WiFi Lessons

Adjacent and Co-Channel Congestion

Hopefully you have learned why channels 1, 6, and 11 are the best choices for operating your wireless network (this is because 1, 6, and 11 are the only WiFi channels in the 2.4 GHz spectrum that don't overlap with one another). An open WiFi channel is one of the best gifts to someone setting up a wireless network, but you may not have that option. Recall that overlap is bad because of the "conversational" way that WiFi operates, and often times you will find yourself having to manage congestion. In this newsletter, you'll learn about adjacent and co-channel congestion, using conversation as a metaphor.

Adjacent channel congestion is the worst type of WiFi interference. To illustrate, think about being at a concert – there's a band playing really loud, and tons of people, each with their own group of friends. With this much going on, it's difficult to talk to your friends, and when you start to talk louder, the person next to you has to raise his voice to talk to their group. You're hearing multiple conversations happening, as well as music from the band, and it seems impossible to communicate.

The diagram below shows a theoretical model of how the above conversation scenario looks when access points on channel 4 (red), channel 6 (green), and channel 5 (blue) are all active at the same time. As one of these APs tries to talk to its clients, its transmissions become garbled because of the transmissions of the other two. This drives down the performance of all of the networks.

visual representation of wifi adjacent channel congestion

An illustration of adjacent channel congestion

In order to explain co-channel congestion, we’ll move our imaginary conversation from a concert venue to a classroom. Think back to your school days – chances are you can think of at least one class that had a student who would talk slower than the other kids, and everyone else would have to wait for their turn to ask a question. Co-channel congestion works in a similar manner: the performance is hindered by the wait times, but the bandwidth is managed, and every device will get a chance to talk to its associated AP. The diagram below depicts a wireless access point and its associated clients, which can only talk one at a time.

a wifi access point and its connected devices

An illustration of adjacent channel congestion

Co-channel congestion is preferable to adjacent channel congestion because of the way the wireless conversations are managed. As mentioned in our previous newsletter, when choosing a channel that has other networks active, try to keep at least 20dB between the RSSI levels of the networks, as illustrated below:

co-channel wifi detail

To recap, an open channel will always be best when deploying your wireless network, but if you have to share a channel, that’s okay too. Adjacent channel congestion is the one you’ll want to avoid if at all possible.

visualization of co-channel and open channel wifi

The image below shows what adjacent channel and co-channel congestion look like in inSSIDer Office. Having a visual representation of where neighboring wireless access points are active is an invaluable tool when planning your own network. It’s easy to see how chaotic adjacent channel congestion is compared to co-channel!

wifi scanner showing you channels

Hopefully you now have a good understanding of how WiFi congestion is caused, and how to best deal with it in the 2.4 GHz Band.

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Understanding RSSI

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