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GLOBAL SYSTEM FOR MOBILE COMMUNICATION
By
PALLABI SARKAR
Electronics & Communication Engineering Department
Asansol Engineering College
Kanyapur, Sen Raleigh Road, Asansol 713304, Burdwan, India
INTRODUCTION
GSM development started in the early 1980s to replace first generation
analogue cellular technology.
The essential difference between a cellular and fixed telephony network
is that the subscriber’s terminal (the Mobile Station - MS) is not
linked by a fixed physical connection to the network.
– Connection is a radio based wireless connection
In order to support this terminal mobility, the geographic area which
the mobile network covers, is subdivided into cells.
The proposed system had to meet certain criteria
– Good subjective speech quality
– Low terminal and network equipment costs
– Support of international roaming
– Efficient use of available spectrum
Global system for mobile (GSM) is thus the second generation digital
cellular system. It is the world’s first cellular system to specify
digital modulation network level architectures and services and has
become world’s most popular 2G technology.
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GSM FEATURES
· One of the remarkable features is the Subscriber Identity Module (SIM).SIM
being a memory device stores information such as the subscribers
identification number, list of countries and networks where the
subscriber is entitled to service, privacy keys etc. A SIM consists of
four digit personal identification number to activate service from any
GSM phone. SIMs are available as smart cards that may be inserted into
GSM phone or plug-in modules which are portable and removable.
· The second feature is on-the-air privacy that the GSM system provides.
The privacy is maintained by encryption of the digital data according to
a specific secret cryptographic key that is known only to the cellular
carrier and the key is changed with time.
GSM SYSTEM ARCHITECTURE
The GSM system consists of the following three major interconnected
sub-systems which interact between themselves and with the users in the
network:
1. Base Station System(BSS)
2. Network and Switching Subsystem(NSS)
3. Operation Support Subsystem(OSS)
Description of BSS: Mobile Station (MS) is usually considered as
part of BSS. Radio transmission paths between MS and MSC is managed by
the BSS. The BSS consists of many BSCs which connect to a single MSC and
each BSC typically controls several Base Transceiver Stations (BTSs).Some
of the BTSs may be co-located at the BSC and the other may be remotely
located and connected to the BSC by
· microwave link or
· leased line.
Handoffs between two BTSs under the control of the same BSC are handled
by the BSC.This reduces the load, thus reducing the switching burden of
the MSC.
Description of OSS: The OSS helps in the operation and
maintenance of GSM and allows monitoring.
Description of NSS: The switching functions of the system is
managed by NSS. Communication with other networks like PSTN and ISDN
with the MSCs is yet another function of NSS. The NSS is responsible for
switching of GSM calls between BSCs and external networks. External
access to several customer databases is managed by NSS. The traffic
among all of the BSCs is controlled by MSC which is the core unit of NSS.
There are three databases in the NSS:-
· Home Location Register(HLR)
· Visitor Location Register(VLR)
· Authentication Centre(AUC)
Home Location Register
The subscriber information and location information is present in the
database called HLR. This is mainly for the users who reside in the same
city (service area of the corresponding MSC).
Visitor Location Register
The database temporarily storing IMSI (International Mobile Subscriber
Identity), the identity unique for each user and identifying each home
user is known as VLR. The customer information for each roaming
subscriber visiting the coverage area of that particular MSC, as a
roaming subscriber is stored in the VLR.
Authentication Centre
The authentication centre is a highly protected database handling the
authentication and encryption keys for every single subscriber in the
HLR and VLR. A resistor named Equipment Identity Resistor helps
identifying fraudulently altered phones transmitting identity data which
do not coincide with the information contained in HLR or VLR.
GSM INTERFACES
The different interfaces used in GSM listed as follows:
1. GSM radio air interface: This is the interface between MS and
BTSs.
2. Abis interface: The one connecting the BTS to a BSC is known
as Abis interface. This is responsible for carrying traffic and
maintenance data.
3. A interface: This is the interface between a BSC and a MSC.
GSM CHANNELS:
There are two types of GSM logical channels:
· Traffic Channels: These channels carry digitally encoded user speech
or data.
· Control Channels: Signaling and synchronizing commands between BS and
MS are transmitted through these channels.
GSM SERVICES:
The GSM services in different spheres are listed as follows:
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Data services include computer to computer communication and packet
switched traffic.
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telephone services which include fax services. Videotex and teletex
are also supported by GSM.
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Mobile originated traffic and standard mobile telephony are included
in teleservices supported by GSM.
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Different other services include:
· call diversion
· caller line identification
· call wait
· SMS services
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