ARQ

From HFUnderground

ARQ - Automatic Repeat Request, is a term that refers to a common technique in digital data communications that is used to eliminate errors.

Digital data is usually transmitted in discrete pieces called "packets". If, during the time that the packet is being sent over the transmission channel (here we assume a radio frequency), some sort of corruption occurs, the data received does not match the data sent. This corruption can be a burst of noise on the airwaves, or an equipment failure on either end of the transmission circuit, or other interference. The purpose of ARQ is to remedy this mismatch, by detecting it, and having the packet retransmitted.

Each packet has a digital signature, usually called a "CRC", or "checksum", that is included to uniquely identify the packet. Upon reception of a packet, the receiving station recalculates the signature, and compares it to the one included with the packet. If they don't match, the assumption is that there was data corruption during the transmission, and the receiving station requests that the packet be sent again. This continues until the signatures match, or, enough failures accumulate that the channel is assumed to be unusable.