GSM. Standard for public mobile radio telephony (cellular telephony) of digital type, with a transmission speed of 9.6 kbps. The frequency band assigned to this service occupies a total of 50 MHz; for transmissions from mobile terminals to fixed network infrastructures, the frequency band is between 890 MHZ and 915 MHz, for transmissions in the opposite direction between 935 MHz and 960 MHZ. The method adopted for access to the network is the TDMA (Time Division Multiple Access). The time channel (time slot) lasts about 577 microsecs and gives rise to a weft of about 4.62 msec. Each carrier supports physical channels that serve as support for logical channels. The logical channels are of two types: traffic, to transmit coded voice signals, and control and signaling, between mobile devices and fixed infrastructures. The area in which the radio coverage is secured is divided into cells, so the coverage of a geographical area is created by creating a network of cells that completely covers it; in this way two communications can use the same frequency as long as they take place in different cells that are at a certain distance from each other. Coverage is ensured for each cell by a radio base station (BTS); the mobile terminal (MS) establishes the various connections through the BTS. The GSM system is able to reassign to each mobile terminal (MS) a new BTS without interruptions for the user. The switching stations that route the calls are represented, in GSM, by MSC centers (Mobile Switching Centers) similar to the fixed telephone switching stations with the additional task of managing mobile users. The GSM network centralizes the control of the various cells in several levels: the first in the BSC (Base Station Controller) that control more BTS; another in the MSCs that control multiple BSCs; finally, the supervision is carried out by OMC (Operation and Maintenance Center) and NMC (Network Management Center). In the GSM system, the MSC switching stations interact with public fixed switching networks (PSTN – Public Switched Telephone Network) or GSM mobile networks (PLMN- Public Land Mobile Network). In addition to telephony services, GSM allows messaging services (SMS, Short Message Service) and Internet connections. Enhancements to the GSM standard are: HSCSD, GPRS and EDGE; while the evolution in development is expected with the new UMTS and LMDS standards. Several GSM services are dedicated to FS for land and on-board telephony
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