edVirtus Reference

What Is Automatic Repeat reQuest?

What Is ARQ?

Automatic Repeat reQuest (ARQ) is an error-control technique in which a receiver automatically requests the retransmission of data that have been received with errors. Rather than attempting to correct the errors itself, the receiver detects that an error has occurred—typically using a Cyclic Redundancy Check (CRC)—and asks the transmitter to send the affected data again. ARQ is widely used in computer networks, wireless communications, satellite communications, and data storage systems.

The basic principle is straightforward. The transmitter sends a block of data together with an error-detection code such as a CRC. The receiver checks the received block and, if no errors are detected, returns an acknowledgement (ACK) indicating successful reception. If an error is detected, the receiver sends a negative acknowledgement (NAK) or simply fails to acknowledge the transmission. The transmitter then retransmits the missing or corrupted data.

A useful analogy is sending an important document by mail and asking the recipient to confirm that every page arrived intact. If a page is missing or damaged, the recipient requests another copy of that page rather than attempting to reconstruct it from memory. ARQ performs the same function electronically by requesting retransmission of corrupted data.

Several forms of ARQ are commonly used. In Stop-and-Wait ARQ, the transmitter sends one block and waits for an acknowledgement before sending the next. Although simple, this method is inefficient on long-delay communication links because the transmitter remains idle while waiting for the response.

To improve efficiency, modern systems often use Go-Back-N ARQ, in which several blocks may be transmitted before acknowledgements are received. If an error occurs, the transmitter retransmits the affected block and all subsequent blocks in the transmission window. An even more efficient technique is Selective Repeat ARQ, which retransmits only the specific blocks that were received incorrectly, reducing unnecessary retransmissions.

ARQ performs particularly well on communication channels with relatively low error rates, where retransmissions occur only occasionally. However, on noisy channels or links with long propagation delays—such as deep-space or geostationary satellite communications—frequent retransmissions may significantly reduce throughput. In such situations, ARQ is often combined with Forward Error Correction (FEC) to form a Hybrid ARQ (HARQ) system, allowing many errors to be corrected without retransmission while still requesting retransmission when necessary.

It is important to distinguish Automatic Repeat reQuest from Forward Error Correction. ARQ relies on error detection and retransmission, whereas FEC relies on redundant coding that allows the receiver to correct many errors without requesting additional data. ARQ generally requires less transmission overhead but introduces additional delay whenever retransmissions are necessary.

Today, ARQ is a fundamental feature of modern digital communications. Ethernet, Wi-Fi, cellular networks, satellite communications, Internet protocols, and countless other communication systems employ ARQ or Hybrid ARQ to ensure reliable data delivery. By automatically requesting retransmission only when needed, ARQ provides an efficient and practical means of maintaining data integrity across imperfect communication channels.