Cell phones originated from the radio, and evolved into the telephone. Each city is divided up into cells that allow frequencies to be transferred. Each cell has a hexagonal shape with a tower in the center (base station), and a building that contains radio equipment. Every carrier within a city uses the same central office, the Mobile Telephone Switching Office (MTSO). A cell phone carrier is usually allowed about 832 radio frequencies to use per city. With the analog system, one cell phone uses 2 frequencies per phone call, but each cell has about 56 voice channels available which means that 56 cell phones can be in use at one time. This all increases with digital systems and makes cellular phone work more efficiently.
				When the cell phone is activated it searches for a System Identification Code (SID) on a control channel. A SID is a different 5 digit number that is assigned. The control channel is responsible for allowing the phone and base station to communicate about call set–up and channel changing. If a control channel can not be found then the phone is out of range and has “no service”. When the cell phone gets the SID, it is compared to the SID that has already been set in the phone and if they match then this means two cell phones of the same home system have been connected. Also, the cell phone gives off a registration request. The MTSO keeps a record of the phone’s location in order to know what cell it is in when in needs to ring. Once the phone rings and the cell has been determined, the MTSO chooses a frequency pair that the phone uses to take the call. Over the control channel the MTSO communicates with the cell phone to determine which frequencies to use. Then the cell phone and the tower switch with those frequencies and the call is connected. If the cell phone reaches the end of the cell the strength of the signal diminishes so the MTSO allows the 2 base stations to connect and get on a control channel to decide which frequency to switch to.