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Created page with "'''Automatic Repeat reQuest''' ('''ARQ'''), also known as '''Automatic Repeat Query''', is an error-control method for data transmission that uses ackn..."

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'''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 [[data frame]] or [[Packet (information technology)|packet]]) and [[Timeout (computing)|timeout]]s (specified periods of time allowed to elapse before an acknowledgment is to be received) to achieve reliable data transmission over an unreliable service. If the sender does not receive an acknowledgment before the timeout, it usually [[Retransmission (data networks)|re-transmits]] the frame/packet until the sender receives an acknowledgment or exceeds a predefined number of re-transmissions .

The types of ARQ protocols include [[Stop-and-wait ARQ]], [[Go-Back-N ARQ]], and [[Selective Repeat ARQ]] / Selective Reject.

All three protocols usually use some form of [[sliding window protocol]]
to tell the transmitter to determine which (if any) packets need to be retransmitted.

These protocols reside in the [[Data Link Layer|Data Link]] or [[Transport Layer]]s of the [[OSI model]].

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.

==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]].

The [[ITU-T]] [[G.hn]] standard, which provides a way to create a high-speed (up to 1 [[Data rate units#Gigabit_per_second|Gbit/s]]) [[local area network]] using existing residential wiring ([[Power line communication|power lines]], telephone lines, and [[Ethernet over coax|coaxial cables]]), uses [[Selective Repeat ARQ]] to ensure reliable transmission over noisy media.

ARQ systems were 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.

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

==Source==

[http://wikipedia.org/ http://wikipedia.org/]

Automatic repeat request - Revision history

Admin: Created page with "'''Automatic Repeat reQuest''' ('''ARQ'''), also known as '''Automatic Repeat Query''', is an error-control method for data transmission that uses ackn..."