In
data networking,
telecommunications, and
computer buses
In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This ex ...
, an acknowledgment (ACK) is a
signal that is passed between communicating
processes
A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic.
Things called a process include:
Business and management
*Business process, activities that produce a specific se ...
,
computer
A computer is a machine that can be programmed to Execution (computing), carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform generic sets of operations known as C ...
s, or devices to signify acknowledgment, or receipt of message, as part of a
communications protocol. 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
checksums to verify the integrity of the
payload
Payload is the object or the entity which is being carried by an aircraft or launch vehicle. Sometimes payload also refers to the carrying capacity of an aircraft or launch vehicle, usually measured in terms of weight. Depending on the nature of ...
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 code includes an ACK character (0000110
2 or 6
16) which can be transmitted to indicate successful receipt and a NAK character (0010101
2 or 15
16) which can be transmitted to indicate an inability or failure to receive.
Unicode 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 (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 allow many packets to be transmitted before receiving acknowledgement for any of them, a procedure necessary to fill high
bandwidth-delay product 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 protocols which send a NAK when the receiver detects missing packets. Still other protocols make use of both NAKs and ACKs.
Binary Synchronous Communications (Bisync) and Adaptive Link Rate (for
Energy-Efficient Ethernet) are examples.
Still other protocols such as the
RC-5,
User Datagram Protocol (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.
The acknowledgement function is used in the
automatic repeat request (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
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.
Hardware acknowledgment
Some
computer bus
In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This ex ...
es have a dedicated acknowledge wire in the
control bus used to acknowledge bus operations: DACK used for
ISA DMA; DATACK used in the
STEbus
The STEbus (also called the IEEE-1000 bus) is a non-proprietary, processor-independent, computer bus with 8 data lines and 20 address lines. It was popular for industrial control systems in the late 1980s and early 1990s before the ubiqui ...
, 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
''DTACK Grounded'' was a computer hobbyist newsletter published from July 1981 to September 1985 by Hal W. Hardenberg. Subtitled "The Journal of Simple 68000 Systems", the newsletter was dedicated to the proposition that the Motorola 68000 CPU cou ...
, etc. Some computer buses do not always acknowledge every write; some or all of the writes use a
posted write
A posted write is a computer bus write transaction that does not wait for a write completion response to indicate success or failure of the write transaction. For a posted write, the CPU assumes that the write cycle will complete with zero wait st ...
.
The
I²C serial bus has a time slot for an acknowledgment bit after each byte.
See also
*
C0 and C1 control codes
*
Flow control (data)
*
NACK-Oriented Reliable Multicast
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