StopandWait

Stop-and-wait is a simple automatic repeat request (ARQ) protocol used in data communication and networking. In this scheme, the sender transmits a single data frame and then must stop and wait for an acknowledgment from the receiver before sending the next frame. Each frame typically carries a small sequence number, often a single bit, to help detect duplicates after wraparound.

Operation involves two parties: the sender and the receiver. The sender transmits one frame and starts a timer. If the frame arrives correctly, the receiver sends an acknowledgment (ACK). If the ACK is lost or the frame is corrupted, the receiver discards the frame and the sender, upon timeout, retransmits it. An ACK may be piggybacked on a return data frame to save bandwidth. The receiver buffers at most one outstanding frame and delivers data in order, since only one frame is in transit at any time.

Performance characteristics center on simplicity versus efficiency. The protocol’s utilization is limited by the round-trip time (RTT) of the link. Approximate throughput can be described by the frame transmission time relative to the total cycle time, which includes the one-way propagation delay twice and any ACK transmission time. On links with large RTT compared to frame size, the maximum achievable throughput can be quite low.

Compared with sliding-window ARQ protocols, stop-and-wait uses a window size of one and thus offers poor efficiency on high-latency or noisy links. Its main advantages are simplicity and minimal memory requirements, while its limitations include low utilization and limited scalability for high-bandwidth, long-delay networks. Stop-and-wait remains a foundational concept in the study of reliable data transmission.