Automatic repeat request

Automatic repeat request (ARQ), also known as automatic repeat query, is a method used to make sure data is sent correctly from one place to another. It works by using acknowledgments, which are messages the receiver sends to say they got the data okay, and timeouts, which are waiting periods to see if an acknowledgment arrives. If the sender doesn’t 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 achieve reliable data transmission over connections that might not always work well. It’s especially useful when the communication channel’s ability to send information isn’t 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 something called 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, which are layers 2 and 4.

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.