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Definition: ARQ

(Automatic Repeat reQuest) A set of protocols for handling communications errors in which the receiving station requests retransmission if an error occurs. The ARQ is a layer 2 data link protocol (see OSI model).

Stop-and-Wait ARQ
The sender waits for an acknowledgment (ACK) before sending the next frame. If the sender does not receive an ACK within a certain amount of time, or if it receives a not-acknowledged (NAK) message, the frame is retransmitted.

Go-Back-N ARQ
Multiple frames are sent, and the sender waits for all frames to be acknowledged. If there is a NAK for any frame, or if there is no ACK for all the frames, the entire set is retransmitted.

Selective Repeat ARQ
Similar to Go-Back-N ARQ, except that only the frame that did not receive a NAK is retransmitted.