Automatic Repeat Request (ARQ) is an acknowledgement mode for secured data transmission where the data sink, i.e. the receiving station, detects errors and requests retransmission. In the event of an error or non-arrival of the data packets, an automatic retransmission is triggered.
In normal, error-free operation, the data sink sends a positive acknowledgement( ACK) after each data packet. After receiving it, the data source s ends the next data packet. If a data packet is faulty or completely missing, then the receiver sends a negative acknowledgement( NAK) and the data source sends the same data packet again.
ARQ protocols include protocols with flow control and acknowledged receipt of received data packets. A distinction is made between character-oriented protocols, in which an ARQ is waited for after each transmitted character, and bit-oriented protocols, in which acknowledgement is controlled by controlling the window size, as in High Level Data Link Control( HDLC) and Synchronous Data Link Control( SDLC).