Global System for Mobile Communications

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was only supposed to produce the specifications of the next (third, 3G) generation of mobile networks. However, 3GPP also took over the maintenance and development of the GSM specification. ETSI is a partner in 3GPP.

As of 2004 more than one billion people are using GSM phones.

GSM Security

GSM was also been designed for a moderate level of security. The system is designed to authenticate the subscriber using shared secret cryptography. Communications between the subscriber and the base station can be encrypted.
GSM's original encryption algorithm has been broken, but, in principle at least, the system supports multiple algorithms so operators may replace that cipher with a stronger one.

Radio Interface

GSM employs TDMA between stations on a frequency duplex pair of radio channels, with slow frequency hopping between channels. GSM uses also SDMA and FDMA. It uses a modified Gaussian shift-key modulation.

GSM Frequency Ranges

GSM exists in four main versions, based on the band used: GSM-900, GSM-1800, GSM-850 and GSM-1900. GSM-900 (900 MHz) and GSM-1800 (1.8 GHz) are used in most of the world, excluding the United States and Canada. The United States and Canada use GSM-850 and GSM-1900 (1.9 GHz) instead, since in the U.S. the 900 and 1800 bands were already allocated. GSM-850 is also sometimes called GSM-800.
Another less common GSM version is GSM-450 (450 MHz), sometimes also called GSM-400. It uses the same frequency as and can co-exist with old analog NMT systems. NMT is a first generation (1G) mobile phone system which was in some widespread usage in Europe and other areas. Widespread is of course a relative term since the boom of mobile phones started later with GSM.
• GSM-900 uses 890-915MHz to send information and 935-960MHz to receive information.
• GSM-1800 uses 1710-1785MHz to send information and 1805-1880 to receive information.
Please note, however, that GSM-1800 is known as DCS (Digital C...