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Alternating Bit Protocol


The alternating bit protocol (ABP) is a simple communication protocol for reliable transmission of messages (frames) [BSW69]. Reliability is guaranteed by retransmitting lost or corrupted messages and ensuring the order of messages. The protocol consists of a sender and a receiver, which communicate via two channels: channel M for messages from sender to receiver and channel A for acknowledgements from receiver to sender, see Figure 1.

Fig. 1: Alternating bit protocol.

Each message the sender transmits contains a data part and an additional bit, i.e., a value that is 0 or 1. After transmission, the sender waits for an acknowledgement from the receiver (with the same bit) via A. If no correct acknowledgement arrives, the sender retransmits the message with the same bit. Otherwise, the sender flips the bit and starts transmitting the next message.