Alternating bit protocol
Encyclopedia
Alternating bit protocol (ABP) is a simple network protocol operating at the data link layer
that retransmits lost or corrupted messages.
Message
s are sent from transmitter
A to receiver
B. Assume that the channel
from A to B is initialized and that there are no messages in transit. Each message from A to B contains a data part and a one-bit sequence number, i.e., a value that is 0 or 1. B has two acknowledge character
s that it can send to A: ACK0 and ACK1.
When A sends a message, it resends it continuously, with the same sequence number, until it receives an acknowledgment from B that contains the same sequence number. When that happens, A complements
(flips) the sequence number and starts transmitting the next message.
When B receives a message that is not corrupted and has sequence number 0, it starts sending ACK0 and keeps doing so until it receives a valid message with number 1. Then it starts sending ACK1, etc.
This means that A may still receive ACK0 when it is already transmitting messages with sequence number one. (And vice-versa.) It treats such messages as negative-acknowledge character
s (NAKs). The simplest behaviour is to ignore them all and continue transmitting.
The protocol may be initialized by sending bogus messages and acks with sequence number 1. The first message with sequence number 0 is a real message.
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
that retransmits lost or corrupted messages.
Message
Message
A message in its most general meaning is an object of communication. It is a vessel which provides information. Yet, it can also be this information. Therefore, its meaning is dependent upon the context in which it is used; the term may apply to both the information and its form...
s are sent from transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
A to receiver
Receiver (Information Theory)
The receiver in information theory is the receiving end of a communication channel. It receives decoded messages/information from the sender, who first encoded them. Sometimes the receiver is modeled so as to include the decoder. Real-world receivers like radio receivers or telephones can not be...
B. Assume that the channel
Channel (communications)
In telecommunications and computer networking, a communication channel, or 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...
from A to B is initialized and that there are no messages in transit. Each message from A to B contains a data part and a one-bit sequence number, i.e., a value that is 0 or 1. B has two acknowledge character
Acknowledge character
In telecommunications, an acknowledge character is a transmission control character transmitted by the receiving station as an acknowledgement, i.e...
s that it can send to A: ACK0 and ACK1.
When A sends a message, it resends it continuously, with the same sequence number, until it receives an acknowledgment from B that contains the same sequence number. When that happens, A complements
Negation
In logic and mathematics, negation, also called logical complement, is an operation on propositions, truth values, or semantic values more generally. Intuitively, the negation of a proposition is true when that proposition is false, and vice versa. In classical logic negation is normally identified...
(flips) the sequence number and starts transmitting the next message.
When B receives a message that is not corrupted and has sequence number 0, it starts sending ACK0 and keeps doing so until it receives a valid message with number 1. Then it starts sending ACK1, etc.
This means that A may still receive ACK0 when it is already transmitting messages with sequence number one. (And vice-versa.) It treats such messages as negative-acknowledge character
Negative-acknowledge character
* In telecommunications, a negative-acknowledge character is a transmission control character sent by a station as a negative response to the station with which the connection has been set up....
s (NAKs). The simplest behaviour is to ignore them all and continue transmitting.
The protocol may be initialized by sending bogus messages and acks with sequence number 1. The first message with sequence number 0 is a real message.
See also
- Acknowledge characterAcknowledge characterIn telecommunications, an acknowledge character is a transmission control character transmitted by the receiving station as an acknowledgement, i.e...
- Information TheoryInformation theoryInformation theory is a branch of applied mathematics and electrical engineering involving the quantification of information. Information theory was developed by Claude E. Shannon to find fundamental limits on signal processing operations such as compressing data and on reliably storing and...
- Negative-acknowledge characterNegative-acknowledge character* In telecommunications, a negative-acknowledge character is a transmission control character sent by a station as a negative response to the station with which the connection has been set up....
- Stop-and-wait ARQStop-and-wait ARQStop-and-wait ARQ is a method used 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 in the correct order. It is the simplest kind of automatic repeat-request method...