Choosing the lesser evil
Let’s say you are setting up a Wi-Fi access point. You launch some simple analyzer tool to check which channel to configure your AP for and see something like this:
Given the fact that there is no way to avoid any interference, which channel would you choose?
There is a temptation to put your new AP on channel 3 as it looks like it has the lowest Received Signal Strength from foreign APs.On the other hand we have a recommendation to use channels 1, 6 and 11 of the 2.4GHz ISM band for the majority of countries. So maybe we just use channel 1 and hope that our signal will be heard well?
Two types of Wi-Fi interference
If we set our AP to channel 3 the signal spectrum will partially overlap with the AP configured for channel 1. This is called Adjacent Channel Interference:
If we set it to channel 1 then what we will have is called Co-Channel Interference. The whole spectrum of both signals will overlap:
Wi-Fi is often called polite protocol. In practice, this means that APs and clients do their best to avoid collisions during data transmission. In short if someone is already using the channel wireless device backs off and tries to transmit its data later. All this works when one wireless device “hears” other devices data (i.e. is able to see the actual Layer-2 frames).
What happens in case of adjacent channel interference is that you have disruption on part of the channel spectrum. The signal from an AP operating on an overlapping channel is perceived as noise. The collision avoidance mechanism don’t work, both APs keep sending data whenever they want, but large part of the data comes corrupt to its destination. AP tries to change data rate and coding scheme to adapt to noisy environment and then resend the data good part of which will probably come corrupt causing the whole cycle to get repeated. More than that, the interfering AP will certainly have periods of more and less intense conversations to its clients. So your AP will try to switch to lower and higher datarates during these periods which will make the situation even worse.
Of course we all would like to operate in an interference free environment where all the channel spectrum is ours, but nowadays it is virtually impossible. So if you have to choose between co-channel and ajdacent-channel interferer, choose the first one. In the presented case channel 1 looks like the best option.
Also keep in mind that there are some other factors that will affect the performance, sometimes even more. For example, how intensely the other AP and its clients utilize the channel, the presence of non-Wi-Fi interference sources and so on.