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Architecture
A GSM based PLMN (public land mobile network) can broadly be broken up into 3 parts:-
The Mobile Station
The MS is the subscriber end of the network and consists of the GSM device and the SIM (Subscriber identity module). each mobile device has a globally unique IMEI (international mobile Equipment identity) and each SIM has an IMSI (international mobile subscriber identity), which is used to identify and authorize the subscriber for various services. The SIM card may be protected against unauthorized number using a PIN (Personal Identity Number).
Base Station Subsystem (BSS)/ Base Station Controller (BSC)
Mobile networks consist of may BTS (Base Transceiver Stations), each of which form a “cell” covering a certain geographical area. The BTSs are the ones responsible for directly communicating with the mobile devices. Several BTSs are controlled together by a BSC (Base Station Controller). The provides connectivity to the network subsystem.
Network subsystem/ Mobile Station Switching Office (MTSO)
The MTSO (Mobile Station Switching Office) is the central component of the network subsystem. It communication with all the BSCs and is responsible for providing all the functionalities to mobile devices like, registration, authentication, call routing etc. A network also, usually, has GMSC (Gateway MSCs) to communicate with external (landline) networks like ISDN, PSTN etc.
Many registers are also maintained which contain information necessary for smooth functioning of the network. The HLR (Home Location Register) stores information about the current location of all subscribers of the network. This information is necessary for routing all calls/messages their intended destinations. A VLR (Visitor Location Register) covers one or more cells and stores information about the subscribers currently under its area of influence. The EIR (Equipment Identity Register is used to authenticate and store equipment, rather than subscriber data, while the AUC or AC (Authentication Center) stores and validates the user’s PIN. The MSC communicates with these registers whenever it needs some information.
