Published on Dec 12, 2016
Global system for mobile(GSM) is a second generation cellular system standard that was developed to solve the fragmentation problems of the first cellular systems in Europe's is the world's first cellular system to specify the digital modulation and network level architecture and services. Before GSM, European countries used different cellular standards throughout the continent, and it was not possible for customer to use a single subscriber unit throughout Europe. GSM's success has exceeded the expectations of virtually everyone, and it is now the world's most popular standard for new cellular radio and personal communication equipment throughout the world.A variety of data service is offered by GSM. GSM users can send and receive data, at rates up to 9600 bps. A unique feature of GSM is short message services (SMS). SMS is bidirectional service for alphanumeric (up to 160 bytes) messages. The access method chosen by GSM is combination of time and frequency division multiple access (TDMA/FDMA).
Description of GSM
The FDMA part involves the division by frequency of the (maximum) 25 MHz bandwidth of 124 carrier frequencies spaced 200KHz apart. One or more carrier assigned to each base station .each of this frequency is then divided in time, using a TDMA scheme. Eight burst periods are grouped into TDMA frames, which form the basic unit for definition of logical channels. The type of switching used in GSM network is circuit switching. GSM (group special mobile or general system for mobile communications) is the Pan-European standard for digital cellular communications. The Group Special Mobile was established in 1982 within the European Conference of Post and Telecommunication Administrations (CEPT). A Further important step in the history of GSM as a standard for a digital mobile cellular communications was the signing of a GSM Memorandum of Understanding (MoU) in 1987 in which 18 nations committed themselves to implement cellular networks based on the GSM specifications. In 1991 the first GSM based networks commenced operations. GSM provides enhanced features over older analog-based systems.
The central component of the Network Subsystem is the Mobile services Switching Center (MSC). It acts like a normal switching node of the PSTN or ISDN, and additionally provides all the functionality needed to handle a mobile subscriber, such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber. These services are provided in conjuction with several functional entities, which together form the Network Subsystem. The MSC provides the connection to the fixed networks (such as the PSTN or ISDN). Signaling between functional entities in the Network Subsystem uses Signaling System Number 7 (SS7), used for trunk signaling in ISDN and widely used in current public networks.
The Home Location Register (HLR) and Visitor Location Register (VLR), together with the MSC, provide the call-routing and roaming capabilities of GSM. The HLR contains all the administrative information of each subscriber registered in the corresponding GSM network, along with the current location of the mobile. The location of the mobile is typically in the form of the signaling address of the VLR associated with the mobile station. The actual routing procedure will be described later. There is logically one HLR per GSM network, although it may be implemented as a distributed database.
There are five classes of mobile stations defined, according to their peak transmitter power, rated at 20, 8, 5, 2, and 0.8 watts. To minimize co-channel interference and to conserve power, both the mobiles and the Base Transceiver Stations operate at the lowest power level that will maintain an acceptable signal quality. Power levels can be stepped up or down in steps of 2 dB from the peak power for the class down to a minimum of 13 dBm (20 mill watts).