alternatingbit

The alternating bit protocol (ABP) is a simple data-link layer protocol used to achieve reliable transmission of a sequence of frames over an unreliable communication channel. It uses a one-bit sequence number that alternates between 0 and 1 to distinguish successive frames.

In operation, the sender transmits a frame with sequence bit s (0 or 1) and starts a timer. The receiver maintains an expected bit e, initially 0. If a frame with bit equal to e arrives, the receiver accepts it, delivers the payload to the upper layer, toggles e to 1 − e, and sends an acknowledgment (ACK) back to the sender containing the bit s. If a frame with a bit different from e arrives, the receiver discards it but may resend the last ACK, acknowledging the last correctly received frame. The sender, upon receiving an ACK with bit s, treats the corresponding frame as successfully delivered, stops its timer, and transmits the next frame with bit 1 − s. If the sender times out before an ACK is received, it retransmits the same frame.

ABP is a fundamental form of automatic repeat request (ARQ) and is effectively a stop-and-wait protocol with a two-state, one-bit sliding window. It guarantees in-order, duplicate-free delivery despite occasional frame or ACK loss, provided that ACKs and frames are eventually delivered by the channel. The protocol is valued for its simplicity and is commonly presented in introductory networking texts as a stepping stone to more advanced sliding-window ARQ schemes. Its main limitation is throughput, which can be suboptimal on long-delay links due to the requirement to wait for an ACK before sending the next frame.