GSM represents today's most successful digital mobile telecommunication system. Back in the early 1980s, Europe was facing the problem of many coexisting analog mobile phone system, which were often based on similar standards, but running on slightly different carrier frequencies.
To avoid this situation for a second-generation fully digital system, the group special mobile (GSM) was founded in 1982. Today, the system acquired by this group is called the Global system for mobile communication (GSM), with the specification process lying in the hands of ETSI. The primary aim of GSM was to provide your mobile for a system that enables the roaming of users through Europe and give voice services compatible with ISDN and other PSTN systems.
The specification for the initial system already covers more than 5000 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 much as possible.
GSM is a typical second-generation system, replacing the first generation analog systems, but not offering worldwide high data rates as the third generation systems, such as UMTS, are promising. GSM has initially been deployed in Europe using 890-915 MHz for the uplinks and 935-960 MHz for downlinks. This system is now also sometimes called GSM 900 to distinguish it from the later versions.
These versions include GSM at 1800 MHs, also termed digital cellular system (DCS) 1800, and the GSM system largely used in the US at 1900 MHs, also called PCS (Personal Communications Service) 1900. GSM has mainly been created for voice services and voice services still constitute the main use of GSM systems, one can anticipate that meaning future applications for mobile communications will be data-driven. Thus, the relation of data to voice traffic is shift more and more towards data.
GSM permits the integration of different voice and Data Services and the interworking with existing networks. Services make a network interesting for customers. GSM has established three different kinds of services: Bearer, Tele, and supplementary services. A mobile station MS is attached to the GSM public land mobile network through the Um interface. This network is attached to transit networks, e.g., integrated services Digital Network (ISDN) or traditional public turned telephone network (PSTN).
There might be an additional network, this source or destination network before another terminal TE is connected. Bearer services now comprise all services that enable the transparent transmission of data between the interfaces to the network, i.e., S in the case of the mobile station, and a similar interface for the other terminal. In the classical GSM model, bearer services are connection-oriented and circuit or packet-switched. These services only need the lower three layers of the ISO reference model.
GSM specifies Different mechanisms for data transmission, the original GSM allowing for data rates of up to 9600 bits for non-voice services. Bearer services permit transparent and non-transparent, synchronous, or asynchronous data transmission. Transparent bearer services only use a function of the physical layer to transmit data. Data transmission, thus, has a constant delay and throughput if no transmission errors occur.
The only tool to enhance transmission quality use of a data stream and thus serves to reproduce the original data in case of Transmission failures. Depend on the FEC, data rates of 2.4, 4.8, or 9.6 Kbits are possible. Transparent bearer services do not try to recover lost data in case of, for example, shadowing or interruptions due to handover.
GSM mainly forces on voice-oriented teleservices. These comprise encrypted voice transmission, message services, and basic data communication with terminals as known the PSTN or ISDN. However, as the main service is telephony, the primary goal of GSM what's the permission of high-quality digital voice transmission, offering at least the typical bandwidth of 3.1 kHz of the analog phone system.
A useful service for very simple message transfer is the short message service (SMS), which offers transmission of messages of up to 160 characters. SMS messages do not use standard data channels. Thus, sending and receiving of SMS is possible during data or voice transmission. SMS is typically used today for displaying Road conditions, email headers, stock quotes, etc.
In addition to tele and bearer services, GSM providers can offer supplementary Services. Similar to the ISDN network, these services offer various enhancements for the standard telephony service. The services offered may vary from provider to provider. Typical services are user identification, call redirection, or forwarding of ongoing calls. Furthermore, standard ISDN features such as closed user groups and multiparty Communication may be available. Closed user groups are of special interest for companies because they allow, for example, a company-specific GSM subnetwork, to which only members of the group have access.
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