Automatic repeat request (ARQ), also known as automatic repeat query, is an error-control method for

data transmission Data communication, including data transmission and data reception, is the transfer of data, signal transmission, transmitted and received over a Point-to-point (telecommunications), point-to-point or point-to-multipoint communication chann ...

that uses acknowledgements (messages sent by the receiver indicating that it has correctly received a message) and timeouts (specified periods of time allowed to elapse before an acknowledgment is to be received) If the sender does not receive an acknowledgment before the timeout, it re-transmits the message until it receives an acknowledgment or exceeds a predefined number of retransmissions. ARQ is used to achieve reliable data transmission over an unreliable

communication channel A communication channel refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel in telecommunications and computer networking. A channel is used for infor ...

. ARQ is appropriate if the communication channel has varying or unknown capacity. Variations of ARQ protocols include

Stop-and-wait ARQ Stop-and-wait ARQ, also referred to as alternating bit protocol, is a method in telecommunications to send information between two connected devices. It ensures that information is not lost due to dropped packets and that packets are received ...

Go-Back-N ARQ Go-Back-''N'' ARQ is a specific instance of the automatic repeat request (ARQ) protocol, in which the sending process continues to send a number of frames specified by a ''window size'' even without receiving an acknowledgement (ACK) packet fr ...

, and

Selective Repeat ARQ Selective Repeat ARQ or Selective Reject ARQ is a specific instance of the automatic repeat request (ARQ) protocol used to manage sequence numbers and retransmissions in reliable communications. Summary Selective Repeat is part of the automati ...

. All three protocols usually use some form of sliding window protocol to help the sender determine which (if any) packets need to be retransmitted. These protocols reside in the

data link A data link is a means of telecommunications link, connecting one location to another for the purpose of transmitting and receiving digital information (data communication). It can also refer to a set of electronics assemblies, consisting of a t ...

transport layer In computer networking, the transport layer is a conceptual division of methods in the layered architecture of protocols in the network stack in the Internet protocol suite and the OSI model. The protocols of this layer provide end-to-end c ...

s (layers 2 and 4) of the

OSI model The Open Systems Interconnection (OSI) model is a reference model developed by the International Organization for Standardization (ISO) that "provides a common basis for the coordination of standards development for the purpose of systems inter ...

Examples

The

Transmission Control Protocol The Transmission Control Protocol (TCP) is one of the main communications protocol, protocols of the Internet protocol suite. It originated in the initial network implementation in which it complemented the Internet Protocol (IP). Therefore, th ...

uses a variant of

to ensure reliable transmission of data over the

Internet Protocol The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet. IP ...

, which does not provide guaranteed delivery of packets; with Selective Acknowledgement (SACK), it uses

IEEE 802.11 IEEE 802.11 is part of the IEEE 802 set of local area network (LAN) technical standards, and specifies the set of medium access control (MAC) and physical layer (PHY) protocols for implementing wireless local area network (WLAN) computer com ...

wireless networking uses ARQ retransmissions at the data-link layer. ITU-T G.hn uses hybrid ARQ, a mixture of high-rate

forward error correction In computing, telecommunication, information theory, and coding theory, forward error correction (FEC) or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels. The centra ...

(FEC) and ARQ. It is a high-speed

local area network A local area network (LAN) is a computer network that interconnects computers within a limited area such as a residence, campus, or building, and has its network equipment and interconnects locally managed. LANs facilitate the distribution of da ...

standard that can operate at data rates up to 1 Gbit/s over existing home wiring ( power lines, phone lines and coaxial cables). G.hn uses

CRC-32C A cyclic redundancy check (CRC) is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to digital data. Blocks of data entering these systems get a short ''check value'' attached, based on th ...

for Error Detection,

LDPC Low-density parity-check (LDPC) codes are a class of error correction codes which (together with the closely-related turbo codes) have gained prominence in coding theory and information theory since the late 1990s. The codes today are widely ...

for FEC and selective repeat for ARQ. ARQ systems are widely used on shortwave radio to ensure reliable delivery of data such as for telegrams. These systems came in forms called

ARQ-E ARQ-E is a radio transmission method used to send data over short wave radio. ARQ-E is a full duplex synchronous data communications system that requests repeats if data is not received correctly. It uses an alphabet that can detect errors. Anothe ...

and

ARQ-M ARQ-M, short for Automatic Repeat reQuest, Multiplex, is a radio telegraphy protocol used to reliably forward telex messages over partially reliable radio links. It is a low-speed system designed to match the performance of landline telex systems an ...

, which also included the ability to multiplex two or four channels. A number of patents exist for the use of ARQ in live video contribution environments. In these high throughput environments negative acknowledgements are used to drive down overheads.

References

*Peterson and Davie, ''Computer Networks: A Systems Approach'', Third Edition, 2003 * - Advice to link designers on link Automatic Repeat reQuest (ARQ)
Negative acknowledgement ARQ
for live video * {{Refend Logical link control Error detection and correction

Examples

See also

References