---
title: "Network congestion and its parameters"
subject: "Computer Networks and Security"
chapter: 5
key: "semester_7/computer_networks_and_security:network-congestion-parameters"
source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/network-congestion-parameters
site: BCT Engineering
---

# Network congestion and its parameters

Computer Networks and Security · Chapter 5: Transport Layer

- Asked in: 2073 Shrawan, 2+2 marks
- Video: [Network Performance](https://www.youtube.com/watch?v=5AVuUgEJu1E) — 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/network-congestion-parameters
- Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-5

## Question

Discuss network congestion. Explain how different network parameters affect the congestion.

## Short answer

Congestion occurs when offered load exceeds capacity, causing queue buildup, delay, and packet drops. Higher arrival rate, low bandwidth, small buffers, slow router processing, long latency, retransmissions, bursty traffic, and short timeouts all worsen congestion.

## Answer

#### Network congestion
**Congestion** occurs when the load offered to the network (or a router) is greater than it can handle, so queues grow, delay increases and packets are dropped. Dropped packets are retransmitted, adding more load, which can lead to **congestion collapse**.

```text
Throughput
   |      ______ ideal
   |     /   ___ with congestion control
   |    / /
   |   / /\____ congestion collapse
   +---------------------- Offered load
```

#### Parameters affecting congestion
| Parameter | Effect |
|---|---|
| Arrival rate (load) | When the rate exceeds link capacity, queues build up. |
| Link bandwidth | Low-capacity links become bottlenecks. |
| Router buffer size | Small buffers overflow and drop packets; very large buffers cause long delay (bufferbloat). |
| Router processing speed | A slow CPU cannot forward at line rate, so queues grow. |
| Delay (latency) | Long delay makes feedback slow, so senders react late. |
| Packet loss and retransmission | Retransmissions add to the load. |
| Burstiness of traffic | Sudden bursts overload queues even when average load is low. |
| Timeout settings | Too short timeouts cause unnecessary retransmissions. |

Queueing delay grows sharply with utilisation ρ. For a simple M/M/1 queue, delay = service time / (1 − ρ):

| Utilisation ρ | 0.5 | 0.8 | 0.9 | 0.99 |
|---|---|---|---|---|
| Delay (× service time) | 2 | 5 | 10 | 100 |

So congestion is controlled by keeping the load below capacity (admission control, traffic shaping, window-based control).

## Check yourself

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

1. When does congestion occur?
   A. When offered load exceeds the capacity of the network or router
   B. When a link is idle
   C. When TTL is large
   D. When packets use UDP
   Answer: A. Load greater than capacity makes queues grow, delay increase and packets drop. (Key point: Definition)

2. How can drops lead to congestion collapse?
   A. ACKs are removed
   B. Dropped packets reduce load
   C. Retransmissions of dropped packets add more load
   D. Routers speed up
   Answer: C. Retransmissions increase load further, lowering useful throughput. (Key point: Congestion collapse)

3. What is the effect of router buffer size?
   A. Small buffers cause bufferbloat
   B. Larger is always better
   C. Buffers do not matter
   D. Small buffers overflow and drop packets; very large ones cause long delay
   Answer: D. Both extremes cause problems; large buffers cause bufferbloat. (Key point: Buffer size)

4. Using delay = service time/(1 - rho), what is the delay at rho = 0.9?
   A. 10 x service time
   B. 5 x service time
   C. 2 x service time
   D. 100 x service time
   Answer: A. 1/(1-0.9) = 10. (Key point: Utilisation and delay)

## Related in other subjects

- [S-parameters and three-port network S-matrix](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3As-matrix-three-port-network) — RF and Microwave Engineering, chapter 3, asked 4×
- [T and Y parameters of bridged-T network](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_circuit_theory%3At-and-y-parameters-3ohm-bridge) — Electrical Circuit Theory, chapter 7
- [Two-port network model and parameters](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Atwo-port-network-model) — RF and Microwave Engineering, chapter 3
- [Importance of S-parameters and two-port derivation](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3As-parameters-two-port-derivation) — RF and Microwave Engineering, chapter 3, asked 3×