INTRODUCTION TO GSM

GSM Standardization and Service Aspects

The GSM standard was developed by the Group Special Mobile, which was an initiative of
the Conference of European Post and Telecommunications (CEPT) administrations. The underlying
aim was to design a uniform pan-European mobile system to replace the existing incompatible
analog systems. Work on the standard was started in 1982, and the first full set of specifications
(phase I) became available in 1990. The responsibility for GSM standardization now resides with
the Special Mobile Group (SMG) under the European Telecommunication Standards Institute
(ETSI), and revisions/enhancements to various aspects of GSM standard are being carried out in
SMG technical subcommittees. The characteristics of the initial GSM standard include the following
:

 Fully digital system utilizing the 900 MHz frequency band.

 TDMA over radio carriers (200 kHz carrier spacing)
 8 full-rate or 16 half-rate TDMA channels per carrier
 user terminal authentication for fraud control
 encryption of speech and data transmissions over the radio path
 full international roaming capability
 low speed data services (up to 9.6 kb/s)
 compatibility with ISDN for supplementary services
 support of short message service (SMS)

SERVICES SUPPORTED BY GSM

GSM supports a range of basic and supplementary services, and these services are
defined in terms analog to those for ISDN (i.e, bearer services, tele services and supplementary
services). The most important service supported by GSM is telephony. Other services derived from
telephony included in the GSM specification are emergency calling and voice messaging.
Bearer services supported in GSM include various asynchronous and synchronous data
services for information transfer between GSM and other networks (i.e. PSTN, ISDN, CSPDN,
PSPDN) at rates from 300 to 9600 b/s.

Teleservices based on these bearer services include group 3 fax and short message
service (SMS). The data capabilities of GSM have now been enhanced to include high speed
circuit-switched data (HSCSD) and general packet radio service (GPRS).
 The common ISDN-like supplementary services supported by GSM include the following.

 all offering services-call forwarding

 Call restriction services-call barring
 Call waiting service
 Call hold service
 Multi party service-tele conferencing
 Calling line presentation restriction services
 Advice of charge service
 Closed user group service

Whereas the first two categories of supplementary services (call offering and call
restriction) are defined in the original GSM specification (phase 1), the remaining services were
recently added as part of enhancements to GSM (phase 2). Many of these services are similar (at
least in their description) to services currently available in fixed networks. However, because of the
mobile radio link, their implementation may impose special requirements. Mobile data
communication services provided by cellular systems are addressed in Chapter 5.

The GSM standard has been undergoing continuous extensions and enhancements to
support more services and capabilities like high speed circuit-switched data (HSCSD), general
packet radio service (GRPS), and CAMEL (customized applications for mobile network-enhanced
logic).
 GSM ARCHITECTURE

Mobile Station

The mobile station (MS) consists of the mobile equipment (the terminal) and a smart card
called the Subscriber Identity Module (SIM). The SIM provides personal mobility, so that the user
can have access to subscribed services irrespective of a specific terminal. By inserting the SIM
card into another GSM terminal, the user is able to receive calls at that terminal, make calls from
that terminal, and receive other subscribed services.

The mobile equipment is uniquely identified by the International Mobile Equipment Identity
(IMEI). The SIM card contains the International Mobile Subscriber Identity (IMSI) used to identify
the subscriber to the system, a secret key for authentication, and other information. The IMEI and
the IMSI are independent, thereby allowing personal mobility. The SIM card may be protected
against unauthorized use by a password or personal identity number.

SIM
A subscriber identity module (SIM) is required to activate and operate a GSM terminal.
The SIM may be contained within the MS, or it may be a removable unit that can be inserted by the
user. In the latter case, any GSM terminal (capable of receiving a detachable SIM card) can
become the user’s MS upon plugging in the SIM card. The international mobile subscriber identity
(IMSI) is programmed into the SIM at the time of service provisioning, along with the appropriate
security parameters and algorithms .
Base Station System (BSS)
The base station system comprises a base station controller (BSC) and one or more
subtending base transceiver stations (BTS). The BSS is responsible for all functions related to the
radio resource (CHANNEL) management. This includes the management of radio channel
configuration with respect to use as speech, data, or signalling channels, allocation and release of
channels for call setup and release; control of frequency hopping, and transmit power at the mobile
station (MS). The range of functions performed by the BSS therefore include the following.

 Radio resource control

Configuration of radio channels
Selection, allocation, and deallocation of radio channels
Monitoring of radio channel busy/idle status
Encryption of radio interface

 Frequency hopping and power control

Assignment of frequency-hop sequence and start time
Assignment of effective radiated power (ERP) values to mobile stations.

 Handoff management
Collect signal quality data from adjacent BSSs
Analyze signal quality data and determine handoff need
Keep MSC informed regarding handoff activity

 Digital signal processing

Transcoding and rate adaption
Channel coding and decoding

Table
Functions and/or Physical Elements Located in a BSC and a BTS

Base Station Controller (BSC) Base Transceiver Station (BTS)

BSC processor BTS processor
Control of BTS(s) Radio transceivers
Radio resource management Equalizers
Handoff management and control Channel coders
Transcoders
Encryption unit
Interleaving and deinterleaving
Combiners
Preselectors
Mobile Switching Center (MSC)
The mobile switching center for GSM can be viewed as a local ISDN switch with additional
capabilities to support mobility management functions like terminal registration, location updating,
and handoff. Further, unlike a local switch in a fixed network, the MSC does not contain the mobile
subscriber parameters [which are dynamic and are held in the visitor location register (VLR)
associated with the MSC]. Thus, the MSC performs the following major functions :

 Call setup, supervision, and release

 Digit collection and translation
 Call routing
 Billing information collection
 Mobility management
Registration
Location updating
Inster-BSS and inter-MSC call handoffs
 Paging and alerting
 Management of radio resources during a call
 Echo cancellation
 Manage connections to BSS, other MSCs and PSTN/ISDN
 Interrogation of appropriate registers (V/HLRs)

Home Location Register (HLR)

The HLR represents a centralized database that has the permanent datafill about the
mobile subscribers in a large service area (generally one per GSM network operator). It is
referenced using the SS7 signalling capabilities for every incoming call to the GSM network for
determining the current location of the subscriber [i.e., for obtaining the mobile station routing
number (MSRN) so that the call may be routed to the mobile station’s serving MSC]. The HLR is
kept updated with the current locations of all its mobile subscribers, including those who may have
roamed to another network operator within or outside the country. The routing information is
obtained from the serving VLR on a call-by-call basis, so that for each incoming call the HLR
queries the serving VLR for an MSRN.

Usually one HLR is deployed for each GSM network for administration of subscriber
configuration and service. Besides the up-to-date location information for each subscriber, which is
dynamic, the HLR maintains the following subscriber data on a permanent basis :
 International mobile subscriber identity (IMSI)
 Service subscription information
  Service restrictions
 Supplementary services (subscribed to)
 Mobile terminal characteristics
 Billing/accounting information

Visiting Location Register (VLR)

The VLR represents a temporary data store, and generally there is one VLR per MSC. This
register contains information about the mobile subscribers who are currently in the service area
covered by the MSC/VLR. The VLR also contains information about locally activated features, such
as call forward on busy. Thus, the temporary subscriber information resident in a VLR includes :
 Features currently activated
 Temporary mobile station identity (TMSI)
 Current location information about the MS (e.g., location area and cell identities).

Authentication Center (AC)

Generally, associated with the HLR, the authentication center contains authentication
parameters that are used on initial location registration, subsequent location updates, and on each
call setup request from the MS. In case of GSM, the AC maintains the authentication keys and
algorithms, and provides the security triplets (RAND, SRES and Kc) to the VLR so that the user
authentication and radio channel encryption procedures may be carried out within the visited
network. The authentication center for GSM contains the security modules for the authentication
keys (Ki) and the authentication and cipher key generation algorithms A3 and A8, respectively.

Equipment Identity Register (EIR)

The EIR maintains information to authenticate terminal equipment so that fraudulent, stolen,
or non-type-approved terminals can be identified and denied service. The information is in the
form of white, gray, and black lists that may be consulted by the network when it wishes to confirm
the authenticity of the terminal requesting service.