Understanding RF Interference In Wi-Fi Deployments
In my last post, I looked at how to pick the best access point. This week, we will be looking at what it takes to deploy a network that actually works.
A good Wi-Fi deployment looks like something out of science fiction; you simply put the access points up on the ceiling, and suddenly you are connected to the network through the air. It is vital to realize, however, that Wi-Fi is not magic; it’s radio waves. And like radio waves, Wi-Fi is subject to interference.
When Wi-Fi signals run into something, the result is not the scratchy signal you get from your car radio; the result is silence. That is because Wi-Fi works like a walkie-talkie in that the client and the AP can both transmit, but not at the exact same time. If a device runs in to interference, it will back off, wait for a certain amount of time, and then try to transmit again. The result is that your throughput can become very slow.
Many people are aware of the fact that Wi-Fi can be subject to interference, and they will try to get around the problem by deploying more access points. Ironically, that can often be the source of the problem. While it is well known that a concrete staircase interferes with Wi-Fi, it is less well understood that other Wi-Fi devices also create interference. So if access points are spaced very closely, they will actually interfere with each other!
This potential problem can become even worse if there is not some attention paid to the channels that the APs are running on. Wi-Fi signals are 20-22 MHz wide, and the channels overlap each other. For example, while there are eleven channels available in the 2.4GHz band in the U.S., there are only three that don’t share some spectrum.
While you want coverage cells to overlap a bit so that mobile clients can roam, if the coverage cells’ channels overlap (for example AP1 is on channel 1, and AP2 is on channel 2), there will be what is called adjacent channel interference. That is because both APs are trying to use channels that share a lot of that 22 MHz space; for example, channel 1 runs from 2401 to 2423 MHz, and channel 2 runs from 2404 to 2428 MHz. What you want is non-overlapping channels. This is particularly important in the 2.4GHz band, where there are only three non-overlapping channels—in the U.S., that means channels 1, 6, and 11. As you can imagine, the more APs you stuff into a space, the harder it gets to ensure that there is no channel overlap.
Interference becomes a huge problem when you consider how many access points you will need for your deployment. The standard wisdom used to be to plan for each student to have a Wi-Fi device. Current projections suggest that you should plan for between three and five devices per student, given that you should expect the equipment you deploy today to last for at least four years. If you try to solve this issue with more access points, you can make the problem much worse. The key is to put the right equipment in the right place.
Next week, I will look at steps you can take to avoid interference.
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