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<title>Automatic repeat request</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Automatic repeat request</span></span>

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<p><b>Automatic repeat request</b> (<b>ARQ</b>), also known as <b>automatic repeat query</b>, is an <a href="Error_control" class="mw-redirect" title="Error control">error-control</a> method for <a href="Data_transmission" class="mw-redirect" title="Data transmission">data transmission</a> that uses <a href="Acknowledgement_(data_networks)" title="Acknowledgement (data networks)">acknowledgements</a> (messages sent by the receiver indicating that it has correctly received a message) and <a href="Timeout_(computing)" title="Timeout (computing)">timeouts</a> (specified periods of time allowed to elapse before an acknowledgment is to be received) If the sender does not receive an acknowledgment before the timeout, it <a href="Retransmission_(data_networks)" title="Retransmission (data networks)">re-transmits</a> the message until it receives an acknowledgment or exceeds a predefined number of retransmissions.

</p><p>ARQ is used to achieve <a href="Reliability_(computer_networking)" title="Reliability (computer networking)">reliable data transmission</a> over an unreliable <a href="Communication_channel" title="Communication channel">communication channel</a>. ARQ is appropriate if the communication channel has varying or unknown <a href="Channel_capacity" title="Channel capacity">capacity</a>.

</p><p>Variations of ARQ protocols include <a href="Stop-and-wait_ARQ" title="Stop-and-wait ARQ">Stop-and-wait ARQ</a>, <a href="Go-Back-N_ARQ" title="Go-Back-N ARQ">Go-Back-N ARQ</a>, and <a href="Selective_Repeat_ARQ" title="Selective Repeat ARQ">Selective Repeat ARQ</a>. All three protocols usually use some form of <a href="Sliding_window_protocol" title="Sliding window protocol">sliding window protocol</a> to help the sender determine which (if any) packets need to be retransmitted. These protocols reside in the <a href="Data_link_layer" title="Data link layer">data link</a> or <a href="Transport_layer" title="Transport layer">transport layers</a> (layers 2 and 4) of the <a href="OSI_model" title="OSI model">OSI model</a>.

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<div class="mw-heading mw-heading2"><h2 id="Examples">Examples</h2></div>

<p>The <a href="Transmission_Control_Protocol" title="Transmission Control Protocol">Transmission Control Protocol</a> uses a variant of <a href="Go-Back-N_ARQ" title="Go-Back-N ARQ">Go-Back-N ARQ</a> to ensure reliable transmission of data over the <a href="Internet_Protocol" title="Internet Protocol">Internet Protocol</a>, which does not provide guaranteed delivery of packets; with Selective Acknowledgement (SACK), it uses <a href="Selective_Repeat_ARQ" title="Selective Repeat ARQ">Selective Repeat ARQ</a>.

</p><p><a href="IEEE_802.11" title="IEEE 802.11">IEEE 802.11</a> wireless networking uses ARQ retransmissions at the data-link layer.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup>

</p><p><a href="ITU-T_G.hn" class="mw-redirect" title="ITU-T G.hn">ITU-T G.hn</a> uses <a href="Hybrid_automatic_repeat_request" title="Hybrid automatic repeat request">hybrid ARQ</a>, a mixture of high-rate <a href="Forward_error_correction" class="mw-redirect" title="Forward error correction">forward error correction</a> (FEC) and ARQ. It is a high-speed <a href="Local_area_network" title="Local area network">local area network</a> standard that can operate at data rates up to 1 Gbit/s over existing home wiring (<a href="Power_line_communication" class="mw-redirect" title="Power line communication">power lines</a>, phone lines and <a href="Ethernet_over_coax" title="Ethernet over coax">coaxial cables</a>). G.hn uses <a href="CRC-32C" class="mw-redirect" title="CRC-32C">CRC-32C</a> for Error Detection, <a href="Low-density_parity-check_code" title="Low-density parity-check code">LDPC</a> for FEC and selective repeat for ARQ.

</p><p>ARQ systems are widely used on shortwave radio to ensure reliable delivery of data such as for telegrams. These systems came in forms called <a href="ARQ-E" title="ARQ-E">ARQ-E</a> and <a href="ARQ-M" title="ARQ-M">ARQ-M</a>, which also included the ability to multiplex two or four channels.

</p><p>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.

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<ul><li><a href="Hybrid_automatic_repeat_request" title="Hybrid automatic repeat request">Hybrid automatic repeat request</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://warpproject.org/trac/wiki/802.11/MAC/Lower/Retransmissions">"802.11 Reference Design: Recovery Procedures and Retransmit Limits"</a><span class="reference-accessdate">. Retrieved <span class="nowrap">July 21,</span> 2020</span>.</cite></span>

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<ul><li>Peterson and Davie, <i>Computer Networks: A Systems Approach</i>, Third Edition, 2003</li>

<li>RFC <a rel="nofollow" class="external text" href="https://www.rfc-editor.org/rfc/rfc3366">3366</a> - Advice to link designers on link Automatic Repeat reQuest (ARQ)</li>

<li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20160304032149/http://www.techex.co.uk/other/arq-video-packet-resend">Negative acknowledgement ARQ</a> for live video</li>

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</style><span class="citation FS1037C MS188"><span class="noviewer" typeof="mw:File"><span></span></span> This article incorporates <a href="Copyright_status_of_works_by_the_federal_government_of_the_United_States" title="Copyright status of works by the federal government of the United States">public domain material</a> from <cite class="citation cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20220122224547/https://www.its.bldrdoc.gov/fs-1037/fs-1037c.htm"><i>Federal Standard 1037C</i></a>. <a href="General_Services_Administration" title="General Services Administration">General Services Administration</a>. Archived from <a rel="nofollow" class="external text" href="https://www.its.bldrdoc.gov/fs-1037/fs-1037c.htm">the original</a> on January 22, 2022.</cite> (in support of <a href="MIL-STD-188" title="MIL-STD-188">MIL-STD-188</a>).</span></li></ul>

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This article is issued from <a class="external text" title="Last edited on 2025-08-03" href="https://en.wikipedia.org/wiki/?title=Automatic_repeat_request&oldid=1304094405">Wikipedia</a>. The text is available under <a class="external text" href="https://creativecommons.org/licenses/by-sa/4.0/deed.en">Creative Commons Attribution-Share Alike 4.0</a> unless otherwise noted. Additional terms may apply for the media files.

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