History:
Installation of receivers and transmitters at the same fixed location allowed exchange of messages wirelessly. As early as 1907, two-way telegraphy traffic across the Atlantic Ocean was commercially available. By 1912 commercial and military ships carried both transmitters and receivers, allowing two-way communication in close to real-time with a ship that was out of sight of land. As radio equipment became more powerful, compact, and easier to use, smaller vehicles had two-way radio communication equipment installed. Installation of radio equipment in aircraft allowed scouts to report back observations in real-time, not requiring the pilot to drop messages to troops on the ground below or to land and make a personal report.
In 1933, the Bayonne, New Jersey police department successfully operated a two-way system between a central fixed station and radio transceivers installed in police cars; this allowed rapidly directing police response in emergencies. During World War II hand-held radio transceivers were extensively used by air and ground troops.
Early two-way schemes allowed only one station to transmit at a time while others listened, since all signals were on the same radio frequency - this was called "simplex" mode. Code and voice operations required a simple communication protocol to allow all stations to cooperate in using the single radio channel, so that one station's transmissions were not obscured by another's. By using receivers and transmitters tuned to different frequencies, and solving the problems introduced by operation of a transmitter immediately next to a transmitter, simultaneous transmission and reception was possible at each end of a radio link, in so-called "full duplex" mode.
Early two-way schemes required training operators to learn and use Morse code; in ship-board installations the radio operating officer typically had no other duties than handling radio messages. When voice transmission became possible, dedicated operators were no longer required and two-way use became more common. Today's two-way mobile radio equipment is nearly as simple to use as a household telephone, from the point of view of operating personnel, thereby making two-way communications a useful tool in a wide range of personal, commercial and military roles.
