Information Theory (IV)

Cellular telephone is a wireless telephone that transmits messages with radio signals.  It enables people to communicate over a wide area by using a network of radio antennas and transmitters arranged in small geographical areas called cells.  Cells vary in size and number, based on the network's extent.  Callers use a cellular telephone unit, which is actually a radio transmitter and receiver.  The unit enables a user to make or receive voice calls and, with special equipment, to send or receive computer data and facsimile (fax) transmissions.    

Radio waves send the call to an antenna transmitter in the cell where the call is made.  The transmitter relays the message to a mobile telephone switching office (MTSO).  The MTSO transmits the message to a local telephone company, which sends the call to the receiver or to a long-distance company for completion. As the caller moves, the network automatically passes the call from one cell to the next cell without interruption.  There are three types of cellular phones.  Installed phones are mounted in a vehicle, such as an automobile, and powered by the vehicle's electrical system.  Transportable phones have their own carrying case and are powered by a battery pack.  Portable phones are battery-powered units small enough to keep in a coat pocket.  The first commercial cellular system went into operation in 1983 in the United States. Cellular service is now available in much of the world.  Many developing nations have used cellular technology to provide telephone service in remote areas without installing phone lines.