Simplex and Duplex systems
Simplex communication is a communication channel that sends information in one direction only. A “duplex” communication channel requires two simplex channels operating in opposite directions. The ITU definition of simplex is a communications channel that operates in one direction at a time, but that may be reversible; this is termed “half duplex” in other contexts.
For example, in TV and radio broadcasting, information flows only from the transmitter site to multiple receivers. An RS 232 interface between a computer terminal and a modem is made up of multiple simplex control and data circuits, but information can flow both ways since channels are provided both to and from the terminal. A pair of walkie-talkie two-way radios provide a simplex circuit in the ITU sense; only one party at a time can talk, while the other listens until it can hear an opportunity to transmit. The transmission medium (the radio signal over the air) can carry information in both directions, but the apparatus only allows one direction at a time to be used.
A duplex communication system is a point-to-point system composed of two connected parties or devices that can communicate with one another in both directions. Translated literally, “duplex” means “two paths”; thus, a duplex system has two clearly defined paths, with each path carrying information in only one direction – A to B over one path, and B to A over the other. There are two types of duplex communication systems: full-duplex and half-duplex.
In a full duplex system, both parties can communicate to the other simultaneously. An example of a full-duplex device is a telephone; the parties at both ends of a call can speak and be heard by the other party simultaneously. The earphone reproduces the speech of the remote party as the microphone transmits the speech of the local party, because there is a two-way communication channel between them, or more strictly speaking, because there are two communication paths/channels between them.
In a half-duplex system, there are still two clearly defined paths/channels, and each party can communicate to the other but not simultaneously; the communication is one direction at a time. An example of a half-duplex device is a walkie-talkie two-way radio that has a “push-to-talk” button; when the local user wants to speak to the remote person they push this button, which turns on the transmitter but turns off the receiver, so they cannot hear the remote person. To listen to the other person they release the button, which turns on the receiver but turns off the transmitter.
Duplex systems are employed in many communications networks, either to allow for a communication “two-way street” between two connected parties or to provide a “reverse path” for the monitoring and remote adjustment of equipment in the field.
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, andsurveillance cameras. In these devices the communication is only in one direction.