Alternating bit protocol: Difference between revisions

Revision as of 20:14, 12 October 2009

Revision as of 19:57, 12 March 2010

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'''Alternating bit protocol''' (ABP) means a simple [[data link layer]] [[network protocol]] that retransmits lost or corrupted messages.

[[Message]]s are sent from [[transmitter]] A to [[receiver (Information Theory)|receiver]] B. Assume that the [[channel (communications)|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. 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 whichwhat extent that is possible is the subject of [[coding theory]].

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.