7 - Indira Gandhi Centre for Atomic Research

to the fact that GSM network is a digital network. GSM also supports the short message
service (SMS) and Cell Broadcast Service (CBS). It supports additional services like call
forward, call barring of outgoing or incoming caller, caller identification, call waiting,
multiparty conversations etc.
5.1 Network Architecture
The layout of a GSM network is shown below: The mobile station (MS) and base station
(BS) communicates across the Um interface or radio link. The BS communicates with the
Mobile Switching Center (MSC) across the A interface.
Figure 6 : GSM Network Architecture
5.1.1 Mobile Station (MS)
It consists of the terminal (TE) and a smart card called the subscriber identity module
(SIM). The SIM provides personal mobility so that user can have access to subscribed
services irrespective of a specific terminal. The SIM card is the actual place where the
GSM network finds the telephone number of the user. Thus by inserting the SIM card into
another GSM terminal, the user is able to use the new terminal receive, make calls and use
other subscribed services while using the same telephone number.
The GSM terminal 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 structures of IMEI and IMSI are shown below :
TAC ( 3 digits) FAC (1 or 2 digit) Serial number (upto11
digits)
IMEI structure
1 spare digit
MCC (3 digits) MNC (2 digits) MSIC (upto 10 digits)
IMSI structure
The IMEI can be up to 15 digits and comprises:
• 3 digit Type Approval Code (TAC). This is given to the unit after it passes
conformance tests.
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