---
title: "Flow control in data communication network"
subject: "Telecommunications"
chapter: 8
key: "semester_8/telecommunications:flow-control-network"
source: https://www.bctengineer.com/q/semester_8/telecommunications/flow-control-network
site: BCT Engineering
---

# Flow control in data communication network

Telecommunications · Chapter 8: Data Communication

- Asked in: 2069 Chaitra, 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_8/telecommunications/flow-control-network
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-8

## Question

What is flow control in data communication network? Explain.

## Short answer

Flow control regulates sender data rate to prevent receiver or network buffer overrun, avoiding packet loss and retransmissions. Key methods include stop-and-wait, sliding window (HDLC, TCP), and credit/rate-based schemes used at link, network access, and end-to-end levels.

## Answer

**Flow control** in a data communication network is the technique that controls the rate at which a sender sends data so that the receiver (or an intermediate node) is not flooded with more data than its buffers and processor can handle. Without it, frames are lost, must be retransmitted and throughput falls.

**Why needed**
- Sender and receiver differ in speed and buffer size.
- Receiver must also process data (check errors, pass to higher layers).
- Network nodes have limited buffers; overflow causes congestion.

**Main methods**
1. **Stop-and-wait:** send one frame, wait for ACK, then send the next. Simple but inefficient when propagation delay is large ($U = 1/(1+2a)$, $a = t_{prop}/t_{frame}$).
2. **Sliding window:** sender may have up to W unacknowledged frames; ACKs slide the window. Receiver can send RNR to stop. Efficient; used in HDLC, X.25, TCP.
```text
  [0 1 2][3 4 5 6 7][8 9 ...]
  ACKed   window=5   not yet allowed
```
3. **Start–stop:** XON/XOFF characters or RTS/CTS hardware lines.
4. **Credit / rate based:** receiver grants credits (TCP window); or source is limited to a rate (leaky/token bucket).

**Levels:** hop-by-hop (between adjacent nodes, data link layer), network access (host to network) and end-to-end (between hosts, transport layer).

## Check yourself

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

1. Why is flow control needed?
   A. Sender and receiver may differ in speed and buffer size, causing overflow
   B. To increase noise immunity
   C. To assign addresses
   D. To select the shortest path
   Answer: A. Different speeds and limited buffers mean a fast sender can flood the receiver, causing loss and retransmission. (Key point: Why flow control is needed)

2. Why is sliding window more efficient than stop-and-wait?
   A. It sends no ACKs
   B. The sender may have up to W unacknowledged frames outstanding
   C. It uses a shorter frame
   D. It removes the receiver buffer
   Answer: B. Several frames can be in flight before an ACK, so the link is kept busy on long-delay paths. (Key point: Sliding window vs stop-and-wait)

3. What is the role of RNR in sliding window flow control?
   A. Requests retransmission of a frame
   B. The receiver tells the sender to stop (Receive Not Ready)
   C. Starts a new connection
   D. Increases the window size
   Answer: B. RNR closes the window so the sender stops until the receiver is ready again. (Key point: RNR)

4. Which is end-to-end flow control?
   A. HDLC window between adjacent nodes
   B. X.25 packet level host to network
   C. TCP window between source and destination hosts
   D. XON/XOFF is not flow control
   Answer: C. TCP's window works between hosts; HDLC is hop-by-hop and X.25 is network access. (Key point: Levels of flow control)

## Related in Telecommunications

- [Flow control mechanisms in data communication](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aflow-control-mechanisms) — Telecommunications, chapter 8
- [Routing, flow control and dynamic routing](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Arouting-flow-control-dynamic) — Telecommunications, chapter 8
- [Switching techniques in data communication](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aswitching-techniques-data-comm) — Telecommunications, chapter 8, asked 3×
- [Short note: Routing algorithm in data communication](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Arouting-algorithm-data-comm) — Telecommunications, chapter 8, asked 2×

## Related in other subjects

- [Short note: DLL flow control](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Adll-flow-control) — Computer Networks and Security, chapter 3
- [Flow distribution in five-pipe network](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Anetwork-five-pipes-flow-distribution) — Hydraulics, chapter 3
- [Control methods for interface communication](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Ainterfacing-control-methods) — Embedded System, chapter 5
- [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