• GSM stands for Global System for Mobile Communication. It is the most successful second generation digital mobile telephony system. GSM standard give a detailed account in words of a digital-switched network and circuit-switched network optimization for full duplex speech telephony.
• It firstly digitizes and compresses the signal or data and then sends it to the down the channel where it uses a distinct form of time division multiple access (TDMA) technology. It operates at the frequency band of either 900 MHz or 1800 MHz. It became popular very quickly because it make available for use the improved speech quality.

• GSM include the benefits that are support for international roaming, distinction between user and device identification, excellent speech quality, wide range of services, interworking with ISDN etc., extensive security features, services of SMS, fax, many value added features like call forwarding, caller ID, e-mail and internet connections. A GSM network can be divided into three groups that are mobile station (MS), base station subsystem (BSS) and network subsystem (NSS).

   

GSM CHANNELS:

• GSM network consists of two types of channels that are logical and physical channels. Several logical channels are mapped onto the physical channels. The group of logical channels depends on the application and the direction of information flow uplink or downlink or bidirectional. A logical channel can be either a traffic channel (TCH) which carries user data or a signaling channel as shown below:

Traffic Channel:

• A traffic channel (TCH) is a channel which is used to carry speech and data traffic. Traffic channels are divided into two parts that are full rate and half rate. Every time slots carries 144 bits of data where full rate channel has a transmission rate of 22.8 kbps. Traffic channels are defined using a 26-frame multi-frame or group of 26 TDMA frames.
• The length of a 26-frame multi-frame is 120 msec where the length of a burst period is defined (120 ms divided by 26 frames divided by 8 burst periods per frame). Out of the 26 frames, 24 are used for traffic. It is used for the Slow Associated Control Channel (SACCH) and 1 is currently unused as shown in figure. TCHs for the uplink and downlink are separated in time by 3 burst periods, so that the mobile station does not have to transmit and receive simultaneously.

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