Global System for Mobile communications
GSM (Global System for Mobile communications) is cellular technology used to transmit mobile voice and data services. Originally GSM means Groupe Spécial Mobile. It uses TDMA (time division multiple access approach) to accommodate more users within the available bandwidth. In GSM, an organized group of base transceiver stations (BTS) is controlled by a base station controller (BSC) which is co-located with one of the BTSs. The BSC with its associated BTSs is called as the base station subsystem (BSS). The main switching area i.e. mobile switching centre (MSC) maintains two location registers, namely the home location register (HLR) and the visitor location register (VLR) to track the location of mobiles and enable calls to be routed to them. Authentication Centre (AuC), and the Equipment Identify Register (EIR) are used to authenticate the mobile. The detachable SIM card is the key feature of GSM, it allows the user to retain the information in the card.