What is a network collision?

A collision occurs when two stations transmit onto the same shared medium at the same moment, so their signals overlap and both messages are corrupted.

What does CSMA/CD stand for, and where is it used?

It stands for carrier sense multiple access with collision detection, and it is used on shared wired Ethernet segments.

Describe the steps a station follows under CSMA/CD.

It listens to the medium, transmits if it is idle while continuing to monitor, and if it detects a collision it stops, sends a jam signal, waits a random interval and then listens and tries again.

What is the purpose of the jam signal?

It makes sure every station on the segment notices that the collision happened, so no station accepts the damaged frame as valid.

Why must the wait after a collision be a random length?

If all the colliding stations waited the same length they would begin transmitting together again and collide immediately; random waits make them restart at different moments.

WJEC A-Level Computer Science 601-5345-3

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Networks and Data Transmission

Collisions and shared media

3 things to know
AS Unit 1

Overview

Network collision, steps a station follows under CSMA/CD and purpose of the jam signal.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Explain the hidden node problem.

Key term

How does a switch reduce collisions compared with a hub?

Key term

Why can a collision still happen when every station listens before transmitting?

Questions from this subtopic

Each one with the model answer underneath.

1
What is a network collision?
Model answer
A collision occurs when two stations transmit onto the same shared medium at the same moment, so their signals overlap and both messages are corrupted.
2
Why can a collision still happen when every station listens before transmitting?
Model answer
A signal takes a finite time to travel along the medium, so a second station can hear silence and begin transmitting before the first station's signal has reached it.
3
What does CSMA/CD stand for, and where is it used?
Model answer
It stands for carrier sense multiple access with collision detection, and it is used on shared wired Ethernet segments.
4
Describe the steps a station follows under CSMA/CD.
Model answer
It listens to the medium, transmits if it is idle while continuing to monitor, and if it detects a collision it stops, sends a jam signal, waits a random interval and then listens and tries again.
5
What is the purpose of the jam signal?
Model answer
It makes sure every station on the segment notices that the collision happened, so no station accepts the damaged frame as valid.
6
Why must the wait after a collision be a random length?
Model answer
If all the colliding stations waited the same length they would begin transmitting together again and collide immediately; random waits make them restart at different moments.

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