data networking A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are ...

telecommunication Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...

s, and computer buses, an acknowledgment (ACK) is a

signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...

that is passed between communicating processes, computers, or devices to signify acknowledgment, or receipt of message, as part of a

communications protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synchr ...

. The negative-acknowledgement (NAK or NACK) is a signal that is sent to reject a previously received message or to indicate some kind of error. Acknowledgments and negative acknowledgments inform a sender of the receiver's state so that it can adjust its own state accordingly. Many protocols contain

checksum A checksum is a small-sized block of data derived from another block of digital data for the purpose of detecting errors that may have been introduced during its transmission or storage. By themselves, checksums are often used to verify data ...

s to verify the integrity of the payload and header. Checksums are used to detect data corruption. If a message is received with an invalid checksum (that is, the data received would have a different checksum than the message had), the receiver can know that some information was corrupted. Most often, when checksums are employed, a corrupted message received will either not be served an ACK signal, or will be served a NAK signal.

Acknowledgment characters

ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because ...

code includes an ACK character (0000110₂ or 6₁₆) which can be transmitted to indicate successful receipt and a NAK character (0010101₂ or 15₁₆) which can be transmitted to indicate an inability or failure to receive.

Unicode Unicode, formally The Unicode Standard,The formal version reference is is an information technology standard for the consistent encoding, representation, and handling of text expressed in most of the world's writing systems. The standard, wh ...

provides visible symbols for these characters, U+2406 (␆) and U+2415 (␕).

Protocol usage

Many protocols are acknowledgement-based, meaning that they positively acknowledge receipt of messages. The internet's

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 common ...

(TCP) is an example of an acknowledgement-based protocol. When computers communicate via TCP, received packets are acknowledged by sending back a packet with an ACK bit set. The TCP protocol allows these acknowledgements to be included with data that is sent in the opposite direction. Some protocols send a single acknowledgement per packet of information. Other protocols such as TCP and

ZMODEM ZMODEM is an inline file transfer protocol developed by Chuck Forsberg in 1986, in a project funded by Telenet in order to improve file transfers on their X.25 network. In addition to dramatically improved performance compared to older protocol ...

allow many packets to be transmitted before receiving acknowledgement for any of them, a procedure necessary to fill high

bandwidth-delay product In data communications, the bandwidth-delay product is the product of a data link's capacity (in bits per second) and its round-trip delay time (in seconds). The result, an amount of data measured in bits (or bytes), is equivalent to the maxim ...

links with a large number of bytes in flight. Other protocols are NAK-based, meaning that they only respond to messages if there is a problem. Examples include most

reliable multicast A reliable multicast is any computer networking protocol that provides a '' reliable'' sequence of packets to multiple recipients simultaneously, making it suitable for applications such as multi-receiver file transfer. Overview Multicast is a net ...

protocols which send a NAK when the receiver detects missing packets. Still other protocols make use of both NAKs and ACKs.

Binary Synchronous Communications Binary Synchronous Communication (BSC or Bisync) is an IBM character-oriented, half-duplex link protocol, announced in 1967 after the introduction of System/360. It replaced the synchronous transmit-receive (STR) protocol used with second gene ...

(Bisync) and Adaptive Link Rate (for

Energy-Efficient Ethernet In computer networking, Energy-Efficient Ethernet (EEE) is a set of enhancements to twisted-pair, twinaxial, backplane, and optical fiber Ethernet physical-layer variants that reduce power consumption during periods of low data activity. The ...

) are examples. Still other protocols such as the

RC-5 The RC-5 protocol was developed by Philips in the early 1980s as a semi-proprietary consumer IR (infrared) remote control communication protocol for consumer electronics. It was subsequently adopted by most European manufacturers, as well as by ...

User Datagram Protocol In computer networking, the User Datagram Protocol (UDP) is one of the core communication protocols of the Internet protocol suite used to send messages (transported as datagrams in packets) to other hosts on an Internet Protocol (IP) network ...

(UDP), and X10 protocols perform

blind transmission A blind transmission, in telecommunications, is a transmission made without obtaining a receipt, or acknowledgment of reception, from the intended receiving station. Blind transmissions may occur or be necessary when security constraints, such as ra ...

with no acknowledgement, often transmitting the same message multiple times in hopes that at least one copy of the message gets through. The acknowledgement function is used in the

automatic repeat request Automatic repeat request (ARQ), also known as automatic repeat query, is an error-control method for data transmission that uses acknowledgements (messages sent by the receiver indicating that it has correctly received a packet) and timeouts ...

(ARQ) function. Acknowledgement frames are numbered in coordination with the frames that have been received and then sent to the transmitter. This allows the transmitter to avoid overflow or underrun at the receiver, and to become aware of any missed frames. In

, the NAK is used to indicate that a transmission error was detected in the previously received block and that the receiver is ready to accept retransmission of that block. Bisync does not use a single ACK character but has two control sequences for alternate even/odd block acknowledgement.

Hardware acknowledgment

Some computer buses have a dedicated acknowledge wire in the

control bus In computer architecture, a control bus is part of the system bus and is used by CPUs for communicating with other devices within the computer. While the address bus carries the information about the device with which the CPU is communicating and ...

used to acknowledge bus operations: DACK used for ISA DMA; DATACK used in the STEbus, the data transfer acknowledge pin of the Motorola 68000 that inspired the title of DTACK Grounded, etc. Some computer buses do not always acknowledge every write; some or all of the writes use a posted write. The

I²C I2C (Inter-Integrated Circuit, ), alternatively known as I2C or IIC, is a synchronous, multi-controller/multi-target (master/slave), packet switched, single-ended, serial communication bus invented in 1982 by Philips Semiconductors. It is wi ...

serial bus has a time slot for an acknowledgment bit after each byte.

References

External links

* {{cite web , url=https://ruk.ca/content/ack-vs-nak , title=ACK vs. NAK , author=Peter Rukavina , access-date=2020-03-04 Control characters Data transmission Error detection and correction Flow control (data) Link protocols Network protocols Routing protocols

Acknowledgment characters

Protocol usage

Hardware acknowledgment

See also

References

External links