How does ARQ accomplish error correction?

ARQ stands for Automatic Repeat reQuest (or Automatic Repeat Query). It is a digital communications protocol designed to ensure 100% reliable data transmission over noisy or fading radio links. When a receiving station gets a packet of data, it checks for errors using a mathematical checksum (such as a CRC). If the check indicates the data is corrupted, the receiver automatically sends a short message back to the transmitter asking for that specific packet to be retransmitted. This is distinctly different from Forward Error Correction (FEC). FEC transmits extra redundant data alongside the original message so the receiver can mathematically repair the errors on the fly without ever needing to ask for a retransmission.