Editing Duplex (telecommunications) (section)

== Full-duplex emulation ==
Where [[channel access method]]s are used in [[point-to-multipoint]] networks (such as [[cellular network]]s) for dividing forward and reverse communication channels on the same physical communications medium, they are known as duplexing methods.<ref name="Gyasi-Agyei">{{cite book
 | last1  = Gyasi-Agyei
 | first1 = Amoakoh 
 | title  = Telecommunications Engineering: Principles And Practice
 | publisher = World Scientific Publishing Company
 | date   = 2019
 | pages  = 519–520
 | url    = https://books.google.com/books?id=dmmhDwAAQBAJ&dq=communications+engineering+simplex+duplex&pg=PA519
 | doi    = 
 | id     = 
 | isbn   = 9789811200274
 }}</ref>

=== Time-division duplexing === <!--[[Time-division duplex]] and others link here-->
'''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 [[Telecommunications link|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.<ref>{{cite web |url=http://www.rfwireless-world.com/Terminology/TTG-gap-vs-RTG-gap-in-WiMAX-LTE.html |title=TTG vs RTG-What is TTG and RTG Gaps in WIMAX, LTE |access-date=2021-06-05}}</ref>

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 [[Ethernet hub|hubbed Ethernet]], [[Wireless local area network]]s 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 duplex]] and others link here-->
{{confused|Frequency-division multiplexing}}
'''Frequency-division duplexing''' ('''FDD''') means that the [[transmitter]] and [[receiver (radio)|receiver]] operate using different [[carrier frequencies]]. 

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 (engineering)|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]]) 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]] and [[VDSL]]
* [[Mobile technology]], including [[LTE (telecommunication)|LTE]], [[UMTS]] and [[CDMA2000]]
* [[IEEE 802.16]] [[WiMax]]