Stop-and-Wait ARQ

The simplest ARQ (Automatic Repeat reQuest) flow and error control protocol. The sender transmits one frame at a time and waits for its acknowledgement before sending the next.

Working

1. Sender sends frame 0 and starts a timer; keeps a copy.
2. Receiver checks the frame (CRC). If correct, it sends ACK; the sender then sends the next frame (1), alternating sequence numbers 0 and 1.
3. If the frame is corrupted, the receiver discards it silently; the sender's timer expires and it retransmits.
4. If the ACK is lost, the timer expires, the sender retransmits, and the receiver recognizes a duplicate by the sequence number, discards it and re-sends the ACK.

Features

• Window size: sender = 1, receiver = 1
• Sequence numbers: 1 bit (0, 1)
• Uses timer, ACKs and retransmission

Drawback

Low efficiency on long or high-bandwidth links since the channel sits idle while waiting for the ACK. Efficiency $= \dfrac{1}{1 + 2a}$ where $a = T_p / T_t$. Sliding window protocols (Go-Back-N, Selective Repeat) overcome this.