GSM (Global System for Mobile communications) is the most popular standard for mobile phones in the world . In GSM signaling and speech channels are digital and data communication is easy to build into the system<br /><br />GSM is a cellular network,and mobile phones connect to it by searching for cells in the immediate vicinity.There are five different cell sizes in a GSM network-macro, micro, pico, femto and umbrella cells. The coverage area of each cell varies according to the implementation environment.<br /><br />GSM networks operate in a number of different frequency ranges (separated into GSM frequency ranges for 2G and UMTS frequency bands for 3G). Most 2G GSM networks operate in the 900 MHz or 1800 MHz bands. Most 3G GSM networks in Europe operate in the 2100 MHz frequency band.900MHz GSM uses a combination of TDMA and FDMA. It uses eight time slots, hence one carrier can support eight full rate or sixteen half rate channels. Channel separation is 200kHz with mobile transmit channels in the range 890 to 915MHz and mobile receive channels in the range 935 to 960MHz. Peak output power of the transmitters depends on the class of the mobile station and can be 0.8, 2, 5, 8, or 20 watts.<br /><br />GSM is based on digital cellular networks which have some advantages as listed below<br /><br />Greater spectrum usage efficiency compared to analogue approaches.<br /><br />Improved service quality for users in the form of improved speech quality, improved security through inbuilt encryption (there is none at present), and higher connection reliability.<br /><br />Larger number of advanced user services and easier linkage to private and public ISDN networks.