<P> Systems that do not need the duplex capability may instead use simplex communication, in which one device transmits and the others can only "listen". Examples are broadcast radio and television, garage door openers, baby monitors, wireless microphones, and surveillance cameras . In these devices the communication is only in one direction . </P> <P> A half - duplex (HDX) system provides communication in both directions, but only one direction at a time (not simultaneously). Typically, once a party begins receiving a signal, it must wait for the transmitter to stop transmitting, before replying . </P> <P> An example of a half - duplex system is a two - party system such as a walkie - talkie, wherein one must use "over" or another previously designated keyword to indicate the end of transmission, and ensure that only one party transmits at a time, because both parties transmit and receive on the same frequency . A good analogy for a half - duplex system would be a one - lane road with traffic controllers at each end, such as a two - lane bridge under re-construction . Traffic can flow in both directions, but only one direction at a time, regulated by the traffic controllers . </P> <P> Half - duplex systems are usually used to conserve bandwidth, since only a single communication channel is needed, which is shared alternately between the two directions . For example, a walkie - talkie requires only a single frequency for bidirectional communication, while a cell phone, which is a full - duplex device, requires two frequencies to carry the two simultaneous voice channels, one in each direction . </P>

The transmission of data can only go in one direction at a time