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

data transmission Data transmission and data reception or, more broadly, data communication or digital communications is the transfer and reception of data in the form of a digital bitstream or a digitized analog signal transmitted over a point-to-point or ...

that uses acknowledgements (messages sent by the receiver indicating that it has correctly received a

packet Packet may refer to: * A small container or pouch ** Packet (container), a small single use container ** Cigarette packet ** Sugar packet * Network packet, a formatted unit of data carried by a packet-mode computer network * Packet radio, a fo ...

) and

timeout Time-out, Time Out, or timeout may refer to: Time * Time-out (sport), in various sports, a break in play, called by a team * Television timeout, a break in sporting action so that a commercial break may be taken * Timeout (computing), an enginee ...

s (specified periods of time allowed to elapse before an acknowledgment is to be received) 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 informa ...

. If the sender does not receive an acknowledgment before the timeout, it re-transmits the packet until it receives an acknowledgment or exceeds a predefined number of retransmissions. 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/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 automatic ...

. All three protocols usually use some form of

sliding window protocol A sliding window protocol is a feature of packet-based data transmission protocols. Sliding window protocols are used where reliable in-order delivery of packets is required, such as in the data link layer (OSI layer 2) as well as in the Tra ...

to help the sender determine which (if any) packets need to be retransmitted. These protocols reside in the

data link A data link is the means of 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 transmitter and a recei ...

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-e ...

s (layers 2 and 4) of the

OSI model The Open Systems Interconnection model (OSI model) is a conceptual model that 'provides a common basis for the coordination of SOstandards development for the purpose of systems interconnection'. In the OSI reference model, the communications ...

Examples

The

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

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. ...

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

. IEEE 802.11 wireless networking uses ARQ retransmissions at the data-link layer. ITU-T G.hn uses

hybrid ARQ Hybrid automatic repeat request (hybrid ARQ or HARQ) is a combination of high-rate forward error correction (FEC) and automatic repeat request (ARQ) error-control. In standard ARQ, redundant bits are added to data to be transmitted using an er ...

, a mixture of high-rate

forward error correction In computing, telecommunication, information theory, and coding theory, an error correction code, sometimes error correcting code, (ECC) is used for controlling errors in data over unreliable or noisy communication channels. The central idea i ...

(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, school, laboratory, university campus or office building. By contrast, a wide area network (WAN) not only covers a large ...

standard that can operate at data rates up to 1 Gbit/s over existing home wiring (

power lines Electric power transmission is the bulk movement of electrical energy from a generating site, such as a power plant, to an electrical substation. The interconnected lines that facilitate this movement form a ''transmission network''. This is d ...

, 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 ...

for Error Detection, LDPC for FEC and selective repeat for ARQ. ARQ systems were widely used on shortwave radio to ensure reliable delivery of data such as for telegrams. These systems came in forms called ARQ-E and ARQ-M, 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 *RFC 3366 - Advice to link designers on link Automatic Repeat reQuest (ARQ)
Negative acknowledgement ARQ
for live video *{{FS1037C MS188 Logical link control Error detection and correction

Examples

See also

References