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Global System for Mobile Communications (GSM) Architecture

GSM is a cellular network which means that mobile phones connect to it by searching for cells in the immediate vicinity.

A type of network in which area is divided into cells, each served by its own antenna, transceiver and is allocated a band of frequencies that is dissimilar in adjacent cells to avoid distortion in signals is called a cellular network

GSM Architecture:

The basic architecture of GSM consists of

Mobile Station

Basestation Subsystem

Network Subsystem

The overall GSM architecture of GSM can be shown in the figure below:

GSM architecture

  1. Mobile Station:

Mobile station communicates across Um interface (air interface) with base station transceiver in same cell as mobile unit.A Mobile Station consists of a mobile equipment/unit and a SIM (Subscriber Identity Module).

Mobile equipment (ME)

is a physical terminal, such as a telephone or PCS device. It includes radio transceiver, digital signal processors and a portable Subscriber Identity Module (SIM) card.The SIM is responsible for storing the subscriber's identification number,contact list i.e phone book, the networks the subscriber is authorized to use, encryption keys etc.As SIM is detachable, so user is allowed to retain his or her information after switching handsets. Alternatively, the user can also change operators while retaining the handset simply by changing the SIM. but there are some operators that do not allow this.

2. Base Station Subsystem (BSS);

Base Station Subsystem consists of a Base Station Controller (BSC) and one or more Base Transceiver Station (BTS)

Each BTS defines a single cell (area) and Includes radio antenna, radio transceiver and a link to a base station controller (BSC). A GSM cell can have a radius of between 100 m and 35 km, depending on the environment.

BSC is responsible for reserving radio frequencies, manages hand off of mobile unit from one cell to another within BSS, and controls paging.

3. Network Subsystem (NSS);

NS provides a link between cellular network and public switched telecommunications networks as shown is figure above. The main functions of NS are:

  • Controls hand offs between cells in different BSSs
  • Authenticates users and validates accounts
  • Enables worldwide roaming of mobile users

Central element of NS is the Mobile Switching Center (MSC) which is supported by four databases that it controls. Their functions are briefly explained below :

  • Home Location Register (HLR) database is used to store information about each subscriber that belongs to it(i.e for which the subscriber has its telephone number associated with the switching center).

  • Visitor Location Register (VLR) database is used to determine the position of the subscriber in which the subscriber has entered. It maintains information about subscribers that are currently physically in the region covered by the switching center.
  • Authentication center (AuC) database is used for authentication activities of the system, for example, it holds authentication and encryption keys for all the subscribers in both the home and visitor location registers.
  • Equipment Identity Register database (EIR) keeps track of the type of equipment that exists at the mobile station i.e it can be IMEI number used for tracking of device. Also plays a role in security
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