Each cell phone carrier is assigned about 800 frequencies to use in a particular city. The cell phone carrier divides the area into small regions or "cells," each served by a particular transmitter. Each cell uses only a portion of the available signals, and adjacent stations don't use the same frequencies.
Registration
Your cell phone uses a special channel called the control channel to communicate with its carrier. Each carrier has its own signal identification number (or SID). When you turn on your cell phone, it receives the SID from a mobile station, which tells it that it's in range of a tower. It then sends a registration request to the Mobile Telephone Switching Office (MTSO), a central office that coordinates all the towers in the area. This tells the office where your cell phone is so that it can transmit a signal from the right tower when you get a call.
Calling and Getting Calls
When you place a call, your cell phone communicates with the cell phone tower and the tower assigns the phone two frequencies--one for talking and one for listening. The base station then connects your call. A similar thing happens when you receive a call: The base station contacts your phone on the control frequency and tells it it has a call, then sets up two frequencies for you to accept the call.
Switching Cells
The base stations in your network also monitor the strength of your cell phone's signal. If you move farther away from one base station and nearer to another, both will notice your signal strength change. If you move far enough so that you are closer to a new station versus the one you started on, your phone will receive a signal to switch cells along with assigning a new pair of frequencies.
