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Discovering computer networks: hands on in the Open Networking Lab
Discovering computer networks: hands on in the Open Networking Lab

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8.2 Collision avoidance and multiple access in Wi-Fi

In all networks, ‘access’ is a major consideration. ‘Access’ in this context means ‘access to the underlying communication medium in order to send and receive data’; and by ‘underlying communication medium’ we mean things like cables, radio channels and optical fibres.

If many people use the same medium, the term ‘multiple access’ is used. Designers of multiple access systems have to deal with questions like ‘How do we ensure that one person’s data doesn’t interfere with someone else’s?’ and ‘How do we ensure that all users get fair access to the medium?’

The procedure used for multiple access in Wi-Fi is effective, in the sense that it can cope with many users in a small area, but at the cost of increased latency, or delay. As you will see in the following video, delays are part of the system used for multiple access in Wi-Fi. This connects with the video you saw at the end of Section 2.1.

Now watch the video below, which is about 6 minutes long. It is narrated by a colleague.

Collision avoidance and multiple access in Wi-Fi

Download this video clip.Video player: 53_collision_avoidance.mp4
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Activity 2 Test yourself

15 minutes

  • 1. After the first frame of someone’s data has been successfully transmitted, subsequent frames in the same data transfer are given a short check period. What is achieved by using a short check period in this case?

  • It should ensure that all frames are sent in unbroken succession, without the need to enter a contention period again. Going into a contention period would probably result in loss of control of the radio channel, and hence cause delay.

  • 2. An advice column in a magazine says: ‘If your neighbour is using the same Wi-Fi channel as you, change your Wi-Fi channel so that you don’t interfere with each other.’ The advice is good, but the reason is wrong. Explain.

  • The CSMA/CA system used in Wi-Fi ensures that two users on the same channel who are within range of each other do not use it simultaneously. So there will be no interference. However, there could be additional delays before each user gets access to the channel if both neighbours use the Wi-Fi intensively. To reduce the risk of delay, it would be sensible to switch to a channel that is not being used (although this is not always possible in very busy areas).