Figure: GSM Network Architecture.
The GSM network consists of the following components:
a. Mobile Station (MS)
b. Base Transceiver Station (BTS)
c. Base Station Controller (BSC)
d. Mobile Switching Center (MSC)
e. Home Location Register (HLR)
f. Visitor Location Register (VLR)
g. Authentication Center (AuC)
h. Equipment Identity Register (EIR)
Functions of Base Transceiver Station (BTS):
The BTS is a part of the Base Station Sub system (BSS) and it contacts Mobile Station (MS) through the radio interface. The BTS is in charge of management of simultaneous transmission and reception on the radio interface.
Functions of Base station Controller (BSC):
The BSC is connected on one side to one or several Base transceivers (BTS) and on the other side to the Mobile Switching Center (MSC). The main function of the Base Station Controller (BSC) is allocation and release of radio channels(or allocated ARFCH) and the handover (HO) management.
Functions of Mobile Switching Center (MSC)
The Mobile Switching Center (MSC) responsible for the basic function of switching. The main function of the Mobile Switching Center (MSC) is to co-ordinate the setting up of calls to and from GSM users and the external network The Mobile Switching Center (MSC) has interface with the Base Station Subsystem (BSS) on one side and the external network on the other side.
Functions of Home Location Register (HLR)
The Home Location Register (HLR) together with the Mobile Switching Center (MSC), provide the call routing and roaming capabilities of GSM. The Home Location Register (HLR) contains all the administrative information of each subscriber registered in the related to GSM network, along with the current location of the mobile.
Functions of Visitor Location Register (VLR):
The Visitor Location Register(VLR) contains roaming information. Once the visited system automatically detects the mobile, the Visitor Location Register (VLR) of that system enquires the Home location Register (HLR) to make sure you are a valid subscriber. It temporarily stores the last location area visited by the Mobile Station (MS), the power the mobile uses, the special services the Mobile Station (MS) is subscribed to and so on.
Functions of Equipment Identity Register (EIR):
The Equipment Identity Register (EIR) is a database that stores data related to the mobile equipment. It contains a list of all valid mobile equipment on the network, where each mobile station (MS) is identified by International Mobile Equipment Identity(IMEI). This is useful when searching for stolen mobile equipment or when monitoring misuse of Mobile Station (MS). If a Mobile operators want to prevent the misuse of Mobile phone; It's not too difficult for them.
Functions of Authentication Centers (AuC):
The Authentication Center is a database that stores a copy of the secret key stored in each subscriber's in Subscriber Identity Module (SIM) card, which is used for authentication and ciphering of the radio channel.
