Automatic repeat request

Automatic repeat request (ARQ)

Automatic repeat request (ARQ), also called automatic repeat query, is a way to make sure data gets from one place to another correctly. It uses messages called acknowledgments. These are sent by the receiver to say they got the data okay. It also uses timeouts, which are waiting periods. If the sender does not get an acknowledgment before the timeout ends, it sends the message again. This keeps happening until the message is received or until a certain number of tries is reached.

ARQ is important because it helps make sure data transmission is reliable, even when connections are not always good. It is especially useful when the communication channel’s ability to send information is not clear or changes a lot.

There are different types of ARQ protocols, like Stop-and-wait ARQ, Go-Back-N ARQ, and Selective Repeat ARQ. These protocols often use a sliding window protocol to help the sender know which packets need to be sent again. ARQ works in the data link or transport layers of the OSI model.

Examples

The Transmission Control Protocol uses a type of ARQ called Go-Back-N to make sure data reaches its destination safely over the Internet Protocol. Another type, called Selective Repeat ARQ, is also used with something named SACK.

ARQ is also used in wireless networks like IEEE 802.11. It helps send data reliably even when signals might be weak or interrupted. Some home networking systems, like ITU-T G.hn, mix ARQ with other methods to send data really fast over home wiring like power lines and coaxial cables.