The GSM initially born as acronym of French (Groupe Special Mobile), was subsequently transformed in (Global System for Mobile communications). The GSM is a mobile radio system.

With this system the territory is divided in the small zones said cells, each of which is served by a base radio station. While the user moves from a cell to other, the GSM system keeps the active communication with a change of frequency, called "handover".

GSM uses the "division of time" technique to transmit the voice from the station Radio Base to the end user, in this way more users syncronized on a same frequency have served in separate moments.

With the GSM digital technology, the voice is sampled and turned into numbers, allowing a smaller trouble on the signal and a greater protection of the information that are directly criptate from the mobile terminal.

The GSM system uses magnetic card called SIM (Subscriber Identity Module) to keep data and information about user.

The GSM network is organized on three levels:

• commutation system
• base stations system
• mobile terminal system

The commutation system manages the addressing of the calls, while the radio base station manages the radio transmissions.