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{{Use mdy dates|date = March 2019}}
'''Alternating bit protocol''' ('''ABP''') is a simple [[network protocol]] operating at the [[data link layer]] ([[OSI model|OSI]] layer 2){{cn|reason=this looks more like a transport layer protocol|date=April 2023}} that retransmits lost or corrupted messages using FIFO semantics. It can be seen as a special case of a [[sliding window protocol]] where a simple timer restricts the order of messages to ensure receivers send messages in turn while using a window of 1 bit.<ref>{{cite book|last=Tel|first=Gerard|title=Introduction to distributed algorithms|year=2000|publisher=Cambridge|isbn=0521794838|pages=85}}</ref>
== Design ==
[[Message]]s are sent from [[transmitter]] A to [[Receiver (information theory)|receiver]] B. Assume that the [[Communication channel|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 codes 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 [[Negation|complements]] (flips) the sequence number and starts transmitting the next message.<ref name="foldoc">{{foldoc|Alternating+bit+protocol}}</ref>
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.
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=== Bounded Retransmission Protocol ===
'''Bounded Retransmission Protocol''' (BRP) is a variant of the alternating bit protocol introduced by [[Philips]].<ref>{{cite journal |last=Burnett |first=D.J. |author2=Sethi, H.R. |year=1977 |title=Packet Switching at Philips Research Laboratories |url=http://rogerdmoore.ca/PS/NPLPh/PhilipsA.html |url-status=dead |journal=Computer Networks |volume=1 |issue=6 |pages=341–348 |doi=10.1016/0376-5075(77)90010-1 |archive-url=https://web.archive.org/web/20131020142510/http://rogerdmoore.ca/PS/NPLPh/PhilipsA.html |archive-date=2013-10-20 |access-date=2013-08-30|url-access=subscription }}</ref> The service it delivers is to transfer in a reliable manner, if possible, large files (sequence of data of arbitrary length) from a sender to a receiver. Unlike ABP, BRP deals with sequence numbers of datum in the file and interrupts transfer after fixed number of retransmissions for a datum.<ref>{{cite web|url=https://www.irif.fr/~sighirea//trex/demos/brp.html|title=TreX's Examples -- Bounded Retransmission Protocol|website=www.irif.fr}}</ref>
== History ==
[[Donald Davies|Donald Davies']] team at the [[National Physical Laboratory (United Kingdom)|National Physical Laboratory]] introduced the concept of an alternating bit protocol in 1968 for the [[NPL network]].<ref name=":3">{{Cite journal |last=Cambell-Kelly |first=Martin |date=1987 |title=Data Communications at the National Physical Laboratory (1965-1975) |url=https://archive.org/details/DataCommunicationsAtTheNationalPhysicalLaboratory |journal=Annals of the History of Computing |language=en |volume=9 |issue=3/4 |pages=221–247 |doi=10.1109/MAHC.1987.10023
==See also==
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