ARQ

Automatic repeat request (ARQ) (or automatic repeat-query) is an error-control method for data transmission which uses acknowledgements and requests to achieve reliable data transmission over an unreliable service. An acknowledgement is a message sent by the receiver to the transmitter to indicate that it has correctly received a data frame or packet. A request is a message sent by the receiver to the transmitter to indicate that the sender has to re-transmit the frame/packet; if the sender does not receive an acknowledgement before a timeout, it usually re-transmits the frame/packet until it receives an acknowledgement or exceeds a predefined number of re-transmissions.