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Communication flowing simultaneously in both directions

A duplex communication system is a point to point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow for simultaneous communication in both directions between two connected parties or to provide a reverse path for the monitoring and remote adjustment of equipment in the field. There are two types of duplex communication systems: full duplex (FDX) and half duplex (HDX). In a full duplex system, both parties can communicate with each other simultaneously. An example of a full duplex device is plain old telephone service; 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. There is a two way communication channel between them, or more strictly speaking, there are two communication channels between them. In a half duplex or semiduplex system, both parties can communicate with each other, but not simultaneously; the communication is one direction at a time. An example of a half duplex device is a walkie talkie, a 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 and turns off the receiver, preventing them from hearing the remote person while talking. To listen to the remote person, they release the button, which turns on the receiver and turns off the transmitter. This terminology is not completely standardized, and some sources define this mode as simplex. The International Telecommunication Union definition 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. 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 only one direction. The Western Union company used the term simplex when describing the half duplex and simplex capacity of their new transatlantic telegraph cable completed between Newfoundland and the Azores in 1928. The same definition for a simplex radio channel was used by the National Fire Protection Association in 2002.

A half duplex (HDX) system provides communication in both directions, but only one direction at a time, not simultaneously in both directions. This terminology is not completely standardized between defining organizations, and in radio communication some sources classify this mode as simplex. Typically, once one party begins a transmission, the other party on the channel must wait for the transmission to complete, before replying. An example of a half duplex system is a two party system such as a walkie talkie, wherein one must say "over" or another previously designated keyword to indicate the end of transmission, to ensure that only one party transmits at a time. A good analogy for a half duplex system would be a one lane road that allows two way traffic, traffic can only flow in one direction at a time. Half duplex systems are usually used to conserve bandwidth, at the cost of reducing the overall bidirectional throughput, since only a single communication channel is needed and is shared alternately between the two directions. For example, a walkie talkie or a DECT phone or so called TDD 4G or 5G phones requires only a single frequency for bidirectional communication, while a cell phone in the so called FDD mode is a full duplex device, and generally requires two frequencies to carry the two simultaneous voice channels, one in each direction. In automatic communications systems such as two way data links, time division multiplexing can be used for time allocations for communications in a half duplex system. For example, station A on one end of the data link could be allowed to transmit for exactly one second, then station B on the other end could be allowed to transmit for exactly one second, and then the cycle repeats. In this scheme, the channel is never left idle. In half duplex systems, if more than one party transmits at the same time, a collision occurs, resulting in lost or distorted messages.

A full duplex (FDX) system allows communication in both directions, and, unlike half duplex, allows this to happen simultaneously. There is a technical distinction between full duplex communication, which uses a single physical communication channel for both directions simultaneously, and dual simplex communication which uses two distinct channels, one for each direction. From the user perspective, the technical difference does not matter and both variants are commonly referred to as full duplex. Many Ethernet connections achieve full duplex operation by making simultaneous use of two physical twisted pairs inside the same jacket, or two optical fibers which are directly connected to each networked device: one pair or fiber is for receiving packets, while the other is for sending packets. Other Ethernet variants, such as 1000BASE T use the same channels in each direction simultaneously. In any case, with full duplex operation, the cable itself becomes a collision free environment and doubles the maximum total transmission capacity supported by each Ethernet connection. Full duplex has also several benefits over the use of half duplex. Since there is only one transmitter on each twisted pair there is no contention and no collisions so time is not wasted by having to wait or retransmit frames. Full transmission capacity is available in both directions because the send and receive functions are separate. Some computer based systems of the 1960s and 1970s required full duplex facilities, even for half duplex operation, since their poll and response schemes could not tolerate the slight delays in reversing the direction of transmission in a half duplex line.

Full duplex audio systems like telephones can create echo, which is distracting to users and impedes the performance of modems. Echo occurs when the sound originating from the far end comes out of the speaker at the near end and re enters the microphone there and is then sent back to the far end. The sound then reappears at the original source end but delayed. Echo cancellation is a signal processing operation that subtracts the far end signal from the microphone signal before it is sent back over the network. Echo cancellation is important technology allowing modems to achieve good full duplex performance. The V.32, V.34, V.56, and V.90 modem standards require echo cancellation. Echo cancelers are available as both software and hardware implementations. They can be independent components in a communications system or integrated into the communication system's central processing unit.

Where channel access methods are used in point to multipoint networks (such as cellular networks) for dividing forward and reverse communication channels on the same physical communications medium, they are known as duplexing methods.