Alternating bit protocol (ABP) means a simple data link layer network protocol that retransmits lost or corrupted messages. The data link layer is layer two of the seven-layer OSI model. ... In networking, a communications protocol or network protocol is the specification of a set of rules for a particular type of communication. ...

Messages 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 characters that it can send to A: ACK0 and ACK1. We assume that the channel may corrupt a message and that there is a way in which A and B can decide whether or not they have received a correct message. How and to which extent that is possible is the subject of coding theory. Message in its most general meaning is an object of communication. ... Antenna tower of Crystal Palace transmitter, London A transmitter (sometimes abbreviated XMTR) is an electronic device which with the aid of an antenna propagates an electromagnetic signal such as radio, television, or other telecommunications. ... The receiver in information theory is the receiving end of a communication channel (in particular the binary symmetric channel) in information theory. ... Channel, in communications (sometimes called communications channel), refers to the medium through which information is transmitted from a sender (or transmitter) to a receiver. ... Acknowledge character (ACK): A transmission control character transmitted by the receiving station as an affirmative response to the sending station. ... Coding theory deals with the properties of codes, and thus with their fitness for a specific application. ...

When A sends a message, it sends 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 characters (NAKs). The simplest behaviour is to ignore them all and continue transmitting. In telecommunications, a negative-acknowledge character (NAK) is a transmission control character sent by a station as a negative response to the station with which the connection has been set up. ...

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 character
• Negative-acknowledge character
• Information Theory

Acknowledge character (ACK): A transmission control character transmitted by the receiving station as an affirmative response to the sending station. ... In telecommunications, a negative-acknowledge character (NAK) is a transmission control character sent by a station as a negative response to the station with which the connection has been set up. ... Information theory is the mathematical theory of data communication and storage, generally considered to have been founded in 1948 by Claude E. Shannon. ...

References