GSM is the most successful digital mobile telecommunication system in the world today. It is used by over 800 million people in more than 190 countries.

In the early 1980s, Europe had numerous coexisting analog mobile phone systems, which were often based on similar standards (e.g., NMT 450), but ran on slightly different carrier frequencies. To avoid this situation for a second genera-
tion fully digital system, the groupe spéciale mobile (GSM) was founded in 1982. This system was soon named the global system for mobile communications (GSM), with the specification process lying in the hands of ETSI (ETSI, 2002), (GSM Association, 2002). In the context of UMTS and the creation of 3GPP (Third generation partnership project, 3GPP, 2002a) the whole development process of GSM was transferred to 3GPP and further development is combined with 3G development. 3GPP assigned new numbers to all GSM standards.

The primary goal of GSM was to provide a mobile phone system that allows users to roam throughout Europe and provides voice services compatible to ISDN and other PSTN systems. The specification for the initial system already covers more than 5,000 pages; new services, in particular data services, now add even more specification details. Readers familiar with the ISDN reference model will recognize many similar acronyms, reference points, and interfaces. GSM standardization aims at adopting as much as possible. GSM is a typical second generation system, replacing the first generation analog systems, but not offering the high worldwide data rates that the third generation systems, such as UMTS, are promising. GSM has initially been deployed in Europe using 890–915 MHz for uplinks and 935–960 MHz for downlinks – this system is now also called GSM 900 to distinguish it from the later versions. These versions comprise GSM at 1800 MHz (1710–1785 MHz uplink, 1805–1880 MHz downlink), also called DCS (digital cellular system) 1800, and the GSM system mainly used in the US at 1900 MHz (1850–1910 MHz uplink, 1930–1990 MHz
downlink), also called PCS (personal communications service) 1900. Two more versions of GSM exist. GSM 400 is a proposal to deploy GSM at 450.4–457.6/478.8–486 MHz for uplinks and 460.4–467.6/488.8–496 MHz for
downlinks. This system could replace analog systems in sparsely populated areas. A GSM system that has been introduced in several European countries for railroad systems is GSM-Rail (GSM-R, 2002), (ETSI, 2002). This system does not only use separate frequencies but offers many additional services which are unavailable using the public GSM system. GSM-R offers 19 exclusive channels for railroad operators for voice and data traffic. Special features of this system are, e.g., emergency calls with acknowledgements, voice group call service (VGCS), voice broadcast service (VBS). These so-called advanced speech call items (ASCI) resemble features typically available in trunked radio systems only. Calls are prioritized: high priority calls pre-empt low priority calls. Calls have very short set-up times: emergency calls less than 2 s, group calls less than 5 s. Calls can be directed for example, to all users at a certain location, all users with a certain function, or all users within a certain number space. However, the most sophisticated use of GSM-R is the control of trains, switches, gates, and signals. Trains going not faster than 160 km/h can control all gates, switches, and signals themselves. If the train goes faster than 160 km/h (many trains are already capable of going faster than 300 km/h) GSM-R can still be used to maintain control.

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