Duplex (telecommunications)
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A duplex
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
Systems that do not need duplex capability may instead use
Simplex
Simplex communication is a
The
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.[4] The same definition for a simplex radio channel was used by the National Fire Protection Association in 2002.[5]
Half duplex
A half-duplex (HDX) system provides communication in both directions, but only one direction at a time, not simultaneously in both directions.[6]
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
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.[6][7][8] Land-line
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
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
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.
Full-duplex emulation
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.[13]
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
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.[14]
Examples of time-division duplexing systems include:
- UMTS-TDD for data communications on 3Gmobile networks
- LTE-TDD for data communications on 4Gmobile networks
- DECTwireless telephony
- Half-duplex Wireless local area networks and Bluetooth, can be considered as time-division duplexing systems, albeit not TDMA with fixed frame-lengths.
- WiMAX
- PACTOR
- ISDN BRI U interface, variants using the time-compression multiplex (TCM) line system
- G.fast, a digital subscriber line (DSL) standard developed by the ITU-T
Frequency-division duplexing
Frequency-division duplexing (FDD) means that the
The method is frequently used in
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
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) 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:
See also
Notes
- ^ This feedback path may be acoustic, through the air, or it may be mechanically coupled, for example in a telephone handset.
References
- ^ a b Lindley, Matthew (12 February 2023). "What is a Two-Way Radio?". Technology. WiseGeek website. Retrieved 27 February 2023.
- ^ ISBN 9780750637404.
- ^ "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.
- ^ ISBN 9788131731871.
- ^ ISBN 9780128118795.
- ^ a b "Duplex". Terms and Definitions Database. International Telecommunications Union (ITU) website. Retrieved 27 February 2023.
- ^ "Half-duplex". ATIS Telecom Glossary. Alliance for Telecommunications Industry Solutions. Retrieved 27 February 2023. This definition is accredited by the American National Standards Institute (ANSI)
- ^ "half-duplex". www.pcmag.com. Retrieved 20 June 2023.
- ^ "Cell phone Frequencies". HowStuffWorks. Retrieved 2019-02-14.
- ISBN 978-1-139-46075-0.
- ISBN 9789811200274.
- ^ "TTG vs RTG-What is TTG and RTG Gaps in WIMAX, LTE". Retrieved 2021-06-05.
Further reading
- ISBN 0-13-038488-7.
- Riihonen, Taneli (2014). Design and Analysis of Duplexing Modes and Forwarding Protocols for OFDM(A) Relay Links. Aalto University publication series DOCTORAL DISSERTATIONS, 81/2014. ISBN 978-952-60-5715-6.