Editing Automatic repeat request

{{Short description|Error-control method for data transmission}}
{{Use American English|date = March 2019}}
{{Use mdy dates|date = March 2019}}
{{no footnotes|date=June 2018}}

'''Automatic repeat request''' ('''ARQ'''), also known as '''automatic repeat query''', is an [[error control|error-control]] method for [[data transmission]] that uses [[acknowledgement (data networks)|acknowledgement]]s (messages sent by the receiver indicating that it has correctly received a message) and [[Timeout (computing)|timeout]]s (specified periods of time allowed to elapse before an acknowledgment is to be received) to achieve [[Reliability (computer networking)|reliable data transmission]] over an unreliable [[communication channel]]. ARQ is appropriate if the communication channel has varying or unknown [[Channel capacity|capacity]]. If the sender does not receive an acknowledgment before the timeout, it [[Retransmission (data networks)|re-transmits]] the message until it receives an acknowledgment or exceeds a predefined number of retransmissions. 

Variations of ARQ protocols include [[Stop-and-wait ARQ]], [[Go-Back-N ARQ]], and [[Selective Repeat ARQ]]. All three protocols usually use some form of [[sliding window protocol]] to help the sender determine which (if any) packets need to be retransmitted. These protocols reside in the [[Data link layer|data link]] or [[transport layer]]s (layers 2 and 4) of the [[OSI model]].

==Examples==
The [[Transmission Control Protocol]] uses a variant of [[Go-Back-N ARQ]] to ensure reliable transmission of data over the [[Internet Protocol]], which does not provide guaranteed delivery of packets; with Selective Acknowledgement (SACK), it uses [[Selective Repeat ARQ]].

[[IEEE 802.11]] wireless networking uses ARQ retransmissions at the data-link layer.<ref>{{cite web |url=https://warpproject.org/trac/wiki/802.11/MAC/Lower/Retransmissions |title=802.11 Reference Design: Recovery Procedures and Retransmit Limits |access-date=2020-07-21}}</ref>

[[ITU-T G.hn]] uses [[Hybrid automatic repeat request|hybrid ARQ]], a mixture of high-rate [[forward error correction]] (FEC) and ARQ. It is a high-speed [[local area network]] standard that can operate at data rates up to 1&nbsp;Gbit/s over existing home wiring ([[Power line communication|power lines]], phone lines and [[Ethernet over coax|coaxial cables]]). G.hn uses [[CRC-32C]] for Error Detection, [[Low-density parity-check code|LDPC]] for FEC and selective repeat for ARQ.

ARQ systems are widely used on shortwave radio to ensure reliable delivery of data such as for telegrams.  These systems came in forms called [[ARQ-E]] and [[ARQ-M]], which also included the ability to multiplex two or four channels.

A number of patents exist for the use of ARQ in live video contribution environments. In these high throughput environments negative acknowledgements are used to drive down overheads.

== See also ==
* [[Hybrid automatic repeat request]]

== References ==
{{reflist}}
{{Refbegin}}
*Peterson and Davie, ''Computer Networks: A Systems Approach'', Third Edition, 2003
*{{IETF RFC|3366|link=no}} - Advice to link designers on link Automatic Repeat reQuest (ARQ)
*[https://web.archive.org/web/20160304032149/http://www.techex.co.uk/other/arq-video-packet-resend Negative acknowledgement ARQ] for live video
*{{FS1037C MS188}}
{{Refend}}

[[Category:Logical link control]]
[[Category:Error detection and correction]]