What Causes Wi-Fi Interference?
“The Wi-Fi sucks!”
“My Wi-Fi isn’t working.”
“Every time someone uses the microwave, my internet goes down.”
These complaints plague IT admins all over the world and usually stem from interference issues. Interference in a wireless network occurs when the signal sent by a transmitting device is somehow disrupted in transit to the receiving device causing a degradation of the wireless experience (and performance).
There are two classifications of interference, non-Wi-Fi sources and (you guessed it) Wi-Fi sources. Of the two, non-Wi-Fi sources are more often discussed (especially in the 2.4GHz spectrum) yet they are tougher to suppress. Wi-Fi sources of interference, however are much simpler to control for, but are also easier to miss.
Here, most discussions would turn to 2.4 GHz and cite the myriad non-Wi-Fi devices that can cause interference on your network. Because we already covered this topic recently, I will focus more Wi-Fi sources of interference and how co-channel interference (CCI) comes into play with respect to 802.11ac.
What do you know about Wi-Fi sources of interference? Consider the following scenario:
At a hotel conference, people go into two ballrooms that are next to each other and start up conversations with each other. With only two people per ballroom, it is very easy for them to hear what the other is saying. Also, there is good noise separation between each ballroom at this point.
Now let’s add more people to the ballrooms. The sound grows. If two people want to hear each other, they must either get closer or talk louder. As the room gets louder still, fewer people can actually have complete conversations. Not only can the people in one ballroom no longer hear, but the sound also starts to bleed over into nearby rooms, making it harder for those people to hear each other as well.
Extrapolating from this scenario, replace the people with wireless devices and the ballrooms with channels. The group of people in one room trying to talk --yet making so much noise that nobody can hear-- is called co-channel interference (CCI). Noise bleeding through the walls and interrupting conversations in other rooms is akin to adjacent-channel interference (ACI).
To go a little deeper, adjacent-channel interference arises due to the nature of phase modulation. When a device transmits, it does not create a clean signal, but instead uses a width of the frequency band (or bandwidth) as shown below. In doing so, said device can overstep the boundaries between channels (i.e. use a frequency more than 10 MHz away from the channel center) and create some noise for a neighboring device. ACI results in a lower SNR for clients and slightly reduced throughput.
Figure 1: Channel mask overlap (the cause of ACI)
Co-channel interference occurs when two devices (generally APs) in range of each other are set to use the same channel. This setup results in more contentions for the medium. Whenever AP1 needs to transmit, it must wait for AP2 (or any other device on the channel) to finish transmitting first. Therefore, instead of both APs transmitting at throughput T, they both end up transmitting at about .5T (or less in higher density situations).
Figure 2: CCI visualized
Interestingly, CCI is not making a huge comeback with the advent of 802.11ac and bigger channel widths. Instead, there are new challenges.
Next time I will talk about getting the most out of your 802.11ac deployment.
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