GSM standard published by the European Telecommunications Standards Institute (European Telecommunications Standard Institute ETSI). 
GSM system covers the territory, dividing it into the cell - the cell. Hence the name - cellular communication. Each cell is served by a BTS (Base Transceiver Station) - the base station transmitter, simply put the transmitter. Individual cells may overlap (overlap). When a user moves from one cell phone to another cell, the cell passes control to the next cell - the one in which the user has moved. 
What would work the whole system GSM, it is very important to keep track of the coordinates of the user. Multiple cells form the field. Network constantly updated information on the location of the user. If you need to find a user for example, all the cells are checked a certain area. 
GSM standard uses two frequencies: 
• Primary frequency - 900 MHz. Bandwidth - 25 MHz, ie, the range from 890 to 915 MHz or 935 to 960 MHz. 
• The secondary frequency - 1800 MHz. Used two bands from 1710 to 1785 and from 1805 MHz to 1880 MHz, i.e. the bandwidth three times higher than the GSM 900 (amounts to 75 MHz). 
Allocated bandwidth are divided into sections of 200 kHz is used as stations BTS, and mobile phones themselves. Theoretically, primary 124 comprises a frequency frequency. Since the two extremes are generally not used, then we are left with 122 frequency. 
One cell can use 122/9 frequency, ie 13. In practice, one takes BTS, and the rest (4 to 12) used mobile phones. 
GSM infrastructure comprises: 
1) Radio interface - is used for communication between the BTS and the mobile telephone. 
2) BTS (Base Transceiver Station) - transmitter station. 
3) BSC (Base Station Controller) - station controlling a pair of stations BTS. 
4) NSS (Network and Switching System) - a system switching (switching) calls. Each call, even within a single cell, the system is switched NSS. NSS system can transfer calls to other networks; for her work, she uses signaling SS7, which was considered when considering technology ISDN.