Global System for Mobile communications (GSM) is the most widely used standard for cell phones in the world. The GSM Association promotes its use and claims that 80 percent of all mobile phones are using the GSM standard. GSM is the network these cell phones use. This means each of these phones search for a cell in the area in which they're being used. This means there must be a GSM network to handle all GSM-based phone calls.
The GSM network is a cellular network. Cellular networks are radio networks that consist of cells. Each cell is a cell site that consists of an elevated tower that contains transceivers (transmitters and receivers), signal processors, a timing receiver and electrical power sources. The GSM network refers to these towers as base stations or Base Transceiver Stations (BTS). The size of each cell refers to the coverage area it's designed for. There are five different cell sizes. The cell size, called a macro, is the size used in GSM mobile phone systems.
Subscriber Identity Module
The subscriber identity module (or SIM card) is a key feature of a GSM cell phone. The SIM contains the subscription profile and address book. Although the SIM card contains account information that prevents or enables certain services, the handset (cell phone) is the device that locks the user into a specific phone. The phone can be locked from accepting any other SIM card or multiple SIM cards.
Time division multiple access (TDMA) is the access method GSM uses for shared networks. In the GSM network, more than one cell phone user can use the same frequency channel. This can occur because the signal is divided into time slots. A cell phone user in this system uses only a small portion of the base station channel capacity. Each TDMA frame contains eight time slots. GSM uses TDMA to raise the efficiency of the network.
Using A GSM Phone
Just because a cell phone can be turned on doesn't mean it can be used to make a phone call. A GSM phone must connect to a base station via a signal. When a phone is turned on, the phone searches for a signal to connect with. GSM technology assigns a maximum range for its macro base stations of about 25 miles. In highly populated areas, base stations are grouped and even overlapped so that their phones are never without an available signal.
Sharing GSM Signals
Behind the scenes, a cell phone is in constant contact with the available base station, moving in and out of range of one and into the range of the next one. This connecting and reconnecting happens through the relationship of a GSM controller, located on the base station, and a GSM phone's inner mechanism. When a GSM cell phone moves toward a tower or mast, it checks for the strongest signal available and connects. Simultaneously, the weaker mast signal is released and becomes available for the next user.