What Is Automatic Repeat Request?

Automatic repeat request (ARQ) is an error-control method used in data communications that achieves reliable delivery over unreliable channels by combining error detection with retransmission. When a receiver detects an error in a received frame, it signals the sender to retransmit that frame, rather than attempting to correct the error locally. ARQ protocols are implemented at the data-link layer and the transport layer in many network architectures, including TCP/IP and IEEE 802.11 wireless LAN. The technique draws from information theory, queuing theory, and coding theory, and was formalized in the foundational IEEE paper on ARQ error-control schemes by Lin and Costello published in the IEEE Communications Magazine.

ARQ schemes depend on two mechanisms working in concert: a cyclic redundancy check (CRC) or similar algebraic code appended to each transmitted frame, which lets the receiver detect corruption, and an acknowledgment (ACK) or negative acknowledgment (NACK) signal sent back to the transmitter. If a positive ACK does not arrive within a timeout period, the sender retransmits. This simple feedback loop makes ARQ self-correcting without requiring the receiver to solve a decoding problem.

Stop-and-Wait and Sliding-Window Protocols

Stop-and-wait ARQ is the simplest variant: the sender transmits one frame and waits for an ACK before transmitting the next. The channel efficiency of stop-and-wait is low on high-bandwidth-delay-product links because the sender sits idle while awaiting the acknowledgment. Sliding-window protocols address this inefficiency by allowing the sender to transmit multiple unacknowledged frames simultaneously, up to a window size W. Two sliding-window variants are in common use: Go-Back-N, in which an error causes the sender to retransmit the erroneous frame and all frames transmitted after it, and Selective Repeat, in which only the erroneous frame is retransmitted while correctly received subsequent frames are buffered. Selective Repeat achieves higher throughput at the cost of larger receiver buffers. The IETF RFC 793 specification for TCP incorporates a sliding-window ARQ mechanism at the transport layer, using sequence numbers and cumulative acknowledgments to achieve reliable byte-stream delivery.

Hybrid ARQ

Hybrid ARQ (HARQ) combines ARQ retransmission with forward error correction (FEC), allowing the receiver to attempt to decode a received frame using redundancy already present in the transmission before requesting a retransmit. In HARQ Type I, each retransmission is a complete self-contained coded frame. In HARQ Type II and Type III (incremental redundancy), each retransmission adds new parity information that the receiver combines with earlier transmissions to progressively improve decoding success. HARQ is a core mechanism in 3GPP LTE and 5G NR physical layer standards, where the short round-trip time of the scheduler feedback loop makes it practical to achieve near-optimal link adaptation. The 3GPP specifications for HARQ in LTE and NR define the timing, process numbering, and redundancy-version sequencing used in modern cellular systems.

Performance and Flow Control

ARQ performance is characterized by throughput efficiency, which depends on the window size, round-trip propagation delay, frame size, and residual error rate. Link-level ARQ interacts with higher-layer congestion control: excessive retransmissions at the link layer can mask channel errors from the transport layer, causing TCP to misinterpret packet loss as network congestion. Accurate performance models account for the correlation structure of channel errors, burst error behavior, and the interplay between retransmission delay and application latency requirements.

Applications

Automatic repeat request has applications in a range of fields, including:

• Reliable data transfer in TCP/IP networks over wired and wireless links
• Cellular radio access in LTE and 5G NR through HARQ protocols
• Satellite link communications where long propagation delays require selective-repeat windows
• Bluetooth and IEEE 802.11 Wi-Fi data-link error recovery
• Industrial fieldbus and process-control network reliability mechanisms