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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. Systems that do not need 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 microphone A wireless microphone, or cordless microphone, is a microphone without a physical cable connecting it directly to the sound recording or amplifying equipment with which it is associated. Also known as a radio microphone, it has a small, battery ...
s, and surveillance cameras. In these devices, the communication is only in one direction.


Half duplex

A ''half-duplex'' (HDX) system provides communication in both directions, but only one direction at a time, not simultaneously in both directions. Don Lancaster
"TV Typewriter Cookbook"
( TV Typewriter). 1978. p. 175.
Typically, once a party begins receiving a signal, it 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 Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
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.


Full duplex

A ''full-duplex'' (FDX) system allows communication in both directions, and, unlike half-duplex, allows this to happen simultaneously. Land-line
telephone A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...
networks are full-duplex since they allow both callers to speak and be heard at the same time. Full-duplex operation is achieved on a two-wire circuit through the use of a hybrid coil in a telephone hybrid. Modern cell phones are also full-duplex. 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 doesn't matter and both variants are commonly referred to as ''full duplex''. Many
Ethernet Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 1 ...
connections achieve full-duplex operation by making simultaneous use of two physical
twisted pair Twisted pair cabling is a type of wiring used for communications in which two conductors of a single circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a single conductor or an untwisted b ...
s 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 In computer networking, Gigabit Ethernet (GbE or 1 GigE) is the term applied to transmitting Ethernet frames at a rate of a gigabit per second. The most popular variant, 1000BASE-T, is defined by the IEEE 802.3ab standard. It came into use ...
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.


Echo cancellation

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
modem A modulator-demodulator or modem is a computer hardware device that converts data from a digital format into a format suitable for an analog transmission medium such as telephone or radio. A modem transmits data by modulating one or more c ...
s 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 A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, a ...
.


Full-duplex emulation

Where channel access methods are used in point-to-multipoint networks (such as
cellular network A cellular network or mobile network is a communication network where the link to and from end nodes is wireless. The network is distributed over land areas called "cells", each served by at least one fixed-location transceiver (typically th ...
s) for dividing forward and reverse communication channels on the same physical communications medium, they are known as duplexing methods.


Time-division duplexing

Time-division duplexing (TDD) is the application of time-division multiplexing to separate outward and return signals. It emulates full-duplex communication over a half-duplex communication link. Time-division duplexing is flexible in the case where there is asymmetry of the uplink and downlink data rates or utilization. As the amount of uplink data increases, more communication capacity can be dynamically allocated, and as the traffic load becomes lighter, capacity can be taken away. The same applies in the downlink direction. The ''transmit/receive transition gap'' (TTG) is the gap (time) between a downlink burst and the subsequent uplink burst. Similarly, the ''receive/transmit transition gap'' (RTG) is the gap between an uplink burst and the subsequent downlink burst. Examples of time-division duplexing systems include: * UMTS-TDD for data communications on 3G mobile networks * LTE-TDD for data communications on 4G mobile networks * DECT wireless telephony * Half-duplex packet switched networks based on carrier sense multiple access, for example 2-wire or hubbed Ethernet, Wireless local area networks and
Bluetooth Bluetooth is a short-range wireless technology standard that is used for exchanging data between fixed and mobile devices over short distances and building personal area networks (PANs). In the most widely used mode, transmission power is limi ...
, can be considered as time-division duplexing systems, albeit not TDMA with fixed frame-lengths. *
WiMAX Worldwide Interoperability for Microwave Access (WiMAX) is a family of wireless broadband communication standards based on the IEEE 802.16 set of standards, which provide physical layer (PHY) and media access control (MAC) options. The WiMAX ...
* PACTOR * ISDN BRI U interface, variants using the time-compression multiplex (TCM) line system * G.fast, a
digital subscriber line Digital subscriber line (DSL; originally digital subscriber loop) is a family of technologies that are used to transmit digital data over telephone lines. In telecommunications marketing, the term DSL is widely understood to mean asymmetric dig ...
(DSL) standard developed by the
ITU-T The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Co ...


Frequency-division duplexing

Frequency-division duplexing (FDD) means that the transmitter and receiver operate using different
carrier frequencies In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has a m ...
. The method is frequently used in ham radio operation, where an operator is attempting to use a repeater station. The repeater station must be able to send and receive a transmission at the same time and does so by slightly altering the frequency at which it sends and receives. This mode of operation is referred to as ''duplex mode'' or ''offset mode''. Uplink and downlink sub-bands are said to be separated by the ''frequency offset''. Frequency-division duplex systems can extend their range by using sets of simple repeater stations because the communications transmitted on any single frequency always travel in the same direction. Frequency-division duplexing can be efficient in the case of symmetric traffic. In this case, time-division duplexing tends to waste bandwidth during the switch-over from transmitting to receiving, has greater inherent latency, and may require more complex circuitry. Another advantage of frequency-division duplexing is that it makes radio planning easier and more efficient since base stations do not ''hear'' each other (as they transmit and receive in different sub-bands) and therefore will normally not interfere with each other. Conversely, with time-division duplexing systems, care must be taken to keep guard times between neighboring base stations (which decreases
spectral efficiency Spectral efficiency, spectrum efficiency or bandwidth efficiency refers to the information rate that can be transmitted over a given bandwidth in a specific communication system. It is a measure of how efficiently a limited frequency spectrum is ut ...
) or to synchronize base stations, so that they will transmit and receive at the same time (which increases network complexity and therefore cost, and reduces bandwidth allocation flexibility as all base stations and sectors will be forced to use the same uplink/downlink ratio). Examples of frequency-division duplexing systems include: *
ADSL Asymmetric digital subscriber line (ADSL) is a type of digital subscriber line (DSL) technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. ...
and
VDSL Very high-speed digital subscriber line (VDSL) and very high-speed digital subscriber line 2 (VDSL2) are digital subscriber line (DSL) technologies providing data transmission faster than the earlier standards of asymmetric digital subscriber line ...
* Mobile technology, including LTE, UMTS and
CDMA2000 CDMA2000 (also known as C2K or IMT Multi‑Carrier (IMT‑MC)) is a family of 3G mobile technology standards for sending voice, data, and Signaling (telecommunication), signaling data between mobile phones and cell sites. It is developed by 3GP ...
*
IEEE 802.16 IEEE 802.16 is a series of wireless broadband standards written by the Institute of Electrical and Electronics Engineers (IEEE). The IEEE Standards Board established a working group in 1999 to develop standards for broadband for wireless metrop ...
WiMax Worldwide Interoperability for Microwave Access (WiMAX) is a family of wireless broadband communication standards based on the IEEE 802.16 set of standards, which provide physical layer (PHY) and media access control (MAC) options. The WiMAX ...


See also

* Communication channel * Crossband operation * Double-track railway *
Duplex mismatch On an Ethernet connection, a duplex mismatch is a condition where two connected devices operate in different duplex modes, that is, one operates in half duplex while the other one operates in full duplex. The effect of a duplex mismatch is a l ...
*
Duplexer A duplexer is an electronic device that allows bi-directional ( duplex) communication over a single path. In radar and radio communications systems, it isolates the receiver from the transmitter while permitting them to share a common antenna. ...
* Four-wire circuit * Multiplexing * Push-to-talk *
Radio resource management Radio resource management (RRM) is the system level management of co-channel interference, radio resources, and other radio transmission characteristics in wireless communication systems, for example cellular networks, wireless local area networks, ...
* Simplex communication


Notes


References


Further reading

* * {{DEFAULTSORT:Duplex (Telecommunications) Communication circuits