In data networking,

telecommunication Telecommunication, often used in its plural form or abbreviated as telecom, is the transmission of information over a distance using electronic means, typically through cables, radio waves, or other communication technologies. These means of ...

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

signal A signal is both the process and the result of transmission of data over some media accomplished by embedding some variation. Signals are important in multiple subject fields including signal processing, information theory and biology. In ...

that is passed between communicating processes,

computer A computer is a machine that can be Computer programming, programmed to automatically Execution (computing), carry out sequences of arithmetic or logical operations (''computation''). Modern digital electronic computers can perform generic set ...

s, 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 variation of a physical quantity. The protocol defines the rules, syntax, semantics (computer science), sem ...

. Correspondingly a 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.

Acknowledgment signal types

The

ASCII ASCII ( ), an acronym for American Standard Code for Information Interchange, is a character encoding standard for representing a particular set of 95 (English language focused) printable character, printable and 33 control character, control c ...

code point A code point, codepoint or code position is a particular position in a Table (database), table, where the position has been assigned a meaning. The table may be one dimensional (a column), two dimensional (like cells in a spreadsheet), three dime ...

for ACK is 0x06 (binary 0000 0110). By convention a receiving device sends an ACK to indicate it successfully received a message. ASCII also provides a NAK code point (0x15, binary 0001 0101) which can be used to indicate the receiving device cannot, or will not, comply with the message.

Unicode Unicode or ''The Unicode Standard'' or TUS is a character encoding standard maintained by the Unicode Consortium designed to support the use of text in all of the world's writing systems that can be digitized. Version 16.0 defines 154,998 Char ...

provides visible symbols for these ASCII characters, U+2406 (␆) and U+2415 (␕). ACK and NAK symbols may also take the form of single bits or

bit field A bit field is a data structure that maps to one or more adjacent bits which have been allocated for specific purposes, so that any single bit or group of bits within the structure can be set or inspected. A bit field is most commonly used to repre ...

s depending on the protocol

data link layer The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer p ...

definition or even as a dedicated wire at

physical layer In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer: the layer most closely associated with the physical connection between devices. The physical layer provides an electrical, mechani ...

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 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 ...

(TCP) is an example of an acknowledgement-based protocol. When computers communicate via TCP, received packets are acknowledged by sending a return packet with an ACK bit set. While some protocols send an acknowledgement per each packet received, other protocols such as TCP and ZMODEM allow many packets to be transmitted before sending an acknowledgement for the set of them, a procedure necessary to fill high bandwidth-delay product links with a large number of bytes in flight. Some protocols are NAK-based, meaning that they only respond to messages if there is a problem. Examples include many reliable multicast protocols which send a NAK when the receiver detects missing packets or protocols that use

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 dat ...

s to verify the integrity of the payload and header. Still other protocols make use of ''both'' NAKs and ACKs. Binary Synchronous Communications (Bisync) and Adaptive Link Rate (for Energy-Efficient Ethernet) are examples. 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 message) and timeout ...

(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 IBM Binary Synchronous Communications, 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. ACK and NAK based methodologies are not the only protocol design paradigms. Some protocols such as the RC-5,

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 Network packet, packets) to other hosts on an Internet Protoco ...

(UDP), and X10 protocols perform blind transmission with no acknowledgement, often transmitting the same message multiple times in hopes that at least one copy of the message gets through.

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 central processing unit, CPUs for communicating with other devices within the computer. While the address bus carries the information about the device with which the ...

used to acknowledge bus operations: DACK used for ISA DMA; DATACK used in the STEbus, the data transfer acknowledge pin of the

Motorola 68000 The Motorola 68000 (sometimes shortened to Motorola 68k or m68k and usually pronounced "sixty-eight-thousand") is a 16/32-bit complex instruction set computer (CISC) microprocessor, introduced in 1979 by Motorola Semiconductor Products Sector ...

that inspired the title of DTACK Grounded, etc. Some computer buses do not wait for acknowledgement of every transmission, see for instance posted write. The

I²C I2C (Inter-Integrated Circuit; pronounced as "" or ""), alternatively known as I2C and IIC, is a synchronous, multi-master/multi-slave, single-ended, serial communication bus invented in 1980 by Philips Semiconductors (now NXP Semiconduct ...

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) Network protocols Routing protocols

Acknowledgment signal types

Protocol usage

Hardware acknowledgment

See also

References

External links