/ˌeɪɑːrˈkjuː/
noun — "a protocol that ensures reliable data delivery by retransmitting lost or corrupted packets."
ARQ (Automatic Repeat reQuest) is an error-control mechanism used in digital communication systems to guarantee the reliable delivery of data across noisy or unreliable channels. ARQ operates at the data link or transport layer, detecting transmission errors through techniques such as Cyclic Redundancy Check (CRC) or parity checks, and automatically requesting retransmission of corrupted or missing packets. This ensures that the receiver reconstructs the original data accurately, which is essential for applications like file transfers, streaming media, network protocols, and satellite communications.
Technically, ARQ protocols combine error detection with feedback mechanisms. When a data packet is sent, the receiver checks it for integrity. If the packet passes validation, an acknowledgment (ACK) is sent back to the transmitter. If the packet fails validation or is lost, a negative acknowledgment (NAK) triggers retransmission. Common ARQ variants include:
- Stop-and-Wait ARQ: the sender transmits one packet and waits for an acknowledgment before sending the next, simple but potentially low throughput.
- Go-Back-N ARQ: the sender continues sending multiple packets up to a window size, but retransmits from the first erroneous packet when a failure is detected, balancing efficiency and reliability.
- Select-Repeat ARQ: only the erroneous packets are retransmitted, maximizing throughput and minimizing redundant transmissions.
Key characteristics of ARQ include:
- Error detection: ensures that corrupted packets are identified before processing.
- Feedback-driven retransmission: leverages ACK/NAK signaling to trigger recovery.
- Windowing and flow control: optimizes throughput while avoiding congestion.
- Reliability assurance: guarantees that all transmitted data is eventually delivered correctly.
- Protocol integration: used in combination with IP, TCP, and other transport-layer protocols to maintain end-to-end integrity.
In practical workflows, ARQ is integral to reliable communications over networks subject to packet loss or interference. For example, a TCP/IP file transfer uses ARQ-like mechanisms to detect missing segments, request retransmission, and reassemble the file accurately. In wireless sensor networks or satellite links, ARQ ensures that telemetry data or command instructions are delivered correctly despite high bit error rates (BER), interference, or fading.
Conceptually, ARQ is like a meticulous courier system: if a package is lost or damaged, the sender is automatically informed and resends it until it reaches its destination intact.
Intuition anchor: ARQ acts as the reliability safeguard of communication systems, turning imperfect, noisy channels into trustworthy conduits for precise data delivery.