ARQ — Automatic Repeat Request — Architecture & Protocol Behavior

Introduction

Automatic Repeat Request (ARQ) is a family of reliability mechanisms that ensure correct data delivery by combining error detection with retransmission.
Unlike FEC, which corrects errors proactively, ARQ relies on:

detecting corrupted or missing packets
requesting retransmission
maintaining ordering and completeness

ARQ is fundamental in protocols where latency is acceptable and where a return channel is available (e.g., TCP, wireless MACs, storage links).

Core Principles of ARQ

ARQ operates on three basic components:

Error detection — typically CRC or checksum
Feedback channel — ACK/NACK messages
Retransmission policy — rules for resending lost or corrupted packets

The combination of these elements defines the behavior and performance of the ARQ system.

ARQ Mechanisms

Stop‑and‑Wait ARQ

The simplest form:

sender transmits one packet
waits for ACK
retransmits on timeout or NACK

Pros: simple, low memory
Cons: low throughput, high idle time

Used in low‑speed or highly constrained systems.

Go‑Back‑N ARQ

A sliding window mechanism:

sender transmits multiple packets without waiting
receiver accepts packets in order only
on error, sender retransmits from the failed packet onward

Pros: higher throughput
Cons: wasteful on long links or high error rates

Used in older link‑layer protocols.

Selective Repeat ARQ

The most efficient ARQ variant:

receiver accepts packets out of order
only corrupted/lost packets are retransmitted
requires reordering buffers

Pros: high efficiency, minimal retransmission
Cons: more complex, requires larger buffers

Used in modern wireless and transport protocols.

ARQ Timing and Control

Acknowledgments

ACKs confirm successful reception.
NACKs explicitly signal failure.

Timeouts

If no ACK is received within a timeout window, the sender retransmits.

Timeout tuning affects:

throughput
latency
congestion
energy consumption (wireless)

Sliding Windows

Windows define how many packets can be “in flight” simultaneously.

Window size impacts:

throughput
memory requirements
link utilization

ARQ in Real Protocols

TCP

TCP uses a sophisticated ARQ system with:

cumulative ACKs
selective acknowledgments (SACK)
congestion control
retransmission timers
fast retransmit

TCP’s ARQ is tightly integrated with flow control and congestion management.

Wireless MAC Protocols

Wi‑Fi and cellular systems use ARQ to compensate for:

fading
interference
collisions

Often combined with FEC (Hybrid ARQ).

Storage and Interconnects

Some storage protocols (e.g., SATA, NVMe‑over‑Fabrics) use ARQ for reliability over noisy or long‑reach links.

Hybrid ARQ (HARQ)

HARQ combines:

FEC (error correction)
ARQ (retransmission)

Two main types:

Type I HARQ — FEC + ARQ independently
Type II/III HARQ — incremental redundancy (IR), where retransmissions add new parity bits

HARQ is used in:

LTE
5G NR
Wi‑Fi 6/7

It provides excellent performance in noisy wireless channels.

Performance Considerations

Latency

ARQ introduces round‑trip delays due to:

feedback
retransmission
reordering

Throughput

Throughput depends on:

window size
error rate
timeout tuning
congestion

Energy Efficiency

In wireless systems, retransmissions increase energy consumption.

Buffering

Selective Repeat requires:

reordering buffers
per‑packet state
larger memory footprints

Comparison: ARQ vs FEC

Mechanism	Corrects Errors?	Needs Feedback?	Latency	Typical Use
ARQ	No (detect only)	Yes	Higher	TCP, wireless MAC
FEC	Yes	No	Low	PHYs, high‑speed links
HARQ	Yes + Retransmit	Yes	Medium	LTE, 5G

ARQ is ideal when retransmission is acceptable; FEC is ideal when it is not.

Summary

ARQ provides reliability through detection and retransmission.
It is simple, robust, and widely used in transport and wireless protocols.
Modern systems often combine ARQ with FEC (HARQ) to balance throughput, latency, and robustness.

DexterLab