Global System for Mobile communication (GSM) architecture comprises of the following components −
Mobile Station : The mobile station is the mobile phone, which comprises of the mobile handset and SIM card. The mobile handset comprises of the transceiver, the display and its processor. SIM stands for SubscriberIdentity Module. It is a removable chip that contains account information of the subscriber and connects the handset to the mobile network system.
Air Interface : The air interface is the interface between the mobile station and the Base Transceiver Station. It is also called the UM interface as it is analogous to U interface of ISDN. GSM runs on a range of frequencies including 900, 1800 and 1900 MHz. It uses frequency division duplex (FDD) that divides the channel into several sub-bands. Each of the sub-bands are shared by multiple mobiles through time division multiplexing (TDM).
Base Station Subsystem : It is a connection between the mobile station and the mobile switching center. It comprises of two parts −
Base Transceiver Station (BTU) − It handles the protocols for communication with the mobile stations using radio transceivers.
Base Station Controller (BSC) − It controls the radio resources of the cell, interfaces with the mobile switching center and also handles handoff.
Mobile Switching Center (MSC) : It provides the basic network connection to the mobile network and gives access to other networks like PSTN, ISDN and the Internet. It maintains databases to locate mobiles. The databases are −
Visitor Location Register (VLR) − It is a database of nearby mobiles that are managed by a cell.
Home Location Register (HLR) − It is a database of last known location of each mobile.
International Mobile Equipment Identity (IMEI) − It is an account of all the mobiles wherein each mobile is identified by its own IMEI number.