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In page Alternating bit protocol:

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Alternating bit protocol (ABP) is a simple network protocol operating at the data link layer (OSI layer 2)[citation needed] 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.[1]