GSM stands for Global System for Mobile Communication. It is the most successful second generation digital mobile radio system. GSM became popular very quickly because it provided improved speech quality and through a uniform international standard, made it possible to use a single telephone number and mobile unit around the world. The European Telecommunications Standardization Institute (ETSI) adopted the GSM standard in 1991, and GSM is now used in 135 countries.

The benefits of GSM include support for international roaming, distinction between user and device identification, excellent speech quality, wide range of services, interworking i.e. with ISDN, DECT etc., extensive security features etc.

GSM also stands out from other technologies with its wide range of services i.e.

• Telephony
• Asynchronous and synchronous data services (2.4/ 4.8/ 9.6 kbit/s)
• Access to packet data network (X.25)
• Telematic services (SMS, fax, videotext etc.)
• Many value added features ( call forwarding, caller ID, voice mailbox)
• Email and internet connections
• Supplementary services i.e. barring of outgoing calls, barring of incoming calls, call hold, call wait, multi party service, advice of charge etc.

The following subscriber services are supported by GSM are Dual Tone multi-frequency (DTMF), facsimile group-III, short message services, cell broadcast, voice mail, fax mail etc.

GSM main features are:

1. Modulation technique: Gaussian minimum shift keying (GMSK) modulation is used in GSM.
2. Frequency bands: GSM networks operate in different carrier frequency ranges i.e. 900 MHZ or 1800 MHZ bands
3. Multiple access techniques: TDMA and FDMA both are used in GSM.
4. Duplexing techniques: GSM uses frequency division duplexing techniques where transmitter and receiver operates at different frequencies.
5. Channel coding: GSM uses Block code and convolution code for channel coding.