ARQ

ARQ stands for Automatic Repeat reQuest, a family of error-control protocols used in data communication to provide reliable transmission over channels that may lose or corrupt packets. In an ARQ system, the transmitter sends data in frames and awaits an acknowledgment from the receiver. If the receiver detects an error or a frame is not acknowledged within a timeout, the sender retransmits the affected frame. Some ARQ schemes also use negative acknowledgments to indicate specific problems, while others rely on timeout and acknowledgment sequencing to trigger retransmissions.

The simplest form is Stop-and-Wait ARQ, where the sender transmits one frame and waits for its acknowledgment before sending the next. While easy to implement, Stop-and-Wait can underutilize bandwidth on longer or higher-latency links. More efficient variants use sliding windows. Go-Back-N ARQ allows the sender to transmit multiple frames up to a window limit, but on a error or NACK, it may retransmit from the erroneous frame onward. Selective Repeat ARQ improves efficiency further by only retransmitting the frames that were actually lost or corrupted, while correctly received frames are kept.

Hybrid ARQ combines ARQ with forward error correction, enabling the receiver to decode some information from partially received frames and request additional redundancy as needed. This approach is particularly common in wireless systems, where changing channel conditions benefit from adapting coding and retransmission strategies (incremental redundancy is a typical technique).

ARQ concepts appear in various communication layers and standards, including link-layer protocols such as HDLC and PPP, and in wireless and satellite communications where error rates are higher. In addition to technical usage, ARQ is the title of a 2016 Canadian science fiction thriller film directed by Tony Elliott.