Automatic Repeat Request

ARQ (from English Automatic Repeat-reQuest) are protocols used to control errors in data transmission, guaranteeing their integrity. These are usually used in systems that do not act in real time, since the time that is lost in forwarding can be considerable and it is usually more useful to issue a bad message at the moment, than to do it correctly some time later. This can be seen very clearly with a video conferencing application where it is not useful to emit the correct pixel of the image 2 seconds after having seen the image.

This error control technique is based on the resending of information packets that are detected as erroneous (This means that not all information packets are detected as erroneous).

To control the correct reception of a packet, ACK's (acknowledge) and NACK's are used so that when the receiver receives a packet correctly, the receiver agrees with an ACK and if it is not correct, it responds with a NACK. During the protocol that controls packet reception, multiple problems can arise (ACK loss, receiving an incorrect ACK, etc.), thus complicating the content of the ACK and giving rise to new concepts such as timeout.

If the sender does not receive information about the reception of the packet during a set time (timeout), it is automatically resent.

Essentially there are three types of ARQ although in practice they are combined looking for the optimal system for each specific channel or traffic state.

Among other ARQ protocols, we find Stop-and-wait ARQ, Go-Back-N ARQ and Selective Repeat ARQ; these protocols belong to the Data Link Layer and/or Transport Layer of the OSI model