1.3 Overview of the Global System for Mobile (GSM)
Communications
In the 1980s most mobile cellular systems were based on analog systems. The GSM
system can be considered as the first digital cellular system. GSM is the international
standard in Europe, Australia and much of Asia and Africa.
Matrix of Worldwide Wireless Telecommunication Standard Characteristics
Analog Cellular Telephones
Standard
Digital Cellular Telephone
IS-54/AMPS
IS-95
TACS
NMT
136
GSM
Advanced
North
Total Access
Nordic
North
Global System
Mobile
American
Communication Mobile American
for Mobile
Phone
Digital
System
Telephone Digital
Communications
Service
Cellular
Cellular
Mobile
Frequency
Range
(MHz)
Rx:869894
Tx:824849
ETACS:
Rx:916-949
Tx:871-904
NTACS:
Rx:860-870
Tx:915-925
NMT450:
Rx:463468
Tx:453458
NMT900:
Rx:935960
Tx:890915
Multiple
Access
Method
FDMA
FDMA
FDMA
TDMA/
FDM
Duplex
Method
FDD
FDD
FDD
FDD
Number of
Channels
Channel
Spacing
Modulation
832
30kHz
FM
Rx:925-960
Tx:880-915
Rx: 810826 Tx:
Rx:
940-956
1805Rx:
1880 Tx:
142917101453 Tx:
1785
14771501
CDMA/
FDM
TDMA/ FDM
TDMA/ TDMA/
FDM
FDM
FDD
FDD
Rx: 869- Rx: 869894 Tx: 894 Tx:
824-849 824-849
NMT832 (3
20(798
ETACS:1240 450: 200
users
users
NTACS:400
NMT/channel) /channel)
900: 1999
NMT450:
ETACS: 25kHz
25kHz
NTACS:
NMT12.5kHz
900:
12.5kHz
FM
FM
30kHz
DCS
1800
PDC
Personal
Digital
Cellular
1250kHz
124 (8 users
/channel)
200kHz
( /4
QPSK
GMSK (0.3
DQPSK) /OQPSK Gaussian Filter)
FDD
FDD
374 (8 1600 (3
users
users
/channel) /channel)
200kHz
25kHz
GMSK
( /4
(0.3 DQPSK)
Gaussian
Filter)
Channel
Bit Rate
1.2288
270.833 kb/s
Mb/s
Reference: Philips Semiconductors (Innovation in Wireless Communication)
n/a
1.3.1 History
n/a
n/a
48.6 kb/s
270.833
kb/s
42 kb/s
1.3.2 Architecture of the GSM network
The GSM network can be divided into four main parts:




The Mobile Station (MS).
The Base Station Subsystem (BSS).
The Network and Switching Subsystem (NSS).
The Operation and Support Subsystem (OSS).
The architecture of the GSM network is presented in figure 1.
1.3.2.1 Mobile Station
A Mobile Station consists of two main elements:


The mobile equipment or terminal.
The Subscriber Identity Module (SIM).
(a) The Terminal
There are different types of terminals distinguished principally by their power and
application:



The `fixed' terminals are the ones installed in cars. Their maximum allowed
output power is 20 W.
The GSM portable terminals can also be installed in vehicles. Their maximum
allowed output power is 8W.
The handhels terminals have experienced the biggest success thanks to thei
weight and volume, which are continuously decreasing. These terminals can
emit up to 2 W. The evolution of technologies allows to decrease the
maximum allowed power to 0.8 W.
(b) The SIM
The SIM is a smart card that identifies the terminal. By inserting the SIM card into the
terminal, the user can have access to all the subscribed services. Without the SIM
card, the terminal is not operational.
The SIM card is protected by a four-digit Personal Identification Number (PIN). In
order to identify the subscriber to the system, the SIM card contains some parameters
of the user such as its International Mobile Subscriber Identity (IMSI).
Another advantage of the SIM card is the mobility of the users. In fact, the only
element that personalizes a terminal is the SIM card. Therefore, the user can have
access to its subscribed services in any terminal using its SIM card.
1.3.2.2 The Base Station Subsystem
The BSS connects the Mobile Station and the NSS. It is in charge of the transmission
and reception. The BSS can be divided into two parts:


The Base Transceiver Station (BTS) or Base Station.
The Base Station Controller (BSC).
(a) The Base Transceiver Station
The BTS corresponds to the transceivers and antennas used in each cell of the
network. A BTS is usually placed in the center of a cell. Its transmitting power
defines the size of a cell. Each BTS has between one and sixteen transceivers
depending on the density of users in the cell.
(b)The Base Station Controller
The BSC controls a group of BTS and manages their radio ressources. A BSC is
principally in charge of handovers, frequency hopping, exchange functions and
control of the radio frequency power levels of the BTSs.
1.3.2.3 The Network and Switching Subsystem
Its main role is to manage the communications between the mobile users and other
users, such as mobile users, ISDN users, fixed telephony users, etc. It also includes
data bases needed in order to store information about the subscribers and to manage
their mobility. The different components of the NSS are described below.
(a) The Mobile services Switching Center (MSC)
It is the central component of the NSS. The MSC performs the switching functions of
the network. It also provides connection to other networks.
(b) The Gateway Mobile services Switching Center (GMSC)
A gateway is a node interconnecting two networks. The GMSC is the interface
between the mobile cellular network and the PSTN. It is in charge of routing calls
from the fixed network towards a GSM user. The GMSC is often implemented in the
same machines as the MSC.
(c) Home Location Register (HLR)
The HLR is considered as a very important database that stores information of the
suscribers belonging to the covering area of a MSC. It also stores the current location
of these subscribers and the services to which they have access. The location of the
subscriber corresponds to the SS7 address of the Visitor Location Register (VLR)
associated to the terminal.
(d) Visitor Location Register (VLR)
The VLR contains information from a subscriber's HLR necessary in order to provide
the subscribed services to visiting users. When a subscriber enters the covering area of
a new MSC, the VLR associated to this MSC will request information about the new
subscriber to its corresponding HLR. The VLR will then have enough information in
order to assure the subscribed services without needing to ask the HLR each time a
communication is established.
The VLR is always implemented together with a MSC; so the area under control of
the MSC is also the area under control of the VLR.
(e) The Authentication Center (AuC)
The AuC register is used for security purposes. It provides the parameters needed for
authentication and encryption functions. These parameters help to verify the user's
identity.
(f) The Equipment Identity Register (EIR)
The EIR is also used for security purposes. It is a register containing information
about the mobile equipments. More particularly, it contains a list of all valid
terminals. A terminal is identified by its International Mobile Equipment Identity
(IMEI). The EIR allows then to forbid calls from stolen or unauthorized terminals
(e.g, a terminal which does not respect the specifications concerning the output RF
power).
(g) The GSM Interworking Unit (GIWU)
The GIWU corresponds to an interface to various networks for data communications.
During these communications, the transmission of speech and data can be alternated.
1.3.2.4 The Operation and Support Subsystem (OSS)
The OSS is connected to the different components of the NSS and to the BSC, in
order to control and monitor the GSM system. It is also in charge of controlling the
traffic load of the BSS.
However, the increasing number of base stations, due to the development of cellular
radio networks, has provoked that some of the maintenance tasks are transfered to the
BTS. This transfer decreases considerably the costs of the maintenance of the system.
1.3.3 GSM Radio interface
Frequency allocation
Two frequency bands, of 25 Mhz each one, have been allocated for the GSM system:


The band 890-915 Mhz has been allocated for the uplink direction
(transmitting from the mobile station to the base station).
The band 935-960 Mhz has been allocated for the downlink direction
(transmitting from the base station to the mobile station).
Multiple access scheme
The multiple access scheme defines how different simultaneous communications,
between different mobile stations situated in different cells, share the GSM radio
spectrum. A mix of Frequency Division Multiple Access (FDMA) and Time Division
Multiple Access (TDMA), combined with frequency hopping, has been adopted as the
multiple access scheme for GSM.
In GSM, a 25 Mhz frequency band is divided, using a FDMA scheme, into 124 carrier
frequencies spaced one from each other by a 200 khz frequency band. Normally a 25
Mhz frequency band can provide 125 carrier frequencies but the first carrier
frequency is used as a guard band between GSM and other services working on lower
frequencies. Each carrier frequency is then divided in time using a TDMA scheme.
This scheme splits the radio channel, with a width of 200 khz, into 8 bursts. A burst is
the unit of time in a TDMA system, and it lasts approximately 0.577 ms. A TDMA
frame is formed with 8 bursts and lasts, consequently, 4.615 ms. Each of the eight
bursts, that form a TDMA frame, are then assigned to a single user.
Channel structure
A channel corresponds to the recurrence of one burst every frame. It is defined by its
frequency and the position of its corresponding burst within a TDMA frame. In GSM
there are two types of channels:


The traffic channels used to transport speech and data information.
The control channels used for network management messages and some
channel maintenance tasks.
The final choice for the GSM speech codec is a codec named RPE-LTP (Regular
Pulse Excitation Long-Term Prediction). This codec uses the information from
previous samples (this information does not change very quickly) in order to predict
the current sample. The speech signal is divided into blocks of 20 ms. These blocks
are then passed to the speech codec, which has a rate of 13 kbps, in order to obtain
blocks of 260 bits. The final choice for the GSM speech codec is a codec named RPELTP (Regular Pulse Excitation Long-Term Prediction). This codec uses the
information from previous samples (this information does not change very quickly) in
order to predict the current sample. The speech signal is divided into blocks of 20 ms.
These blocks are then passed to the speech codec, which has a rate of 13 kbps, in
order to obtain blocks of 260 bits.
GSM Phase 1 features








Call Forwarding
All Calls
No Answer
Engaged
Unreachable
Call Barring
Outgoing - Bar certain outgoing calls(e.g. ISD)
Incoming - Bar certain incoming calls (Useful if in another country)

Global roaming - Visit any other country with GSM and a roaming agreement and use
your phone and existing number* (see section on roaming)
GSM Phase 2 features










SMS - Short Message Service - Allows you to send text messages too and from
phones
Multi Party Calling - Talk to five other parties as well as yourself at the same time
Call Holding - Place a call on Hold
Call Waiting - Notifies you of another call whilst on a call
Mobile Data Services - Allows handsets to communicate with computers
Mobile Fax Service - Allows handsets to send, retrieve and receive faxes
Calling Line Identity Service - This facility allows you to see the telephone number of
the incoming caller on our handset before answering
Advice of Charge - Allows you to keep track of call costs
Cell Broadcast - Allows you to subscribe to local news channels
Mobile Terminating Fax - Another number you are issued with that receives faxes that
you can then download to the nearest fax machine.
GSM Phase 2 + features










Available by 1998
Upgrade and improvements to existing services
Majority of the upgrade concerns data transmission, including bearer services and
packet switched data at 64 kbit/s and above
DECT access to GSM
PMR/Public Access Mobile Radio (PAMR)-like capabilities
GSM in the local loop
Virtual Private Networks
Packet Radio
SIM enhancements
Premium rate services (eg Stock prices sent to your phone)
GSM 96 features

In fact, there is no such thing as GSM 96. In MoU SERG there is a document called
SE.03. In SE.03 you find the date for implementation of services. The date is 'coded'
E in case this is essential at the start of operation of a GSM network.
Services of that kind are: TS11 (basic speech), TS12 (emergency calls/112), SMS
MT, Call forwarding/Call barring services and data/fax. Then there are E96 services,
servrvices to be implemented for roamers before end 1996. The only service in this
section is ODB Phase 2. (ODB=Operator Determined Barring).
E97 is SMS MO (Short Message/Mobile Originated).
The list for E98 is longer. One reason is to put presure on suppliers. Services
included are CAMEL (to support PNP as a start), SOR, USSD, HSCSD and GPRS.
1.4 Third Generation Systems (3G)
1.4.1 Objectives and Requirements
The ITU (International Telecommunication Union) ,
headquartered in Geneva, Switzerland is an international
organization within which governments and the private sector
coordinate global telecom networks and services.
International Mobile Telecommunications-2000 (IMT-2000) is the
ITU vision of global mobile access in the 21st century.
Mobile communication has developed from first-generation analog to secondgeneration digital communications, and finally to the third-generation IMT2000. IMT-2000 will realize high-speed, wireless multimedia service providing
wider roaming services globally.
The key features of IMT-2000 are:
a) high degree of commonality of design worldwide;
b) compatibility of services within IMT-2000 and with
the fixed network;
c) high quality;
d) small terminals for worldwide use;
e) worldwide roaming capability;
f) capability for multimedia applications and a wide
range of services (e.g. video-teleconferencing, high
speed Internet, speech and high rate data).
IMT-2000 user bit rate versus coverage and mobility
IMT-2000 enables users to transmit voice, data, and even moving images. In order to
realize these services, IMT-2000 improves the data transmission speed up to 144Kbps
in a high-speed moving environment, 384Kbps in a low-speed moving environment,
and 2Mbps in a stationary environment. IMT-2000 provides services like Internet
connection, transmission of large-scale data and moving contents photographed by
digital cameras and videos, and software downloading.
IMT-2000 will use 2GHZ, a worldwide common frequency band, allocated by the
ITU. Unlike the size of bands per channel in Korea, which determine the data
transmission speed (1.25MHz for digital mobile phones and PCS) IMT-2000 has
selected 5MHz to provide multimedia services. Under the current system, maximum
data transmission speed is 64Kbps, and toward late 2000, 384Kbps will be possible in
IS-95C services. The size of IS-95C is included in IMT-2000 technology standards.
At the early stage of IMT-2000 services, an 144Kbps-transmission speed of IS-2000
levels is expected. By around 2005 when IMT-2000 is in general use, a maximum
speed of 2Mbps will be possible.
1.4.2 IMT-2000 HARMONIZATION
IMT-2000 STANDARDS DEVELOPMENT INVOLVES EXTENSIVE GLOBAL OLLABORATION
BETWEEN MANY DIFFERENT ORGANIZATIONS.
IMT-2000 is a result of the collaboration of many entities, inside the ITU (ITUR and ITU-T), and outside the ITU (3GPP, 3GPP2, etc.)
10 TERRESTRIAL RADIO TECHNOLOGY PROPOSALS, PLUS 6 SATELLITE ONES, WERE
RECEIVED IN JUNE 1998.
Radio Transmission Technology (RTT) Proposals
Proposal
Description
Environment
Indoor Pedestrian Vehicular
DECT
Digital Enhanced
Cordless
Telecommunications
UWC-136R6a
Universal Wireless
UWC-136- Communications
R6b
WIMS WCDMA
Wireless Multimedia
and Messaging
DECT
Source
Satellite
ETSI Project (EP)
DECT
UWC-136
USA TIA TR45.3
WIMS W-CDMA
USA TIA TR46.1
Services Wideband
CDMA
TD-SCDMA
China Academy of
Telecommunication
Technology (CATT)
W-CDMA
Japan ARIB
W-CDMA II
S. Korea TTA
UMTS Terrestrial
Radio Access
UTRA (W-CDMA & TDCDMA)
ETSI SMG2
NA: WCDMA
North American:
Wideband CDMA
NA:W-CDMA
USA T1P1-ATIS
CDMA2000
Wideband CDMA
(IS-95)
CDMA 2000
USA TIA TR45.5
CDMA I
Multiband
synchronous DSCDMA
W-CDMA I
S. Korea TTA
SAT-CDMA
49 LEO sats in 7
planes at 2000 km
K:SATCDMA
S. Korea TTA
SW-CDMA
Satellite wideband
CDMA
SW-CDMA
ESA
SWCTDMA
Satellite wideband
hybrid CDMA/TDMA
SWCTDMA
ESA
ICO RTT
10 MEO sats in 2
planes at 10390 km
ICO RTT
ICO Global
Communications
Horizons
Horizons satellite
system
HORIZONS
Inmarsat
TD-SCDMA
Time-Division
Synchronous CDMA
W-CDMA
Wideband CDMA
CDMA II
Asynchronous DSCDMA
UTRA
THE SLIDE SHOWS THE TERRESTRIAL PROPOSALS RECEIVED BY THE ITU AND THEIR
PROGRESSIVE CONSOLIDATION INTO THE KEY CHARACTERISTIC RECOMMENDATION IN
MARCH 1999. FURTHER HARMONIZATION IS ONGOING. IMT-2000 OFFERS
SIGNIFICANT FLEXIBILITY WITHIN A SINGLE STANDARD.
Note: The radio interfaces shown in the figure are commonly known by the following names: UTRA FDD
(WCDMA) for IMT-DS; cdma2000 for IMT-MC; UTRA TDD, and TD-SCDMA for IMT-TC; UWC-136 for
IMT-SC; and DECT for IMT-FT.
FLEXIBILITY WITHIN THE IMT-2000 STANDARD MUST, HOWEVER, BE TRANSPARENT TO
USERS, I.E. ONE MOBILE PHONE WORKS EVERYWHERE WITHOUT SIGNIFICANT
PENALTIES IN SIZE, COST OR BATTERY LIFE. SOFTWARE-DEFINED RADIO
TECHNOLOGIES CAN BE EXPECTED TO ALLOW PROGRESSIVELY INCREASING
FLEXIBILITY WITHOUT INCURRING SIGNIFICANT PENALTIES IN HAND-HELD MOBILE
UNITS.