Editing Automatic repeat request (section)

{{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) 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. 

ARQ is used 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]]. 

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]].