---
title: "Short note: DLL flow control"
subject: "Computer Networks and Security"
chapter: 3
key: "semester_7/computer_networks_and_security:dll-flow-control"
source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/dll-flow-control
site: BCT Engineering
---

# Short note: DLL flow control

Computer Networks and Security · Chapter 3: Data Link Layer

- Asked in: 2074 Asoj, 4 marks
- Video: [Flow Control](https://www.youtube.com/watch?v=ReQiSK8W3Ag) — Neso Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/dll-flow-control
- Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-3

## Question

Write a short note on data link layer flow control mechanisms.

## Short answer

Flow control prevents a fast sender from overwhelming a slow receiver. Stop-and-wait sends one frame and waits for ACK. Sliding window allows multiple outstanding frames, with Go-Back-N retransmitting all after a loss, and Selective Repeat retransmitting only the lost frame, for higher efficiency.

## Answer

**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.
```text
 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.

| Point | Stop-and-wait | Go-Back-N | Selective Repeat |
|---|---|---|---|
| Frames in flight | 1 | up to 2ⁿ − 1 | up to 2ⁿ⁻¹ |
| Retransmission | One frame | Error frame and all after | Only the error frame |
| Receiver buffer | 1 | 1 | Window size |
| Efficiency | Low | Medium | High |

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

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does flow control prevent?
   A. Bit errors
   B. A fast sender overwhelming a slow receiver's buffers
   C. Routing loops
   D. Collisions
   Answer: B. The receiver feedback keeps the sender within its capacity. (Key point: Purpose)

2. Stop-and-wait performs poorly when:
   A. Propagation delay is long
   B. Frames are short
   C. Window is large
   D. Errors are rare
   Answer: A. Efficiency = 1/(1+2a); large a (long propagation) wastes the channel. (Key point: Stop-and-wait efficiency)

3. If frame k is damaged in Go-Back-N:
   A. Only k is resent
   B. k and all later frames are resent
   C. Nothing is resent
   D. k-1 is resent
   Answer: B. The receiver accepts only in order (window 1), so the sender goes back to k. (Key point: Go-Back-N)

4. What is the maximum window in Selective Repeat with n-bit sequence numbers?
   A. 2^n - 1
   B. 2^(n-1)
   C. 2^n
   D. n
   Answer: B. Selective Repeat windows are limited to 2^(n-1). (Key point: Selective Repeat)

## Related in Computer Networks and Security

- [Data link layer framing](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aframing-mechanisms) — Computer Networks and Security, chapter 3, asked 3×
- [Functions of data link layer](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Adll-functions) — Computer Networks and Security, chapter 3, asked 3×
- [TCP flow control](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Atcp-flow-control) — Computer Networks and Security, chapter 5
- [Media Access Control and significance](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Amac-significance) — Computer Networks and Security, chapter 3

## Related in other subjects

- [Flow control mechanisms in data communication](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aflow-control-mechanisms) — Telecommunications, chapter 8
- [Flow control in data communication network](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aflow-control-network) — Telecommunications, chapter 8
- [Concurrency control goals and mechanisms](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Acc-goals-mechanisms) — Distributed System, chapter 8
- [Basic data transfer mechanisms in USB](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ausb-transfer-mechanisms) — Instrumentation II, chapter 3