An Overview of GSM
•
•
•
•
Introduction of GSM
GSM Spectrum
Mobile Station
GSM Sub Systems
NSS
BSS
NMS
• GSM Interfaces
• Channel Concepts
GSM:A brief background
• At the beginning of the 1980s a problem was
that the European countries were using many
different, incompatible mobile phone systems.
These systems are referred to as 1G (first
generation) systems.
• In Europe, the most common 1G system was
NMT (Nordic Mobile Telephone) and TACS
(Total Access Communications System). In the
United States, as well as in other American
countries, AMPS (Advanced Mobile Phone
System) was, and still is, a widely established
system.
GSM:A brief background
• With the passage of time, the need for telecom
services was 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.
• GSM is a 2G (second generation) system.
Expectations From GSM
System
• The GSM system must be an open system.
• The system should be able to support several
network operators in each country.
• GSM networks must be built without causing
any major changes to the already existing
Public
Switched
Telephone
Networks
(PSTN).
Expectations From GSM
System
• 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 ISDN
and other data communication specifications.
• The system must maintain good security both
for subscriber and transmitted information.
Advantages Of GSM
• GSM uses radio frequencies efficiently, and due to the
digital radio path, the system tolerates more inter - cell
disturbances.
• The average speech quality is better than in analogue
systems.
• Data transmission is supported throughout the GSM
system.
• Speech is encrypted and subscriber information
security is guaranteed.
• With ISDN compatibility, new services are offered.
• International roaming is technically possible within all
countries using the GSM system.
• The large market increases competition and lowers the
Requirement Of Telecom Network
• 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 the location.
 Routing the call.
 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
Requirement Of Mobile Telecom Network
• 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
network's service area.
• In practice, the network has to find solutions
to three problems before it can even set up
a call.
Requirement Of Mobile Telecom Network
Requirement Of Mobile Telecom Network
• In other words, the subscriber has to be
LOCATED and IDENTIFIED to provide
him/her with the requested services.
• In order to understand how we are able to
serve the subscribers, it is necessary to
identify
the
main
interfaces,
the
subsystems and network elements in the
GSM network, as well as their functions.
GSM
Uplink & Downlink
• Frequency Bands
GSM 900 Mhz
Uplink
890-915 Mhz
Down Link 935-960 Mhz
B
T
S
890Mhz
915Mhz 935Mhz
960Mhz
UP Link
Down Link
......
......
890 890.2
914.8 915
124 carriers
935 935.2
959.8 960
124 carriers
GSM Specifications
RF Spectrum :
GSM 900
Mobile to BS
(UP-LINK)
-
BS to Mobile
MHz
(DOWN -LINK) -
890 to 915 MHz
935 to 960
Bandwidth - 25 MHz
GSM 1800
Mobile to Cell (UP-LINK)
1710 to 1785 MHz
Cell to Mobile (DOWN -LINK) - 1805 to 1880 MHz
Bandwidth - 75 MHz
GSM - MULTIPLE ACCESS
• Each RF carrier 200khz apart
• Total 124 RF Channels available.
One or more carrier assigned to each base station
1
2
3
4
5
6
124
……...
Freq
890.2
890.4 890.6 890.8 891.0
914.8
Mhz.
• Absolute Radio Freq Carrier Number (ARFCN) 1 and 124 not used
until it is co-ordinated with Non -GSM operators in adjacent freq. bands.
• Thus for practical purposes only 122 RF Carriers are available.
• Frequency for any ARFCN ( n) can be calculated from :
F up-link (n) = 890.2 +0.2* ( n-1 ) MHz
F down-link (n) = 935.2 +0.2* ( n-1 ) MHz
Here 1 ≤ n ≤ 124.
GSM
FDMA
890
915
935
960
25 MHz
1
0
25 MHz
2
Mobile to Base
890.2
890.4 890.6
200 kHz
45MHz
1
0
2
Base to Mobile
(MHz)
935.2
935.4
935.6
200 kHz
Channel layout and frequency bands of operation
GSM
TDMA
Amplitude
45 MHz
7 8
5
2
3
6
5
4
2
1
F1
(Cell Rx)
7 8
3
6
4
1
F2
F1’
F2’
(Cell transmit)
Typical TDMA/ FDMA frame structure
Frequency
TDMA Frame
Physical Channel
(0.577ms)
The TDMA frame time slot of 0.577ms is known
as physical channel while the contents of physical
channel according to their roles are called as
logical channel
Mobile Station (MS)
• In GSM, the mobile phone is called Mobile
Station (MS).
• The MS 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.
MS : SIM
SIM Contains:
• Identification numbers of the user (IMSI).
• Tools needed for authentication and ciphering.
• Storage space for messages, such as phone
numbers.
• List of available networks
MS : ME
Subsystems in GSM
The GSM network is called Public Land
Mobile Network (PLMN). It is organised in
three subsystems:
– Base Station Subsystem (BSS)
– Network Switching Subsystem (NSS)
– Network Management Subsystem (NMS)
Subsystems in GSM
Function of NSS
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, and the time and type of the transaction, and
transfers it to the Billing Centre.
• Mobility management :This maintains information about
the location of the subscriber.
• Signalling : 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
Network Switching Subsystem (NSS)
NSS Elements
• Mobile services Switching Centre (MSC)
• Gateway Mobile services Switching Centre
(GMSC)
• Visitor Location Register (VLR)
• Home Location Register (HLR)
• Authentication Centre (AC)
• Equipment Identity Register (EIR)
NSS Element : MSC
Functions of MSC:
Call control : identification of the type of call,
the destination, and the origin of a call. It also
sets up, supervises, and clears connections.
Initiation of paging : Paging is the process of
locating a particular mobile station in case of a
mobile terminated call (a call to a mobile
station).
Charging data collection : The MSC generates
CDRs, Charging Data Records, which contain
information about the subscribers’ usage of the
network.
NSS Element : GMSC
• The GMSC is responsible for the same tasks as the MSC,
except for paging. It is needed in case of mobile terminated
calls.
• In GSM, the MSC, which is serving the MS may vary due
to subscriber’s mobility. Therefore, in a mobile terminated
call, the call is set up to a well defined exchange in the
subscriber’s home PLMN. This exchange is called GMSC.
• The GMSC than interacts with a database called Home
Location Register, which holds the information about the
MSC, which is currently serving the MS. The process of
requesting location information from the HLR is called
HLR Interrogation.
NSS Element : VLR
VLR is a temporary database which contains
information about subscribers currently being
in the service area of the MSC/VLR, such as:
– Identification numbers of the subscribers.
– Security information for authentication of
the SIM card and for ciphering.
– Services that the subscriber can use.
NSS Element : VLR
• The VLR carries out location registrations and
updates. When a mobile station comes to a new
MSC/VLR serving area, it must register itself in
the VLR, in other words perform a location
update.
• A mobile subscriber must always be registered in
a VLR in order to use the services of the network.
• The VLR database is temporary, in the sense that
the data is held as long as the subscriber is within
its service area. It also contains the address to
every subscriber's Home Location Register.
NSS Element : HLR
HLR maintains a permanent database of the
subscribers. The following date related to the
subscriber can be found here:
• Subscriber identification number (IMSI)
• The subscribed services.
• The current location of its subscriber in terms of
VLR address.
NSS Element : AC
• The Authentication Centre provides security
information to the network, so that the mobile
network can verify the SIM cards (authentication
between the MS and the VLR) and cipher the
information transmitted in the air interface
(between the MS and the BTS).
• The Authentication Centre supports the VLR's
work by issuing so-called authentication triplets
upon request (RAND, SRES, and Kc).
NSS Element : EIR
• The Equipment Identity Register is used for
security reasons.
• EIR is responsible for IMEI checking (checking
the validity of the mobile equipment).
• When this optional network element is in use, the
mobile station is requested to provide the
International Mobile Equipment Identity (IMEI)
number. The EIR contains three lists:
- White list
- Grey list.
- Black list.
Base Station Subsystem (BSS)
The BSS is responsible for managing the radio
network, and it 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). The
BSS consists of the following network elements:
• BSC (Base Station Controller )
• BTS (Base Transceiver Station )
• TRAU (Transcoder and Rate Adaptation Unit
referred to as TC (Transcoder))
Base Station Subsystem (BSS)
Function of BSS
• Radio Path Control: BSS takes care of radio
resources, that is, radio channel allocation and quality of
the radio connection.
• Synchronisation : 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.
• Air- and A-interface signalling : In order to establish a
call, the MS must have a connection through he the
BSS.
• Connection establishment between the MS and the
NSS.
• Mobility Management and Speech Transcoding :
BSS mobility management mainly covers the different
BSS Element : BSC
Functions of BSC:
• Connection establishment between the MS and the
NSS: All calls to and from the MS are connected
through the switching functionality of the BSC.
• Mobility management : Keeps track of MS and
responsible for initiating the vast majority of all
handovers. It makes the handover decision based on
measurement reports sent by the MS during a call.
• Statistical raw data collection: Information from the
BTS, Transcoders, and BSC are collected in the BSC
and forwarded via the DCN (Data Communications
Network) to the NMS (Network Management
Subsystem), where they are post-processed into
statistical views, from which the network quality and
status is obtained.
BSS Element : BSC
Functions of BSC:
• Air- and A-interface signalling support: In the AInterface, SS#7 is used as the signalling language, while
the environment in the Air-Interface allows the usage of a
protocol adapted from ISDN standards, namely LAPDm
(Link Access Protocol on the ISDN D Channel, modified
version). Between the BTS and the BSC (Abis Interface), a
more standardised LAPD protocol is used. The BSC also
enables the transparent signalling connection needed
between the MSC/VLR and the MS.
• BTS and TRAU control : Inside the BSS, all the BTSs
and TCs are connected to the BSC(s). The BSC maintains
the BTSs and is capable of separating (barring) a BTS
from the network and collecting alarm information. TRAUs
are also maintained by the BSC.
BSS Element : BTS
BTS is responsible for maintaining the Air-Interface and
minimising the transmission problems. BTS functions are as
follows:
• Air interface signalling
• Ciphering: Both the BTS and the MS must be able to cipher
and decipher information in order to protect the transmitted
speech and data in the air interface.
• Speech processing : All functions performed by BTS in
order to guarantee an error-free connection between the MS
and the BTS. This includes:
– Speech Coding: D/A conversion in the downlink direction
and vice-versa.
– Channel Coding: Used for error protection against fading
dips.
– Interleaving: Spreading the coded speach in many Bursts
to enable a secure transmission.
– Burst Formatting: Adding information to the coded
Speech Processing by BTS
BSS Element : BTS
• The BTS can contain several TRXs
(Transceivers), each supporting one pair of
frequencies for transmitting and receiving
information.
• The BTS also has one or more antennas, which
are capable of transmitting and receiving
information to/from one or more TRXs. The
antennas are either omnidirectional or
sectorised.
• BTS also has control functions for O & M,
Omnidirectional & Sectorised BTS
BSS Element : TRAU
TRAU performs two main functions:
• Conversion from one speech compression
format to another.
• DTX (Discontinuous transmission) which is
used during a call when there is nothing to
transmit (no conversation). It is activated in
order to reduce interference and to save MS
battery.
BSS Element : TRAU
BSS Element : TRAU
• In GSM 900/1800 specifications, bit rate in AirInterface is 13 Kbps (Full rate) and bit rate in the MSC
and PSTN interface is 64 Kbps. Transcoder converts 13
Kbps to 64 Kbps and vice-versa. TC belongs to BTS but
it is kept near MSC to save the transmission media. 64
Kbps from MSC is converted into 16 Kbps(13 Kbps for
speech + 3 Kbps for signaling) after TC and
Submultiplexure puts 4 such 16 Kbps streams on 64
Kbps standard PCM channel to be carried towards BSC.
Thus 4 PCMs from MSC side goes on single PCM
towards BSC after TCSM unit. Due to this
TCSM(TRAU) is kept near MSC.
Network Management Subsystem
(NMS)
• The purpose of the NMS is to monitor various
functions and elements of the network.
• 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.
NMS : 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.
• provides the network operator with information
about the current status of alarms, 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.
NMS : 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.
NMS : 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.
GSM Interfaces
• One of the main purposes behind the GSM
specifications is to define several open interfaces.
Due to this operator maintaining the network may
obtain different parts of the network from different
GSM network suppliers.
• The GSM specifications define two truly open
interfaces within the GSM network. The first one
is between the Mobile Station (MS) and the Base
Station (BS). This open-air interface is called Um.
• The second interface is located between the
Mobile services Switching Centre, MSC and the
Base Station Controller (BSC). This interface is
called the “A-interface”.
MSC/VLR Interfaces
• The MSC/VLR makes use of the signaling
protocols to exchange signaling information with
the other PLMN network nodes. The purpose of
this inter-working is to fulfill the MSC/VLR
function when serving a mobile subscriber.
• The GSM Recommendation 03.02 defines the
MSC/VLR interfaces with the other network
nodes.
A-interface and B-Interface
• The A-interface is the interface between the MSC/VLR
and the base station system (BSS). This interface
contains traffic channels for speech and data traffic, and
signaling channels on which the base station system
applications part (BSSAP) is implemented for the
signaling inter-working.
• The B-interface is the interface between the MSC and
the VLR. Since the MSC and the VLR are integrated in
one functional unit, this interfaces is partly reduced to a
procedural interface. The B-interface is used when the
MSC required VLR data.
C-interface
• The C-interface is the interface between the
MSC and the HLR/AC. On this interface
the
MSC makes use of the mobile
application part (MAP) for the signaling
inter-working related to the following
MSC/VLR functions:
 Interrogation
 Short Message Service.
D-interface
The D-interface is between the VLR and the
HLR/AC. On this interface, the VLR makes use
of MAP for signaling inter-working, related to the
following MSC/VLR functions:
 Authentication
 Location update
 Retrieval of subscriber data during call setup
 Supplementary services
 Restoration of location registers.
E-interface
• The E-interface is the interface between two
MSCs when signaling inter-working is
accomplished by means of MAP. This MAP
interface is used to support the following
functions:
 Handover
 Short message service
 Call controlling tasks after inter-MSC
handover.
F-interface and G-interface
• The F-interface is between the MSC and the EIR.
On this interface, the MSC makes use of MAP for
the signaling inter-working, related to the
identification of the mobile station equipment
(IMEI) check.
• The G-interface is between the VLR and the
previous VLR (PVLR). On this interface, the VLR
makes use of MAP for signaling inter-working,
related to the following MSC/VLR function:
 Location update
 Authentication.
I-interface
• The I-interface is the interface between the MSC and
another exchange (e.g. another MSC) when they are
directly connected to trunks. On this interface, the
MSC makes use of the ISDN user part (ISUP) for
signaling inter-working related to the following
MSC/VLR functions:
• MOC and MTC setup
 Connection establishment between two MSCs
in the case of inter-MSC handover
 Call clearing.
OMC Interface-I
• The MSC/VLR-OMC interface used between the
MSC/VLR and a centralized operation and
maintenance center (OMC) is used for the
MSC/VLR administration by means of Man
Machine Interface (MMI) commands and the
transfer of traffic measurement data.
OMC Interface-II
On this interface, the MSC/VLR makes use of the
common management information service element
(CMISE) and the file transfer access and management
(FTAM) for signaling inter-working and file transfer,
related to the following MSC/VLR functions:
 Alarm reporting
 Administration
 Transfer of charging data & hot-operation data
 Transfer of traffic measurement data
 Transfer of security records
GSM Interfaces
Um
A
Abis
Ater
B
C
D
E
F
G
:
:
:
:
:
:
:
:
:
:
MS – BTS
MSC – BSC
BSC – BTS
BSC – TRAU
MSC – VLR
MSC – HLR
HLR – VLR
MSC – MSC
MSC – EIR
VLR - VLR.
(air or radio interface)
(proprietary interface)
(proprietary interface)
GSM Interfaces
GSM Channel Concepts
Physical Channel
Logical Channel
Physical Channel
 Each timeslot in TDMA frame is called a physical
channel.
 There are 8 physical channels per carrier in GSM.
 Physical channels may be used to carry speech data
or signaling information.
Logical Channel
 The Information carried by physical channels is
known as logical channel.
 There are a several type of logical channels
available in GSM.
GSM
LOGICAL CHANNELS
USER INFORMATION( TRAFFIC)
SIGNALLING INFORMATION (CONTROL)
Logical Channels
Logical
Channels
Dedicated
Channels
Common
Channels
Broadcast
Channels
FCCH
SCH
BCCH
Common
Control
Channels
RACH
PCH
AGCH
Dedicated
Control
Channels
SDCCH
SACCH
FACCH
Traffic
Channels
TCH (Full-Rate)
TCH (Half-Rate)
TCH (EFR)
Common channels are used to broadcast different
information to MS & for setting up signaling
channels between the MSC/VLR & the MS
Common
Channels
Broadcast
Channels
Common
Control
Channels
Broadcast Channels
FCCH (Frequency Correction Channel) – Provides the
frequency correction information used by the mobile
station.
SCH (Synchronization Channel) – Contains the Base
Station Identity Code (BSIC) and the TDMA frame
number used for synchronization of the mobile station to
the frame structure of a new BTS.
BCCH (Broadcast Control Channel) – Used to broadcast
general information, related to network, to all mobile
stations.
Broadcast Channels…..
FCCH:-
This channel do not contain any
information, this have a stream of 142
zeros.
So, the frequency that have this
sequence helps MS to identify the BCCH
frequency among all transmitted
frequency
Broadcast Channels…..
FCCH:Down link
F1
F2
B
T
S
F3
As F3 have 142 zeros
at starting so this
contain BCCH
information
Broadcast Channels….
SCH(Synchronization Channel):- BTS sends
TDMA frame number on SCH by which MS
synchronize it self with GSM system.
Down link
SCH
TDMA Frame Number
BSIC
BSIC:-N/W color cod & Base Station color cod
Broadcast Channels….
Broadcast Control Channel(BCCH):1. System Type 900/1800
Down link
2. LAI
3. Neighbor Cell BCCH Information
4. Frequency hopping used or not?
5. Type of Training sequence used
6. Max power allowed in cell
Broadcast Channels….
BCCH….
7. Paging Group
8. Max Timing advance allowed
Common Control Channels (CCCH)
PCH (Paging Channel) – Used to page the mobile station. PCH
information is transmitted over the downlink It contain IMSI or
TMSI.
RACH (Random Access Channel ) – Used by a mobile station to
Request access to the system. RACH information is transmitted
over the uplink.
AGCH (Access Grant Channel) – Used to assign a SDCCH for
call setup. AGCH information is transmitted over the downlink.
Common Control Channels (CCCH)…
Incoming call for
IMSI/TMSI no:XXXXX, Where
are you?
PCH:Down link
PCH
PCH
PCH
Common Control Channels (CCCH)….
RACH:It used when MS want to uplink first time
1. Used for Emergency Call
2. Answer to paging
3. Initiate O/G call
4. Location update request
Common Control Channels (CCCH)….
AGCH (Access Grant Channel) – Used to
assign a SDCCH for call setup. AGCH
information is transmitted over the downlink
Down link
It show which SDCCH is allotted to MS
Dedicated Control Channels (DCCH)
SDCCH (Stand alone Dedicated Control Channel) – Carries
signaling information during call setup
SACCH (Slow Associated Control Channel) – Transmits call
Control data and measurement reports during the call.
FACCH (Fast Associated Control Channel) – Carries urgent
Signalling information for handover etc.
.
Dedicated Control Channels (DCCH)….
SDCCH (Stand alone Dedicated Control Channel)
1. Call Set up
Both way
2. Authentication
3. Transmission of SMS
4. Location Update
Question
Which channel allocate TMSI?
SDCCH
Question
What is Timing alignment?
How BTS Inform MS about
timing alignment
GSM – RF Interface
Timing Advance : ( To counteract problem of Time Alignment )
- MS instructed to do its transmission certain bit-times earlier
or later – to reach its timeslot at BTS in right time.
- In GSM systems maximum 63 bit-times can be
used.
Start
- This limits the GSM cell size to 35 Km radius. Sending
0 1 2 3 4 5 6 7
Time
0 1 2 3 4 5 6 7
Dedicated Control Channels (DCCH)….
SACCH:- Both way
Uplink:-Transmit Measurement Report
Down Link:- Power To use
TA to use
FACCH:-It is mapped onto a TCH &
replace 20ms of speech
Both way
GSM
THREE TYPES OF CONTROL CHANNELS
Broadcast control channel BCCH
P- MP
For Freq Correction
For Syncronisation
Common control channel CCCH
For ACCESS Management
Dedicated control channel DCCH
P- P
For Registration
,authentication
& Handover
FCCH
SCH
BCCH
PCH
RACH
AGCH
SDCCH
SACCH
FACCH
OPERATIONAL CONCEPTS
IDLE MODE
ACCESS PROCDURE DEDICATED MODE
• IDLE MODE
----FCCH
---- SCH
----BCCH
• MS O/G Call
----RACH
----AGCH
----SDCCH
----TCH
• MS I/C Call
----PCH
----RACH
----AGCH
----SDCCH
----TCH
Common channels are generally transmitted
on TS0 of BCCH carrier frequency
Down Link
F S B B B B C0 C0 C0 C0
F S C1 C1 C1 C1 C2 C2 C2 C2
F S C3 C3 C3 C3 C4 C4 C4 C4
F S C5 C5 C5 C5 C6 C6 C6 C6
F S C7 C7 C7 C7 C8 C8 C8 C8
I
Question
Which information is transmitted
on uplink direction of TS0 of
BCCH carrier frequency?
RACH
0
1
2
3
4
5
6
7
Traffic Information
Broadcast/Control Information
Question
If MS is handling calls then how it
can receive SMS
TCH Structure:TTTTTTTTTTTTATTTTTTTTTTTTI
0
12
It get SMS on SACCH as it
comes after every 12 TCH
25
I am XXX my LAI is
ZZZ….