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Adjacent and Co-Channel Congestion

Hopefully you have learned why channels 1, 6, and 11 are the best choices foroperating your wireless network (this is because 1, 6, and 11 are the only Wi-Fi

channels in the 2.4 GHz spectrum that don’t overlap with one another). An open

Wi-Fi 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 Wi-Fi operates, and often t imes 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 Wi-Fi 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 tocommunicate.

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.

 An illus tration of adjacent channel conges tion

In order to explain co-c hannel 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 c ongestion works in a similar manner: the performance is

hindered by the wait times, but the bandwidth is managed, and every device willget 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.

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 An illus tration of co-channel conges tion

Co-channel congestion is preferable to adjacent channel congestion because of the

way t he 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:

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.

The image below shows what adjacent channel and co-channel congestion look like

in inSSIDer. 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!

 

Hopefully you now have a good understanding of how Wi-Fi congestion is caused,

and how to best deal with it in the 2.4 GHz Band.

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