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Electronics & Communication Engineering Department
Asansol Engineering College
Kanyapur, Sen Raleigh Road, Asansol 713304, Burdwan, India

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.

    · 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.
    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.
    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.

    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.

    The GSM services in different spheres are listed as follows:

  1. Data services include computer to computer communication and packet switched traffic.

  2. telephone services which include fax services. Videotex and teletex are also supported by GSM.

  3. Mobile originated traffic and standard mobile telephony are included in teleservices supported by GSM.

  4. Different other services include:
    · call diversion
    · caller line identification
    · call wait
    · SMS services