Short note: DLL flow control

Flow control is the data link layer mechanism that stops a fast sender from overwhelming a slow receiver with frames that its buffers cannot hold. The receiver gives feedback so that the sender sends only what the receiver can accept.

1. Stop-and-wait

The sender sends one frame and waits for the ACK before sending the next.

 Sender   -- Frame 0 -->  Receiver
          <-- ACK 0  ---
          -- Frame 1 -->

Simple, but poor use of the channel when propagation delay is long. Efficiency = 1/(1 + 2a), where a = propagation time / transmission time.

2. Sliding window

The sender may send up to W frames before an ACK arrives. Frames carry sequence numbers; the receiver acknowledges, and the window slides forward. This keeps the pipe full.

• Go-Back-N: receiver accepts frames only in order; if frame k is lost or damaged, the sender retransmits k and all later frames. Receiver window = 1; sender window ≤ 2ⁿ − 1.
• Selective Repeat: the receiver buffers out-of-order frames, and only the damaged frame is retransmitted. Sender and receiver window ≤ 2ⁿ⁻¹.

Flow control is combined with error control (ARQ) in practice, and acknowledgements can be piggybacked on data frames.