ation conditions, etc. These factors vary the cell size from a couple of hundred metres to a few kilometres. Due to this cell sizes are classified into four kinds in GSM networks; macro, micro, pico and umbrella, with macro cells being the biggest and pico and umbrella cells being the smallest. System Architecture A network mobile system has two major components: the fixed installed infrastructure (network) and the mobile subscribers, who use the services of the network. The fixed installed network can again be subdivided into three subnetworks: radio networks, mobile switching network and management network. These subnetworks are called subsystems. The respective three subsystems are: Base Station Subsystems (BSS); Switching and Management Subsystem (SMSS); Operation and Management Subsystems (OMSS). Radio Network - Base Station Subsystem (BSS) This comprises the Base Station Controller (BSC) and the Base Transceiver Station/Base Station (BTS/BS). The counterpart to a Mobile Station (MS) within a cellular network is the Base Transceiver Station, which is the mobile's interface to the network. A BTS is usually located in the centre of a cell. The BTS provides the radio channels for signalling and user data traffic in the cells. Besides the high frequency part (the transmitter and receiver component) it contains only a few components for signal and protocol processing. A BS has between 1 and 16 transceivers, each of which represents a separate radio frequency channel. The main tasks of the BSC include: frequency administration; control of the BTS; exchange functions. The hardware of the BSC may be located at the same site as the BTS, at its own stand-alone site, or at the site of the Mobile Switching Centre (MSC). Mobile Switching Network The Mobile Switching Subsystem (MSS) consists of Mobile Switching Centres and databases, which store the data required for routing and service provisions. The switching node of a mobile network is called the Mobile Switching Centre (MSC). It performs all the switching functions o