What are ARQ protocols and how do they provide reliable data transmission?

ARQ (Automatic Repeat reQuest) protocols use acknowledgments and retransmissions for reliability. Stop-and-Wait: Sender waits for ACK before sending next frame; simple but inefficient. Go-Back-N: Sender can have multiple outstanding frames, retransmits from error point; better efficiency. Selective Repeat: Only retransmits errored frames; most efficient but complex receiver buffering. ARQ combined with FEC creates Hybrid ARQ (HARQ): Type I discards errored packets, Type II/III combine soft information from retransmissions. Trade-offs: throughput, latency, complexity, and buffer requirements.