Adjacent Channel Congestion

You know 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 Wi-Fi channels in the 2.4 GHz spectrum that don’t overlap with one another). Today you’ll learn about adjacent and co-channel congestion, using conversation as a metaphor.

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 times you will find yourself having to manage congestion.

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 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.

 

An illustration of 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.

An illustration of co-channel congestion

An illustration of co-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:

 

An illustration of co-channel congestion

 

 

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.  

 

Co-Channel vs Adjacent Channel congestion

 

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!

 

Co-Channel and Adjvacent Channel congestion in inSSIDer

 

By now, you have a pretty good understanding of how Wi-Fi congestion is caused, and how to best deal with it in the 2.4 GHz Band. In the next Wi-Fi Tips & Tricks newsletter, we’ll look at the differences between the 2.4 GHz and 5 GHz Bands.

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