GSM Architecture

Alina LewisMScIT-I

(Global System for Mobile Communication)

Mobile Station (MS)

MS is a combination of terminal equipment and subscriber data.

The terminal equipment as such is called the Mobile Equipment(ME)

and the subscriber data is stored in a separate module called

Subscriber Identity Module (SIM).

Therefore ME + SIM=MS

Open-Air Interface

The interface between the Mobile Station and the Base Station is

called “air-interface”.

It is open so that mobile phones of all different brands can

communicate with GSM networks from all different suppliers.

Base Station Subsystem (BSS)

The Base Station Subsystem is responsible for managing the

radio networks and is controlled by a Mobile Switching Center

(MSC).

One MSC contains several BSSs .

A BSS itself can cover a considerably large geographical area

consisting many cells.

The BSS consists of the following elements:

Base Station Controller (BSC)

Base Transceiver Station(BTS)

Transcoder

Base Transceiver Station

It is responsible for maintaining the air interface and transmission

problems.

Each base transceiver station provides radio coverage of a specific

geographical area or cell.

A plurality of base transceiver stations (BTS) are deployed at a

plurality of remote locations to provide wireless telephone coverage.

Each BTS serves a corresponding cell and when a mobile station (MS)

enters the cell, the BTS communicates with the MS.

Typical systems and elements include

a radio link to mobile stations

communication links between the base stations

a controller, typically one or more base station controllers or

centralized base station controllers (BSC/CBSC).

Base Station Controller(BSC)

The BSC is the central network element of the BSS and it controls

the radio network.

The BSC performs the following tasks:

Connection establishment between MS and the NSS.

Mobility management

Statistical raw data management

BTS and TC control

Transcoder

The digital speech signal is compressed over the air-interface.

For transmission over the air interface, the speech signal is

compressed by the mobile station to 13.5 Kbits/s or 5.6 Kbits/s.

In order to communicate with other fixed networks , that have

different compression formats, we use the Transcoder.

The Transcoder converts from one speech compression format to

another.

Network Switching Subsystem(NSS)

The NSS contains the network elements MSC, HLR, VLR,AC and EIR.

The main functions of NSS are:

Call control

Charging

Mobility management

Subscriber data handling

Mobile Services Switching Center(MSC)

MSC is responsible for controlling calls in the mobile network.

It identifies the origin and the destination of the call.

The MSC is responsible for the following important tasks:

Call control

Initiation of paging

Visitor Location Register (VLR)

VLR is the database that contains information of the subscriber

currently in the service area, such as:

Identification numbers of the subscribers

Security information for authentication of SIM card and for

ciphering

Services that the subscriber can use

VLR carries out location registration updates

VLR database is temporary

It contains the address to every subscriber’s Home Location Register

Home Location Register (HLR)

HLR maintains a permanent register of the subscribers , for

instance the subscriber’s identity numbers and the subscribed

services.

It also keeps the current record of its customers.

MSC asks for routing information from the HLR if a call is to be

set up in the mobile station (mobile terminated call). Authentication Center (AC)

AC provides security information to the network so that we can verify the SIM cards.

Equipment Identity Register (EIR)

As for AC , EIR is also used for security reasons.It is used for checking the validity of the mobile equipment.