Editing Duplex (telecommunications) (section)

== <span class="anchor" id="HALF-DUPLEX"></span><span class="anchor" id="Half-duplex"></span>Half duplex ==
[[File:Locale_RS6_HalfDuplex.JPG|thumb|A simple illustration of a half-duplex communication system]]

A '''half-duplex''' ('''HDX''') system provides communication in both directions, but only one direction at a time, not simultaneously in both directions.<ref name="Sudakshina">{{cite book
 | last1  = Sudakshina
 | first1 = Kundu 
 | title  = Analog and Digital Communications
 | publisher = Dorling Kindersley
 | date   = 2010
 | location = New Delhi
 | pages  = 6–7
 | url    = https://books.google.com/books?id=JKfTrRRHT5QC&q=duplex%20simplex
 | doi    = 
 | id     = 
 | isbn   = 9788131731871
 }}</ref>
<ref name="Frenzel">{{cite book
 | last1  = Frenzel
 | first1 = Louis 
 | title  = Electronics Explained: Fundamentals for Engineers, Technicians, and Makers, 2nd Ed.
 | publisher = Elsevier Science
 | date   = 2017
 | pages  = 161
 | url    = https://books.google.com/books?id=uqjRDgAAQBAJ&dq=simplex+duplex&pg=PA161
 | doi    = 
 | id     = 
 | isbn   = 9780128118795
 }}</ref><ref name="ITU">{{cite web
  | title = Duplex
  | work = Terms and Definitions Database
  | publisher = International Telecommunications Union (ITU) website
  | date = 
  | url = https://www.itu.int/br_tsb_terms/#/?q=duplex&sector=T,R&from=2005-01-01&to=2023-02-27&status=1&type=any&page=1
  | format = 
  | doi = 
  | accessdate = 27 February 2023}}</ref> This terminology is not completely standardized between defining organizations, and in radio communication, some sources classify this mode as ''[[Simplex communication|simplex]]''.<ref name="Gosling">{{cite book
 | last1  = Gosling
 | first1 = William
 | title  = Radio Spectrum Conservation
 | publisher = Elsevier Science
 | date   = 2000
 | pages  = 170–171
 | language = 
 | url    = https://books.google.com/books?id=MvbZ2eK7luMC&q=simplex%20duplex
 | doi    = 
 | id     = 
 | isbn   = 9780750637404
 }}</ref>
<ref name="Lindley">{{cite web
  | last = Lindley
  | first = Matthew 
  | title = What is a Two-Way Radio?
  | work = Technology
  | publisher = WiseGeek website
  | date = 12 February 2023
  | url = https://www.wise-geek.com/what-is-a-two-way-radio.htm
  | format = 
  | doi = 
  | accessdate = 27 February 2023}}</ref><ref name="ATIS">{{cite web
  | title = Half-duplex
  | work = ATIS Telecom Glossary
  | publisher = Alliance for Telecommunications Industry Solutions
  | date = 
  | url = https://glossary.atis.org/search-results/?search=duplex
  | format = 
  | doi = 
  | accessdate = 27 February 2023}} This definition is accredited by the American National Standards Institute (ANSI)</ref> Typically, once one party begins a transmission, the other party on the channel must wait for the transmission to complete before replying.<ref>{{cite web |url=https://www.pcmag.com/encyclopedia/term/half-duplex |title=half-duplex |website=www.pcmag.com |access-date=20 June 2023 }}</ref>

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 (signal processing)|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 (telecommunications)|collision]] occurs, resulting in lost or distorted messages.