1. (Global System for Mobile Communication)
GSM Architecture
(Global System for Mobile Communication)
Alina Lewis
MScIT-I

3. 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

4. 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.

5. 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

6. 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).

7. 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

8. 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 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.

9. 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

10. 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

11. 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

12. 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.