What is GSM:
Short for Global System for Mobile Communications, GSM is a digital cellular communications system. It was developed in order to create a common European mobile telephone standard but it has been rapidly accepted worldwide.
GSM is designed to provide a comprehensive range of services and features to the users not available on analogue cellular networks and in many cases very much in advance of the old public switched telephone network (PSTN). In addition to digital transmission, GSM incorporates many advanced services and features like worldwide roaming in other GSM networks.
GSM provides recommendations, not requirements. The GSM specifications define the functions and interface requirements in detail but do not address the hardware. The reason for this is to limit the designers as little as possible but still to make it possible for the operators to buy equipment from different suppliers. The GSM network is divided into three major systems: the switching system (SS), the base station system (BSS), and the operation and support system (OSS).
Architecture of the GSM Network: 
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Base Transceiver Station (BTS)
Base Station Controller (BSC)
Home Location Register (HLR)
Mobile Services switching Center (MSC)
Gateway Mobile Services switching Center (GMSC)
Visitor Location Register (VLR)
Authentication Center (AUC)
Equipment Identity Register (EIR)
Mobile Station:
The Mobile Station (MS) represents the only equipment the GSM user ever sees from the whole system. It actually consists of two distinct entities. The actual hardware is the Mobile Equipment (ME), which is anonymous and consists of the physical equipment, such as the radio transceiver, display and digital signal processors.
Base Station Subsystem:
The Base Station Subsystem is composed of two parts, the Base Transceiver Station (BTS) and the Base Station Controller (BSC). The BTS houses the radio transceivers that define a cell and transmits and receives signals on the cells' allocated frequencies with the mobile station.
Network Subsystem:
The central component of the Network Subsystem is the Mobile services Switching Center (MSC). It acts like a normal switching node of the normal telephones of the land lines and in addition provides all the functionality needed to handle a mobile subscriber, including registration, authentication, location updating and inter-MSC handovers.
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