A cellular telephone system provides a wireless connection to the PSTN for any user location within the radio range of the system.

Cellular systems accommodate a large number of users over a large geographic area, within a limited frequency spectrum.

Cellular radio systems provide high quality service that is often comparable to that of the landline telephone systems. High capacity is achieved by limiting the coverage of each base station transmitter to a small geographic area called a cell so that the same radio channels may be reused by another base station located some distance away.

A sophisticated switching technique called a handoff enables a call to proceed uninterrupted when the user moves from one cell to another.

Figure shows a basic cellular system which consists of mobile stations, base stations and a mobile switching center (MSC). The mobile switching center is sometimes called a mobile telephone switching office (MTSO), since it is responsible for connecting all mobiles to the PSTN in a cellular system.

Each mobile communicates via radio with one of the base stations and may be handed-off to any number of base stations throughout the duration of a call.

The mobile station contains a transceiver, an antenna, and control circuitry, and may be mounted in a vehicle or used as a portable hand-held unit.

The base stations consist of several transmitters and receivers which simultaneously handle full duplex communications and generally have towers which support several transmitting and receiving antennas.

The base station serves as a bridge between all mobile users in the cell and connects the simultaneous mobile calls via telephone lines or microwave links to the MSC.

The MSC coordinates the activities of all of the base stations and connects the entire cellular system to the PSTN. A typical MSC handles 100,000 cellular subscribers and 5,000 simultaneous conversations at a time, and accommodates all billing and system maintenance functions, as well.

In large cities, several MSCs are used by a single carrier.

Communication between the base station and the mobiles is defined by a standard common air interface (CAI) that specifies four different channels.

The channels used for voice transmission from the base station to mobiles are called forward voice channels (FVC), and the channels used for voice transmission from mobiles to the base station are called reverse voice channels (RVC).

The two channels responsible for initiating mobile calls are the forward control channels (FCC) and reverse control channels (RCC).

Control channels are often called setup channels because they are only involved in setting up a call and moving it to an unused voice channel. Control channels transmit and receive data messages that carry call initiation and service requests, and are monitored by mobiles when they do not have a call in progress.

Forward control channels also serve as beacons which continually broadcast all of the traffic requests for all mobiles in the system.

Supervisory and data messages are sent in a number of ways to facilitate automatic channel changes and hand-off instructions for the mobiles before and during a call.