Automatic repeat request (ARQ), also known as automatic repeat query, is an error-control method for data transmission that uses acknowledgements (messages sent by the receiver indicating that it has correctly received a message) and timeouts (specified periods of time allowed to elapse before an acknowledgment is to be received) to achieve reliable data transmission over an unreliable communication channel.
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.
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.
A number of patents exist for the use of ARQ in live video contribution environments.