Editing Error detection and correction (section)

=== Automatic repeat request ===
[[Automatic repeat request]] (ARQ) is an error control method for data transmission that makes use of error-detection codes, acknowledgment and/or negative acknowledgment messages, and [[Timeout (computing)|timeouts]] to achieve reliable data transmission. An ''acknowledgment'' is a message sent by the receiver to indicate that it has correctly received a [[data frame]].

Usually, when the transmitter does not receive the acknowledgment before the timeout occurs (i.e., within a reasonable amount of time after sending the data frame), it retransmits the frame until it is either correctly received or the error persists beyond a predetermined number of retransmissions.

Three types of ARQ protocols are [[Stop-and-wait ARQ]], [[Go-Back-N ARQ]], and [[Selective Repeat ARQ]].

ARQ is appropriate if the communication channel has varying or unknown [[channel capacity|capacity]], such as is the case on the Internet. However, ARQ requires the availability of a [[Backward channel|back channel]], results in possibly increased [[Network latency|latency]] due to retransmissions, and requires the maintenance of buffers and timers for retransmissions, which in the case of [[network congestion]] can put a strain on the server and overall network capacity.<ref name="reliable-erasure-code">A. J. McAuley, ''Reliable Broadband Communication Using a Burst Erasure Correcting Code'', ACM SIGCOMM, 1990.</ref>

For example, ARQ is used on shortwave radio data links in the form of [[ARQ-E]], or combined with multiplexing as [[ARQ-M]].