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Automatic repeat request (ARQ) is a feedback-based error-control method: when a receiver or sender detects that a transmitted frame may have been lost or corrupted, the sender arranges to retransmit it. ARQ improves delivery reliability through acknowledgements, timers, and retries, but its exact behavior depends on the protocol.
How automatic repeat request works
A sender transmits data in units commonly called frames. The receiver checks whether each frame arrived and, depending on the protocol, sends an acknowledgement (ACK), a negative acknowledgement (NACK), or information about which frames it has received. If an expected ACK does not arrive before a timer expires, the sender may treat the frame as lost and retransmit it.
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These mechanisms provide feedback about transmission; they do not guarantee that every frame will eventually arrive. Protocols determine how long the sender waits, what triggers a retry, and whether retries stop after a limit or continue by another rule. For example, ITU-T Recommendation G.9903 specifies a particular ARQ behavior in which an originator that receives no acknowledgement within a waiting period retries during a specified contention period; after several unacknowledged retries, it may terminate the transaction or try again. That is one standard’s behavior, not a universal ARQ rule. ITU-T G.9903 (2017), Amendment 2 (March 2023)
Common ARQ methods
Stop-and-wait
The sender sends one frame and waits for its acknowledgement before sending the next. If the frame is lost or corrupted, the protocol triggers retransmission. RFC 3366 describes this method as simple but inefficient on links with a high bandwidth-delay product, because the sender cannot keep sending while it waits. Its link-layer use is better suited to links with a low bandwidth-delay product.
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Go-Back-N
Go-Back-N is a sliding-window method: the sender can transmit several numbered frames before acknowledgements arrive. If a frame is lost or erroneous, the sender may retransmit that frame and the later outstanding frames, even if some of those later frames arrived successfully.
Selective Repeat
Selective Repeat, also called Selective Reject, also allows multiple numbered frames to be in flight, but selectively retransmits frames that need attention rather than restarting at the error point. It requires careful sequence-number and window design. RFC 3366 notes that, for selective repeat, the sequence space must be larger than twice the link window.
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These descriptions cover common approaches, not every protocol’s exact rules. The IETF’s RFC 3366, Advice to link designers on link Automatic Repeat reQuest (ARQ), published as a Best Current Practice in August 2002, discusses their link-layer design trade-offs.
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How to compare ARQ designs
No ARQ method is best for every link. The useful choice depends on both link conditions and application needs.
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- Throughput and utilization: Stop-and-wait can leave a high-bandwidth-delay link underused; a sliding window can keep more frames in flight.
- Retransmission overhead: Go-Back-N may resend later outstanding frames after an error, while Selective Repeat targets the frames that need retransmission.
- Buffering and sequence numbers: Sliding-window methods need more coordination and buffering; Selective Repeat has particular sequence-space requirements.
- Complexity, ordering, and latency: Protocol rules affect implementation effort and when data can be delivered onward. Assess these alongside the link’s conditions and the application’s tolerance for delay.
- Retry persistence: Retry limits and what happens after retries fail vary by protocol, so check the applicable specification rather than assuming retries continue indefinitely.
What ARQ does—and does not—promise
ARQ is a way to improve reliability by detecting possible loss or corruption and responding with retransmission. “Reliable delivery” is the goal, not a single guarantee shared by every implementation: retry limits, delay, ordering, and performance depend on the protocol and its deployment. The term here refers to the data-communications technique, particularly link-layer ARQ, as described in RFC 3366; the details should not be generalized to every layer or system.
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