A cell phone is a very elaborate radio that uses cells to relay the frequency without wires as a person moves freely. Cities and towns are divided into cells, with a tower and radio equipment (called a base station) in each cell. A cell is about 10 square miles and can hold about 800 frequencies.
Every cell phone has a special code associated with it, which identifies the service provider and the phone’s owner. When you first turn on your phone, it will verify the System Identification Code (SID) on the control channel, which is assigned to every carrier by the FCC. If the control channel cannot be found, the phone will say “no service.” The SID is compared by the control channel and the phone, and if it matches, the phone is in the home cell system, and is not “roaming.”
Each cell phone carrier has a Mobile Telephone Switching Office (MTSO) in every city inwhich it provides service. The MTSO keeps track of your phone location by tracking your phone’s registration request and storing it in a database, along with all the other phones in the area. When someone calls your cell phone, the MTSO will look in this database to find the cell that you are in and will ring your phone. Within milliseconds, the MTSO will choose a frequency pair in the cell where you are located and tell the control channel which frequencies to use. The tower and your phone will switch on the frequencies and your call is connected.
As you get to the edge of the current cell, the base station will notice that your signal strength is fading, while the cell you are approaching will notice that your signal is increasing. These two base stations communicate with each other via the MTSO, and a control channel will tell your phone to change frequencies and hand off your phone to the next cell.