Duplex (telecommunications)
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Aduplexcommunication systemis apoint-to-pointsystem 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 afull-duplexsystem, both parties can communicate with each other simultaneously. An example of a full-duplex device isplain 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 ahalf-duplexorsemiduplexsystem, 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 awalkie-talkie,atwo-way radiothat has apush-to-talkbutton. 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 assimplex.[1][2]
Systems that do not need duplex capability may instead usesimplex communication,in which one device transmits and the others can only listen. Examples arebroadcastradio and television,garage door openers,baby monitors,wireless microphones,andsurveillance cameras.In these devices, the communication is only in one direction.
Simplex
[edit]
Simplex communicationis acommunication channelthat sends information in one direction only.[3]
TheInternational Telecommunication Uniondefinition is a communications channel that operates in one direction at a time, but that may be reversible; this is termedhalf duplexin other contexts.
For example, in TV and radiobroadcasting,information flows only from the transmitter site to multiple receivers. A pair ofwalkie-talkietwo-way radiosprovide 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.
TheWestern Unioncompany used the termsimplexwhen describing the half-duplex and simplex capacity of their newtransatlantic telegraph cablecompleted betweenNewfoundlandand theAzoresin 1928.[4]The same definition for a simplex radio channel was used by theNational Fire Protection Associationin 2002.[5]
Half duplex
[edit]
Ahalf-duplex(HDX) system provides communication in both directions, but only one direction at a time, not simultaneously in both directions.[6] [7][8]This terminology is not completely standardized between defining organizations, and in radio communication some sources classify this mode assimplex.[2] [1][9]Typically, once one party begins a transmission, the other party on the channel must wait for the transmission to complete, before replying.[10]
An example of a half-duplex system is a two-party system such as awalkie-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 conservebandwidth,at the cost of reducing the overall bidirectional throughput, since only a singlecommunication channelis 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 singlefrequencyfor bidirectional communication, while acell phonein 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 multiple xingcan 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, acollisionoccurs, resulting in lost or distorted messages.
Full duplex
[edit]
Afull-duplex(FDX) system allows communication in both directions, and, unlike half-duplex, allows this to happen simultaneously.[6][7][8] Land-linetelephonenetworks are full-duplex since they allow both callers to speak and be heard at the same time. Full-duplex operation is achieved on atwo-wire circuitthrough the use of ahybrid coilin atelephone hybrid.Modern cell phones are also full-duplex.[11]
There is a technical distinction between full-duplex communication, which uses a single physical communication channel for both directions simultaneously, anddual-simplexcommunication 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 asfull duplex.
ManyEthernetconnections achieve full-duplex operation by making simultaneous use of two physicaltwisted pairsinside 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 as1000BASE-Tuse 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.[citation needed]
Echo cancellation
[edit]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[a]there and is then sent back to the far end. The sound then reappears at the original source end but delayed.
Echo cancellationis 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 allowingmodemsto achieve good full-duplex performance. TheV.32,V.34,V.56,andV.90modem standards require echo cancellation.[12]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'scentral processing unit.
Full-duplex emulation
[edit]Wherechannel access methodsare used inpoint-to-multipointnetworks (such ascellular networks) for dividing forward and reverse communication channels on the same physical communications medium, they are known as duple xing methods.[13]
Time-division duple xing
[edit]Time-division duple xing(TDD) is the application oftime-division multiple xingto separate outward and return signals. It emulates full-duplex communication over a half-duplex communication link.
Time-division duple xing is flexible in the case where there isasymmetryof theuplinkanddownlinkdata 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.
Thetransmit/receive transition gap(TTG) is the gap (time) between a downlink burst and the subsequent uplink burst. Similarly, thereceive/transmit transition gap(RTG) is the gap between an uplink burst and the subsequent downlink burst.[14]
Examples of time-division duple xing systems include:
- UMTS-TDDfor data communications on3Gmobile networks
- LTE-TDDfor data communications on4Gmobile networks
- DECTwireless telephony
- Half-duplexpacket switchednetworks based oncarrier-sense multiple access,for example 2-wire orhubbed Ethernet,Wireless local area networksandBluetooth,can be considered as time-division duple xing systems, albeit not TDMA with fixed frame-lengths.
- WiMAX
- PACTOR
- ISDN BRIU interface,variants using the time-compression multiplex (TCM) line system
- G.fast,adigital subscriber line(DSL) standard developed by theITU-T
Frequency-division duple xing
[edit]Frequency-division duple xing(FDD) means that thetransmitterandreceiveroperate using differentcarrier frequencies.
The method is frequently used inham radiooperation, where an operator is attempting to use arepeaterstation. 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 asduplex modeoroffset mode.Uplink and downlink sub-bands are said to be separated by thefrequency 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 duple xing can be efficient in the case of symmetric traffic. In this case, time-division duple xing tends to waste bandwidth during the switch-over from transmitting to receiving, has greater inherentlatency,and may require more complexcircuitry.
Another advantage of frequency-division duple xing is that it makes radio planning easier and more efficient since base stations do notheareach other (as they transmit and receive in different sub-bands) and therefore will normally not interfere with each other. Conversely, with time-division duple xing systems, care must be taken to keep guard times between neighboring base stations (which decreasesspectral efficiency) 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 duple xing systems include:
- ADSLandVDSL
- Mobile technology,includingLTE,UMTSandCDMA2000
- IEEE 802.16WiMax
See also
[edit]Notes
[edit]- ^This feedback path may be acoustic, through the air, or it may be mechanically coupled, for example in a telephone handset.
References
[edit]- ^abLindley, Matthew (12 February 2023)."What is a Two-Way Radio?".Technology.WiseGeek website.Retrieved27 February2023.
- ^abGosling, William (2000).Radio Spectrum Conservation.Elsevier Science. pp. 170–171.ISBN9780750637404.
- ^"Simplex"The IEEE Authoritative Dictionary of Standard Terms, 7th Ed.,2000, Inst. of Electrical and Electronic Engineers, p.1053
- ^Milnor, J.W. and G.A. Randall. "The Newfoundland-Azores High-Speed Duplex Cable".A.I.E.E. Electrical Engineering.May 1931
- ^Report of the Committee on Public Emergency Service Communication.NFPA 1221, May, 2002.
- ^abSudakshina, Kundu (2010).Analog and Digital Communications.New Delhi: Dorling Kindersley. pp. 6–7.ISBN9788131731871.
- ^abFrenzel, Louis (2017).Electronics Explained: Fundamentals for Engineers, Technicians, and Makers, 2nd Ed.Elsevier Science. p. 161.ISBN9780128118795.
- ^ab"Duplex".Terms and Definitions Database.International Telecommunications Union (ITU) website.Retrieved27 February2023.
- ^"Half-duplex".ATIS Telecom Glossary.Alliance for Telecommunications Industry Solutions.Retrieved27 February2023.This definition is accredited by the American National Standards Institute (ANSI)
- ^"half-duplex".pcmag.Retrieved20 June2023.
- ^"Cell phone Frequencies".HowStuffWorks.Retrieved2019-02-14.
- ^Greenstein, Shane; Stango, Victor (2006).Standards and Public Policy.Cambridge University Press. pp. 129–132.ISBN978-1-139-46075-0.
- ^Gyasi-Agyei, Amoakoh (2019).Telecommunications Engineering: Principles And Practice.World Scientific Publishing Company. pp. 519–520.ISBN9789811200274.
- ^"TTG vs RTG-What is TTG and RTG Gaps in WIMAX, LTE".Retrieved2021-06-05.
Further reading
[edit]- Tanenbaum, Andrew S.(2003).Computer Networks.Prentice Hall.ISBN0-13-038488-7.
- Riihonen, Taneli (2014).Design and Analysis of Duple xing Modes and Forwarding Protocols for OFDM(A) Relay Links.Aalto University publication series DOCTORAL DISSERTATIONS, 81/2014.ISBN978-952-60-5715-6.
