freephone and Line Information
Database (LIDB) services using a centralised database. AT&T
demonstrated that these services could be provided more flexibly using
a centralised database compared to dedicated equipment in the PSTN.
Ameritech in 1984 proposed that Bell Communications Research
(Bellcore) study a platform which centralises service logic. Bellcore
developed this concept and prepared a Request for Information (RFI)
for what was then called a Feature Node/Service Interface. It was at this
seminar that the term `Intelligent Network' was first used. The previous
concept of centralising only the service data became termed the
Intelligent Network/1 (IN/1).
The proposed Feature Node/Service Interface was an entity external to
the PSTN which was to provide a software platform for the
implementation of service logic and related service data. The
motivation for the Feature Node idea was the fulfilment of the
following objectives:
•
the ability to rapidly introduce new services into the PSTN,
•
the establishment of interface standards to facilitate equipment
and software vendor independence.
These two objectives became the guiding principles of the Intelligent
Network.
In 1987 the emphasis of Bellcore work on the Intelligent Network
shifted in response to a request from the Regional Bell Operating
Companies (RBOC) for an Intelligent Network architecture that could
be implemented earlier than the IN/2. The result of this work was called
the IN/1+. The objective of the IN/1+ was to develop an architecture
that could be deployed by 1991.
During 1989 Bellcore realised that vendors could not supply the
capability required for IN/1+ for 1991 deployment. In May 1989
Bellcore released a new implementation plan. The new plan replaced
the IN/1+ and IN/2 architectures with architecture termed the Advanced
Intelligent Network (AIN).
The ITU-T and ETSI (European Telecommunications Standards
Institute) started work on defining an Intelligent Network architecture
in late 1990. Their IN architecture has been heavily influenced by
Bellcore’s work. CCITT working party XI/4 have followed Bellcore
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and defined a three stage evolutionary approach. Each stage is defined
by a capability set (CS). The CSs are planned to be fully specified at
two yearly intervals starting from the end of 1991.
8.2.2
Basic telephone network
To explain the IN concept, it is useful to reflect on how a service is
realised. A service is realised by using physical resources within the
network. Physical resources include things such as: dial tone, circuits
(bandwidth), collection of dialled digits. Service logic (i.e., service
intelligence) controls these physical resources in such a way as to
provide the service. In the existing networks (PSTN/GSM), the parts
which provide service intelligence are intertwined and mixed with parts
which provide the physical resources, there is no clear and standardised
interface between the two.
8.2.2.1
Example of introducing new service in the network and
problems it faces
The crucial difference between the existing network architecture and
the IN architecture is that service logic is separated from the actual
physical resources that provide the service. This seemingly small
change is very significant because the service logic can now be
managed and located independently of the physical resources.
Before the introduction of the IN features, the services were allowed by
using a specialised logic implemented on each switch platform of the
network (figure 7.1).
The traditional approach of implementing PSTN services is to
implement the service at every (or most) exchange in the PSTN. This
approach requires regular updating of exchange software and hardware.
This traditional approach is now seen as inadequate to meet the present
and future needs of customers and operating companies.
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This means that when a particular service is introduced all, the switches
had to contain the relative software package; meaning all the switches
were loaded with the same application.
TRADITIONAL WAY
INTELLIGENT WAY
NEW SERVICE FEATURES
FIXED
SERVICES
SCP
CELLULAR
FIXED
SSP-SWITCHES
NEW FEATURES
INTO EXCHANGES
CELLULAR
Figure 8.1 Service logic implementation methods
8.2.2.2
Concept of a central service provider
In the IN the service logic (and service data required to define the service)
is centralised. This means the service can be completely managed from
this one point within the network. This departs significantly from the
existing PSTN where the service logic and service data is distributed
throughout the network at each exchange. From this one centralised
location, new services can be added without the need to modify the
underlying network. This means services can be introduced quickly, and
trial services can be evaluated and modified easily. The main idea of IN
is to be service, switch and equipment-independent.
The logic required to control the IN services is removed from the
switching element to a separate network element to the SCP (Service
Control Point). The remaining IN service related functions in the switch
(functions of the SSP, Service Switching Point) is limited to the
reporting of call progress events to the SCP, and receiving instructions
from the SCP for further actions. Thus introduction of a new service for
network-wide utilisation requires only the provision of new service
logic program to the SCP.
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8.2.2.3
Conceptual view of IN call
Let us take an example of an IN call. Imagine that you need to make a
flight reservation for your holiday. Which office are you calling? In
which city? For you it is easy because the airline has advertised only
one number. But in reality what happens is that once the call arrives at
the MSC, it is detected that this is a call which requires special
handling. So MSC sends the dialled numbers to the central service
provider.
Depending on your town from where you have called, the call will be
routed to the correct IN node that supports the call. Once the correct
point is reached, the service control point will analyse which day of the
week it is, the time of day and so on. Service Logic can choose,
according to the zone/day/hour, to reroute the call to another point
and/or play a courtesy announcement and/or collect digit further
information (e.g. “please, press 5 for booking, 6 for flight information,
9 for .....).
NOKIA DX 200 MSC
1
2
digit “1234”
SSP
6
digit “5”
HP 9000/800 - SCP
7
SSP
IP
digit “5”
4
IP
5Annuncement:
B Numb. 1234
Request for
annuncement
SCP
3
Translation:
- location Milan= 9876
- location Rome= 6789
8
Translation:
- Rome = 6789 +5 --> 6789555
“please, dial the number..”
9
Alert
“6789555“
Figure 8.2 IN call example
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8.3
Intelligent Network Conceptual Models
The Intelligent Network has been made possible through developments
in many fields including: signalling system no.7, stored program
control exchanges, advanced software languages and powerful
computers. The purpose of this section is to focus in on those parts of
the network directly required for the Intelligent Network.
Signalling is required for exchanges, network data bases and other
intelligent nodes in the network to exchange messages related to call set
up, call supervision, call connection control information needed for
distributed application processing and network management information.
Core INAP
MAP
TCAP
ISUP
TUP
SCCP
MTP Layer 3
MTP Layer 2
MTP Layer 1
Figure 8.3 SS7 stack for IN
The SS7 is a signalling system recommended by ITU-T and
specifically designed for telephone networks. The SS7 is the generic
name for a suite of protocols. These protocols are layered and closely
match the Open System Interconnection (OSI) seven layer model. The
layered structure of the SS7 protocol suite is shown in Fig. 5.
The lowest three layers are called the Message Transfer Part (MTP). Its
function is to route a message efficiently and reliably between two
signalling points in the network. Above MTP is the signalling
connection control part (SCCP). The MTP together with SCCP provide
most of the layer services specified in layers 1 through 3 of the OSI
model.
The SS7 application protocols sit immediately above MTP and SCCP.
The Telephony User Part (TUP) is an application protocol for setting
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up and clearing (non-ISDN) telephone calls. The ISDN User part
(ISUP) protocol is an application protocol designed to support ISDN
calling. The Transaction Capabilities and Application part (TCAP) is
an application protocol for invoking applications on a remote network
element. TCAP is used for signalling between the SSP and the SCP of
the Intelligent Network. It is essential therefore distinguish between
functions and products in the Intelligent Network concepts.
8.3.1
Physical Entities
The functional entities (FEs) are allocated to physical entities (PEs) or
physical nodes. A FE must be found in at least one PE, but a PE can
consist of more than one FE. However, there cannot be more than one
“FE of the same type” in one PE.
The roles of the single element are:
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SSP. The Service Switching Point operates as call control
“controlled” point. Sends service requests to IN platform and
receives instructions. Requests are sent when triggers are
invoked.
•
SCP. The database that contains the subscribers data and
subscriptions. The SCP contains also the software to manage the
SSP requests. It’s responsible to manage calls and services. These
platforms are based on HP computer.
•
IP. This Intelligent Peripheral is designed to answer to the
subscriber calls in order to furnish resources for particular
specialised functions (mass calling/televoting). It’s controlled by
SCP and has voice connected to SSP. The IP element is an
intelligent answering machine that is able to play an
announcement and wait both “digit” from the user and instruction
from the SSP/SCP.
•
SMS. Centralised or not, this entity is competence to provides the
subscription provisioning, customisations, managing of the SCPs.
Provides also interfaces towards external systems. These
platforms are based on HP computer.
•
SCE. On this host the service provider may define the related
database schemas. Contains tools for service design, prototype
and testing. Produces Service Logic Program (SLP) which is the
SW “feature” used by the SCPs.
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8.4
Nokia Implementation of IN
NOKIA IN architecture is a platform for fast development and
deployment of new value added services in a telecommunications
network.
The Network Elements (figure 8.4) which make up the IN network are:
SC E
A lfa s k op m 3 4 6
SM S
LA N
Alfa skop m346
SM P
SCP
PS T N /
ISD N
SS#7
TUP /ISU P
Core
IN A P
M SC /VL R/SSP
MSC
SMS
SSP
IP
SS#7
TU P/ISU P
VLR
MSC
VLR
M AP
M AP
AC
Figure 8.4
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EIR
M SC/V LR
HLR
Nokia Intelligent Network Architecture
•
DX 200 SSP is implemented on a DX 200 MSC. IP may be
integrated to the DX 200 exchange.
•
SS7 (Core INAP) network is used to connect SCP and SSP nodes.
•
Service Control Point (SCP) is the source of call control within
IN. It stores the service data, executes the service logic programs
and communicates with the SSP-switches. Nokia SCP uses
standard high availability HP-UNIX technology. For better
availability a Mated Pair configuration is also possible.
•
Service Management Processor (SMP) is the Nokia’s name for
the SMS. It is used for managing, updating and backing up
service data. It provides interfaces towards support systems like
customer care and billing. Nokia SMP is based on high
availability HP-UNIX technology. SMS manages also the Mated
Pair requirements for SwitchOver and alarming.
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•
Service Creation Environment (SCE) is a resource for
programming, simulating and testing new service solutions. It
provides a user friendly graphical service logic editor. The SCE is
running on a standard HP-UNIX platform.
Various services using voice processing, dialling control etc. can be
produced with the Intelligent Peripheral (IP) that can be within DX 200
or as a separate unit connected to SCP and SSP via SS7 network.
8.4.1
SSP and IP (Service Switching Point and Intelligent
Peripheral) in MSC
8.4.1.1
Software
Obviously the IN platform cannot be implemented until a basic
telecommunication network (GSM or PSTN) exists. In the case of
mobile application, IN platform is implemented on the “mobile
network” and accessed through the MSC (with IN functionality), in
such case SSP, with particular SS7 protocol for IN named Core INAP
(Intelligent Network Application Part). Nokia application starts with
the M7 release software on MSC which has the SSP functionality and
equipment needed for the IN introduction.
8.4.1.2
Intelligent Peripheral
Currently the Intelligent Peripheral (IP) functionality is embedded into
the DX 200 MSC/VLR/SSP. However in the near future a dedicated IP
will be implemented as an independent network element. The current
IP contains the implementation of the Specialised Resource Function
(SRF) with the following two distinct requirements:
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capability to provide tailored announcements to the users, and
–
capability to receive DTMF dialling for PSTN users of IN
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8.4.2
SCP - Service Control Point
The Service Control Function (SCF) represents the centralised
authority in an Intelligent Network. The SCF contains the platform for
service logic programs (SLP). A service logic program is executed
when network exchange requests call handling instructions. The service
logic programs basically determine the behaviour of the IN-based
services.
8.4.2.1
Core INAP
The communication between the functional entities of CS-1 is done
using the Core INAP protocol. INAP is standardised by ETSI to
support IN functionality (Capability Set 1 or CS-1) on basis of the
corresponding work done internationally in ITU-T. The ETSI Technical
Specifications define in detail the operations required to support
Capability Set 1 functionality. The protocol contains a set of operations
used between different functional entities. The operations obey the
remote procedure call scheme. This means that they have specified
arguments, results and error responses. Most of the operations are in
fact simply messages passed from one entity or network element to
another. Core INAP provides operations for various purposes:
operations for detection point handling, routing, user interaction,
charging, etc. The protocol interaction follows a simple state model.
The detailed description of the Core INAP state models can be found in
the ETSI recommendation.
However, these specifications are restricted to support basic telephony
related information flow only, and do not fulfil the requirements set by
a mobile environment. Therefore the Core INAP specifications of ETSI
are supplemented by Nokia, using the standard extension possibility to
allow the mobility aspect to be utilised to the fullest extent in the
mobile networks.
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8.4.3
SMP - Service Management Point
The Nokia IN/SMP Platform provides a solid foundation for the
environment needed to manage the services within an Intelligent
Network. The role of the SMP in the Nokia IN architecture consists of:
•
a management application for service and subscriber data
provisioning;
•
a custom-built interface between the operator’s administrative
systems and the SMP platform;
•
custom-built applications running on the SMP platform handling
operator and service specific management activities.
Non real time database
The entire Service Management Point typically consists of the Nokia
IN/SMP Platform which performs one or more of the following
activities:
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to act as a safe repository for service and subscriber data (Oracle
DB)
•
to provide the administrative systems with an interface that hides
the details of the SCPs and the distribution of services into
different SCPs from the administrative systems
•
to provision service and subscriber data updates into the relevant
SCP to receive service and subscriber data updates from the SCPs
and store them in the SMP.
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Service Management
System (SMS)
Service Control Point
(SCP)
Service Control Point
(SCP)
Figure 8.5 SMP database
8.4.4
SCE - Service Creation Environment
In order to deploy intelligent services, Network Operators need easy-touse software tools dedicated to the creation and customisation of these
services. There are many views on service creation. The actual creation
work is concentrated into two phases of the service creation life cycle:
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•
The service application programming and database definitions
take place in the service implementation phase. The SCE-tool is
used to program the logic and to define the SCP database schema.
Commercially available development tools are used to create the
service management applications and the SMP database.
•
The service logic programs and the service database can be
designed to include a certain degree of flexibility in the service
itself allowing the operator to define in the service provisioning
phase, which features and options are used in the final service
that is offered to customers. Several different services can thus be
created out of one generic service logic.
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8.5
IN Service Examples
8.5.1
New and existing services provided within IN
By the virtue of their nature IN services are almost unlimited in the
functions they can provide. In the following sections few examples of
IN services are briefly described, but the services (fig. 5.1) will vary
according to the needs of each operator.
Certain descriptions serve as full-bodied service concepts as such (e.g.
Credit Card Calling), but in most cases by combining multiple
individual IN services together to constitute a higher-level IN
application one can gain better results.
There are many kinds of services designed by standardisation
procedure which are not yet developed by NOKIA but are interesting
for us:
Flexible Billing
• Freephone
• Premium Rate
• Calling Card/Credit Card Calling
• A-number validation
Reachability/
Mobility
• Personal Number
• UPT (Universal Personal Telecommunications)
• Portable Number
Customized user
groups
• VPN (Virtual Private Networks)
• Wide Area Centrex
• Free Numbering Plan (mobile)
Mass Calling Services
• Televoting
Customized services
for mobile users
• Originating/terminating call screening
• Originating/terminating location services
• IN service control
• Service override
Figure 8.6 Possibilities for IN applications.
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8.5.2
Flexible Billing
Flexible Billing services are services provisioned for individual
subscribers. As the name implies, the main functionality of these
services is to modify the charging of the subscriber’s calls and add new
charging possibilities in the network.
8.5.3
Reachability/Mobility
The Reachability/Mobility services provide a general access based on
dialled digits for any caller. The main functionality of these services is
the service number based routing of incoming/outgoing calls and
associated charging control. Subscribers, with several telephone sets in
different networks can define a “hunting order” for their calls.
Additional control based on time schedules and caller’s location is also
available.
8.5.4
Customised User Groups
In Group services, subscriber is a member of a group (a list of
subscribers). Group services are applicable for both incoming and
outgoing calls. Group service features include short numbers for group
members (and other destination as well), parameterised routing for each
member, special tariff for in-group calls and so on.
8.5.5
Customised services for mobile users
Terminating/Originating Call services provisioned for mobile
subscribers and used for outgoing/incoming call cases. These services
allow the SCP to control the routing of calls, apply special tariffs for
these calls or screen the calls. Further service control may be based on
time schedules; dialled digits or location of the caller and access by
passwords are available.
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8.5.6
Final conclusive example:
All these services, mixed in different manner create special service. An
example of using these services could be a “Mobile for Teenager”
subscription option:
8.6
•
Prepaid SIM and/or
•
Alternate billing (the parent pays)
•
Selective barring (special number like 144-00-...)
•
Special Discount (frequently number called)
•
Free short number (e.g. house)
Summary of the Learning Points
Why
1. Marketing
The introduction of Intelligent Networks is in a way a revolutionary
move. Switching and intelligence, from being separated in the early
networks, and from being more and more closely integrated in the
networks of yesterday and today, are once again being separated from
each other.
2. Applicability
The ideal goal for continuous development of telecommunications
networks is to serve all users at all times. In other words, this will mean
providing mobility for the user, i.e. calling from any location, to
provide information services to compete with the different media of
today, and to provide customised value-added services for specific
needs. In addition, it should also be possible for users with different
means of access to reach each other, independent of what type of
network is used for access, ISDN, PSTN or mobile networks.
3. Services
Networks will always differ, mostly of course because of size and the
technology that is being used, but in the future it will be the services
themselves that characterise the networks. And services, from the
creation to deployment, are what Intelligent Networks are all about.
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Where
1. IN in GSM
How
1. Network Elements:
- SSP
- SCP
- SMP
- SCE
- IP
8.7
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References
/1/
Guidelines for CS-1 Standards, ETSI, TCR-TR NA-602.04.
/2/
Intelligent Network Capability Set 1 (CS-1) Core INAP, ETSI,
DRAFT pr ETS 300 374-1:1993.
/3/
Mobile IN Concept for GSM/DCS Networks, Nokia Cellular
Systems Oy, E. Hirviniemi, v. 1.0, 1994.
/4/
NOKIA INTELLIGENT NETWORK PLATFORMS,
General Description, NTCD IGD 015/0.2en
/5/
Intelligent Network, Artech House, London, Jan Thorner, 1994
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8.8
IN Glossary
INTELLIGENT NETWORKS TERMS
Abbreviation list
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AC
Authentication Centre
BSC
Base Station Controller
BTS
Base Transceiver Station
CCF
Call Control Function
CCC
Credit Card Calling, an IN service
CCS
Common Channel Signalling
CCV
Calling Card Validation
CLASS
Custom Local Area Signalling Services
CS1
Capability Set 1
DTMF
Dual Tone Multi Frequency
EIR
Equipment Identity Register
ETSI
European Telecommunications Standards Institute
FE
Functional Entity
HLR
Home Location Register
HP
Hewlett Packard
IN
Intelligent Network
INAP
Intelligent Network Application Protocol
IP
Intelligent Peripheral
ITU-T
International Telecommunication Union
IVR
Interactive Voice Response
LAN
Local Area Network
MA
Management Application
MSC
Mobile Switching Centre
NP
Number Portability (Portable Number)
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NTS
Number Translation Services
OCS
Originating Call Screening, an IN service
OLS
Originating Location Services, an IN service
OSS
Operation Support System
PBX
Private Branch Exchange
PON
Portable Number
PPC
Pre-paid Contract
PSTN
Public Switched Telephone Network
SCE
Service Creation Environment
SCF
Service Control Function
SCP
Service Control Point
SIB
Service Independent Building Block
SIM
Subscriber Identity Module
SLEE
Service Logic Execution Environment
SLP
Service Logic Program
SMS
Service Management System
SMP
Service Management Point
SRF
Specialised Resource Function
SSF
Service Switching Function
SSP
Service Switching Point
TCP/IP
Transmission Control Protocol / Internet Protocol
TPS
Transactions Per Second
UPT
Universal Personal Telecommunications
VLR
Visitor Location Register
VM
Voice Mail
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8.9
Intelligent Network Review
8.9.1
Review Questions
In the following questions, please select one alternative which you
think is the best answer for the particular question. There may not be
a perfect answer, select the one which you think is the most correct.
1. Which of the following can not be said to be a reason for the
evolution of Intelligent Network?
a) improved customer control of their services
b) easier and more flexible service implementation
c) ability to adapt to rapidly changing technical environments
d) none of the above
2. What is the main feature of the Intelligent Network Architecture?
a) Centralised Service logic
b) Distributed Service Switching points
c) use of Intelligent peripherals
d) Ability to span across multiple telecommunication networks
3. If a GSM - IN call is handled in a telecommunication network,
which of the following will be the traffic path?
a) MS - BSC - MSC - SCP - MSC - route out
b) MS - BSC - MSC - SSP - route out
c) MS - BSC - MSC - route out
d) MS - BSC - MSC - SCP - (signalling with SMP) - route out
4. Which of the following is not an element of the IN architecture?
a) SMSC
b) SCP
c) SSP
d) IP
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5. In Nokia’s implementation of IN SSP is implemented in
a) BSC
b) MSC
c) HLR
d) HP work stations
6. Which of the following is the correct protocol stack, in ascending
order, for signalling system in IN?
a) MTP - SCCP - TUP - Core INAP
b) MTP - SCCP - ISUP - Core INAP
c) MTP - SCCP - BSSAP - Core INAP
d) MTP - SCCP - TCAP - Core INAP
7. What is the signalling protocol used to communicate between SSP
and SCP?
a) ISUP
b) Core INAP
c) MAP
d) BSSAP
8. During an IN call
a) the actual service logic is performed by MSC while the SCP
simply routes the call
b) the subscriber database look up is performed on the SMS
c) The routing destination and associated parameters are informed
by SCP to MSC
d) if the core INAP link is broken the call will clear
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9
SYSTRA Course Review
9.1
Review Questions
1. Mark True or False to the following Statements.
a) The MSC in GSM is responsible for collecting charging data.
b) NMT and AMPS mobile networks are not digital at all, where
as GSM is fully digital.
c) Frequency reuse can be better employed theoretically in GSM
than in analogue mobile networks.
d) The level of subscriber security employed in GSM is the same
as in other analogue networks.
2. The function of a VLR is:
a) To keep the database for all the subscribers of it’s PLMN.
b) To keep the database for all those subscribers which are being
served by its MSC and was downloaded from it’s own HLR.
c) Keep a backup of the subscriber data stored in it’s own PLMNs
HLR.
d) Keep the database for all those subscribers which are currently
served by its MSC.
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3. A SIM card is:
a) A credit card used to pay GSM phone bills.
b) An accessory that is absolutely necessary for a GSM mobile
phone to function.
c) A card issued by the operator to a subscriber containing his
subscription information details.
d) a card which can be omitted in cases of international roaming.
4. Which of the following Network Entities do not contain subscriber
data?
a) HLR
b) AC
c) VLR
d) BSC
5. CGI
a) Is different for all the cells in the world.
b) Is the total number of cells served by one BSC.
c) Gives information about base station and frequency.
d) Is one of the parameter in a SIM card.
a) B-C-D
b) E-F-G
c) G-H-I
d) I-J-K
6. Channel coding
a) is used for encrypting user data
b) to eliminate the problem of fading dips
c) is the voice coding mechanism used in the transcoder
d) is for error detecting and correcting purposes
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7. The basic principle of speech coding in a GSM Mobile Station is
a) A-Law PCM with 8 bits per sample
b) µ-Law PCM at 104Kbits/s
c) A-Law PCM with special filtering at 13Kbits/s
d) None of the above
8. Authentication verification is carried out in
a) HLR
b) MSC
c) VLR
d) Authentication Centre
9. Which of the following algorithms are Global, i.e., they have to be
the same in all cases?
a) A5
b) A3 and A8
c) All three have to be global
d) None of the three have to be Global, they are operator
dependent.
10. Which of the following is not included in the cost of a call?
a) Originating MSC to destination MSC, in MO-MT call
b) Use of the network resources
c) Location of the call terminating cell
d) Use of Fast Associated Logical Channel
11. Which of the following can not be true?
a) 892 MHz uplink corresponding to 937 MHz downlink
b) 950 MHz downlink corresponding to 905 MHz uplink
c) GHz uplink corresponding to 1.875 GHz down link
d) 1810 MHz downlink corresponding to 1765 MHz uplink
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12. Two IMSI numbers are required of a subscriber if he/she
a) subscribes to any two different basic services
b) subscribes to speech and data services only
c) roams to another PLMN
d) none of the above
13. Frequency Hopping
a) does not produce optimal gain if there are less than 4
Frequencies for one cell
b) may or may not be employed by an operator
c) both are correct
d) neither of them are correct
14. Which of the following is true?
a) MAP stands for Mobile Access Part
b) LAP-D protocol is used to communicate between MSC and
BSC
c) MAP is used for communication between MSC and HLR
d) BSSAP is used for communicating between BSC and MS
15. If an inter MSC handover occurs during a call, the decision to make
a handover is done by
a) BSC controlling the target cell
b) MSC controlling the target cell
c) BSC controlling the current cell
d) MSC controlling the current cell
16. Which of the following is not an advantage of the GSM network
compared to other networks which use the same frequency band?
a) Lower Carrier to Interference Ratio for signal reception
b) Use of MAP signalling
c) Frequency reuse is more efficient than in other networks
d) Lower bit rate for voice coding
234 (244)
© Nokia Telecommunications Oy
NTC CTXX 1985 en
Issue 2.0
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17. Which of the following task is not performed on the SDCCH
channel?
a) Assignment of Traffic Channel to the MS
b) Transmission of Short Messages
c) Adaptive Power Control information from BTS to MS only
d) Authentication
18. Which of the following in not true? Give reason.
a) GSM 1800 can support more traffic than GSM 900
b) Cell sizes are smaller in GSM 1800 than in GSM 900
c) Base Stations for GSM 900 and GSM 1800 are different
d) The cost of setting up a GSM 900 network is more than GSM
1800
Reason:
NTC CTXX 1985 en
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© Nokia Telecommunications Oy
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19. The transcoder changes the 64Kbits/s data rate from MSC to
16Kbits/s. This 16Kbits/s data consists of 13 kbps speech plus 3
kbps additional information. Which of the following is included in
this 3 kbps data stream?
a) Transcoder control information
b) LAP-D signalling
c) Instruction from Transcoder to SMUX about the multiplexing
method used
d) SS7 signalling information
20. What will happen if the OMC is lost?
a) Local operation and maintenance will have to be used
b) Charging data will not be transferred to billing centre by
FTAM protocol and magnetic tapes will have to be used
c) Remote sessions from MSC to HLR will not be possible
d) BTS will stop functioning
21. Which of the following is not a parameter affecting the cell size
while planning the network?
a) Antenna Height
b) MS Power
c) BTS Power
d) None of the above
22. BSSAP needs the services of SCCP to
a) analyse A subscriber data
b) To perform Connectionless signalling with the MSC
c) Send MAP messages to HLR via the MSC
d) To make a virtual connection between the MS and the MSC
23. Which of the following does not use the services of the MTP
directly?
a) TUP
b) SCCP
c) Both of them use
d) Neither of them use
236 (244)
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NTC CTXX 1985 en
Issue 2.0
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24. For a certain PLMN in a country, MCC=123, MNC=45, CC=321,
NDC=54, SN=12xxxxx, MSIN=01xxxxx. Given this information
match the numbers below on the left column, with the correct name
for each number on the right column.
NTC CTXX 1985 en
Issue 2.0
Number
Answer
1
321541234567
1=
A
IMEI
2
123450123456789
2=
B
CGI
3
1EF70A12
3=
C
MSISDN
4
321546789000
4=
D
IMSI
5
123456541234
5=
E
TMSI
6
490130204614290
6=
F
MSRN
© Nokia Telecommunications Oy
Name
237 (244)
¡Error! Estilo no definido.
9.2
Acronyms
A
An interface that GSM recommendations define
between Network Switching Subsystem and Base
Station Subsystem
A3
Authentication algorithm
A5
Encryption Algorithm
A8
Authentication Algorithm
AB
Access Burst
AC
Authentication Centre
AGCH
Access Grant Channel
ARFCN
Absolute Radio Frequency Channel Number
ARQ
Automatic Request for Retransmission
BC
Billing Centre
BCC
Base Station Colour Code
BCCH
Broadcast Control Channel
BNHO
Barring all outgoing calls except those to home
PLMN
BS
Base Station
BSC
Base Station Controller
BSIC
Base Transceiver Station Identity Code
BSIC-NCELL BSIC of an adjacent cell
238 (244)
BSS
Base Station System
BSSAP
Base Station Subsystem Application Part
BTS
Base Transceiver Station
CBCH
Cell Broadcast Channel (Not a standard logical
channel)
CC
Country Code, Call Control
CCS7, CCS#7
Common Channel Signalling System no. 7
© Nokia Telecommunications Oy
NTC CTXX 1985 en
Issue 2.0
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NTC CTXX 1985 en
Issue 2.0
CDSU
Compact Data Services Unit
CEPT
Conférence Européenne des Postes et
Télécommunications
CGI
Cell Global Identity
CI
Cell Identity
CL(N)IP
Calling Line (Number) Identification Presentation
CL(N)IR
Calling Line (Number) Identification Restriction
CSPDN
Circuit Switched Public Data Networks
DB
Dummy Burst
DCCH
Dedicated Control Channel
DCN
Data Communication Networks
DCS
Digital Cellular System (now replaced by GSM 1800)
DL
Data Link (layer)
DLCI
Data Link Connection Identifier
DLD
Data Link Discriminator
Dm
Control Channel (ISDN terminology applied to
mobile service)
DRX
Discontinuous Reception
DTAP
Direct Transfer Application Part
DTE
Data Terminal Equipment
DTMF
Dual Tone Multi-Frequency (signalling)
DTX
Discontinuous Transmission (Mechanism)
EIR
Equipment Identity Register
ETSI
European Telecommunication Standard Institute
FAC
Final Assembly Code
FACCH
Fast Associated Control Channel
FACCH/F
Full rate Fast Associated Control Channel
FACCH/H
Half rate Fast Associated Control Channel
FB
Frequency correction Burst
GMSC
Gateway Mobile Services Switching Centre
© Nokia Telecommunications Oy
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¡Error! Estilo no definido.
240 (244)
GPA
GSM PLMN Area
GSA
GSM System Area
GSM
Groupe Special Mobile/Global System for Mobile
Communications
GSM PLMN
GSM Public Land Mobile Network
HDLC
High Level Data Link Control
HLR
Home Location Register
HON
Hand Over Number
HPLMN
Home PLMN
HSN
Hopping Sequence Number
IAM
Initial Address Message (ISUP message)
IDN
Integrated Digital Networks
IMEI
International Mobile (station) Equipment Identity
IMSI
International Mobile Subscriber Identity
ISDN
Integrated Services Digital Network
ISUP
ISDN User Part
IWF
Inter Working Function
Kc
Cipher Key
Ki
Identity key (Key used to calculate SRES)
L1
Layer 1 (OSI mode layer 1)
LAC
Location Area Code
LAI
Location Area Identity
LAP-D
Link Access Protocol on the D channel
LAP-Dm
Link Access Protocol on the Dm channel
Lm
Traffic channel with capacity lower than Bm
LPLMN
Local PLMN
MAP
Mobile Application Part
MCC
Mobile Country Code
MD
Mediation Device
MM
Mobility Management, Man Machine
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NTC CTXX 1985 en
Issue 2.0
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MMI
Man Machine Interface
MNC
Mobile Network Code
MOC
Mobile Originated Call
MS
Mobile Station
MS ISDN
Mobile Station ISDN Number
MS_PWR_CLASS
MS Power Class. Parameter defining the
power class of an MS expressed in the
same way as the R parameters
MS_RANGE_MAX
Mobile Station Range Maximum
Handover criterion to determine serving
cell
MS_RXLEV_L
Lower Receiver Level. Threshold of
RXLEV received from the serving BS
below which either power control or
handover must take place to improve the
cell quality
MS_TXPWR_CONF MS
Transmitted RF Power Confirmation
Parameter sent by the MS to indicate its
current transmitted RF power level
MS_TXPWR_MAX_CCH Maximum Allowed Transmitted RF
power for MSs to Access the System until
commanded otherwise
MS_TXPWR_REQUEST MS Transmitted RF Power Request.
Parameter sent by the BSS that commands
the required MS RF Power Level.
NTC CTXX 1985 en
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MSC
Mobile-Services Switching Centre
MSCM
Mobile Station Class Mark
MSIN
Mobile Subscriber Identification Number
MSRN
Mobile Station Roaming Number
MT
Mobile Termination
MTC
Mobile Terminated Call
MTP
Message Transfer Part
MUMS
Multi User Mobile Station
NB
Normal Burst
© Nokia Telecommunications Oy
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NDC
National Destination Code
NE
Network Element
NMC
Network Management Centre
NMS
Network Management Subsystem
NSS
Network Switching Subsystem
O&M
Operations &Maintenance
OMC
Operations & Maintenance Centre
OSI
Open System Interconnection
PAD
Packet Assembler/Disassembler
PAGING GROUP
242 (244)
The set of MSs monitoring a particular paging
block
PCH
Paging Channel
PDN
Public Data Networks
PIN
Personal Identification Number
PLMN
Public Land Mobile Network
PSPDN
Public Switched Public Data Network
PSTN
Public Switched Telephone Network
QoS
Quality of Service
RAND
RANDom Number (authentication)
RFCH
Radio Frequency Channel
RFN
Reduced TDMA Frame Number
RLP
Radio Link Protocol
RXLEV
Received Signal Level.
RXLEV_ACCESS_MIN
The minimum received signal level at a
MS for access to a cell
RXLEV_MIN
The minimum received signal level at a
MS from a neighbouring cell for handover
to be permitted.
RXLEV_NCELL
Received signal level of neighbouring or
current serving cell measured on the
BCCH Carrier.
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NTC CTXX 1985 en
Issue 2.0
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RXLEV_SERVING_CELL Received signal level in the serving cell
measured on the BCCH carrier
RXQUAL
Received Signal Quality
RXQUAL-FULL
Received signal quality assessed over the
full set of TDMA Frames without a
SACCH block
RXQUAL_SERVING_CELL
cell
RXQUAL_SUB
NTC CTXX 1985 en
Issue 2.0
Received signal quality of serving
Received signal quality assessed over a
subset of 12 TDMA Frames
S/W
Software
SABME
Set Asynchronous Balanced Mode (L2 message)
SACCH
Slow Associated Control Channel
SAPI
Service Access Point Indicator
SB
Synchronisation Burst
SCCP
Signalling Connection Control Part
SCH
Synchronisation Channel
SDCCH
Stand alone Dedicated Control Channel
SIM
Subscriber Identity Module
SMSCB
Short Message Service Cell Broadcast
SN
Subscriber Number
SP
Signalling Point
SRES
Signed Response (authentication)
SS
Supplementary Service
STP
Signalling Transfer Point
TA
Terminal Adapter
TAC
Type Approval Code
TC
Transcoder
TCAP
Transaction Capabilities Application Part
TCH
Traffic Channel
TCH/EFR
An Enhanced Full Rate TCH
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244 (244)
TCH/F
A Full rate TCH
TCH/F2.4
A Full rate data TCH (<2.4Kbits/s).
TCH/F4.8
A Full rate data TCH (4.8Kbits/s)
TCH/F9.6
A Full rate data TCH (9.6Kbits/s).
TCH/H
A Half rate TCH
TCH/H4.8
A Half rate data TCH (4.8Kbits/s)
TCSM
Transcoder/Submultiplexer
TE
Terminal Equipment
TEI
Terminal Endpoint Identifier
TMN
Telecommunications Management Network
TMSI
Temporary Mobile Subscriber Identity
TRX
Transceiver
TS
Time Slot
TUP
Telephony User Part
TXPRWR
Transmit power
UI
Unnumbered Information (Frame)
VAD
Voice Activity Detection
VLR
Visitor Location Register.
VMS
Voice mail System
VPLMN
Visited Public Land Mobile Network
© Nokia Telecommunications Oy
NTC CTXX 1985 en
Issue 2.0