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The second generation mobile network is a digital system and has many advantages over its analogue counterpart. It provides capacity gains by allowing one frequency to be used by many users either through code multiplexing or time division. It also supports text messaging and improves security. Due to the absence of a standard specification governing both first and second generations, about four systems (technologies) were developed for the 2G systems. These are: Digital Advanced Mobile Systems (D-AMPS); Global System for Mobile Communications (GSM); Code Division Multiple Access (CDMA); and Personal Digital Cellular (PDC). Among these, GSM is by far the most successful and widely used 2G systems (Korhonen, 2003) as it provides terminal mobility and user seamless roaming between GSM networks. It uses a Subscriber Identity Module (SIM) card which has the subscriber identity and helps both the network and mobile to identify each other and authenticate conversations. The GSM which is a second generation mobile system uses the TDMA/FDMA multiple access techniques with an FDD duplexing method. It has a spectrum allocation of 850 – 915MHz in the uplink and 935 – 960MHz in the downlink with a bandwidth of 200 KHz for each physical channel. Total number of channel available for each direction is 124 and the number of user per channel is 8 with a data rate of 270.833Kb/s, and a bit period of 3.693μs. The time division multiple access TDMA has a frame size of 4.615ms, the number of slots per frame is 8 and the slot duration is 0.576923ms. For its operation, the GSM uses the 0.3GMSK modulation technique with a speech coding of 13Kb/s Regular Pulse Excitation with Long Term Predictor (RPE-LPT) and interleaver period of 40ms maximum, using two consecutive 20ms blocks of data. The user of GSM has a data transfer capability of short messaging service, circuit-switched data and GPRS for packet data.
The GSM architecture can be grouped into three major parts; the Mobile Station (MS), Base Station Subsystem (BSS) and the Mobile Subscription Centre (MSC).
The mobile station is the mobile user and can be recognised by the BTS through the SIM card used. The MS is connected to the BTS (Base Transceiver Station) which consist of radio transmitter and receivers via an air interface. The BTS is connected to the Base Station Controller (BSC) via an A-Interface. The BSC performs radio control function. The BSC is connected to MSC also via an A-interface. The MSC is the heart of GSM network because it controls call by creating route to and from PSTNs and switching controls during handover. It performs its duties with the help of the VLR, HLR, EIR and OMC

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