Adjacent Channel Interference (ACI)
Let’s say North tower is a luxury building with a rich family occupying all four floors. Since having two speakers in N5 resulted in one speaker interfering with another via CCI, the family has decided to move the green speaker to a lower floor. However, as shown on the diagram above, the Green and the Red speaker sound still overlaps. Given there is floor/ceiling in between as a separation of two speakers, actual overlap may (or may not) be less than two speakers on the same floor. For APs, this is same as saying moving it to the different channel can still have potential overlap or interference.
Essentially, think channel as just another dimension where Wi-Fi signals can be separated just like speakers can be separated on the same floor but far apart (physical distance) or separate by floor (channel). The form of separate channel signal interfering with another channel is called Adjacent Channel Interference (ACI). This is actually considered to be worse than the CCI (ref). Why?
In the case of CCI, if something goes wrong it will slow down the system but you still have CCA rule in attempt to minimize data collision.
However, with ACI, two different channels have no way to know what the other is doing, so they will each keep sending data but keep colliding so impact is much more significant.
Also, another way to look is to consider ACI as a constant source of noise for your AP. Since Wi-Fi connection performance depends Signal to Noise Ratio (SNR), increase in the noise will make SNR lower; hence, overall, range and throughput performance of the Wi-Fi will be reduced.
Adjacent Channel Interference (ACI) is worse than Co-Channel Interference (CCI) due to lack of clear channel assessment (CCA) rule institution and raising noise level i.e. decreasing Signal to Noise Ratio (SNR).
Orthogonal Channel
So how far is far enough channel distance/separation so there is no overlap?
Orthogonal channel simply means non-overlap channel. In US, there are 11 channels for 2.4 GHz band at channel width of 20 MHz. So here we are saying the apartment complex has 11 floors.
Three of them do not overlap to each other: 1, 6 and 11. In the analogy, you have to be 5 floors apart to completely eliminate any chance of hearing the neighbor.
With 5 floor away (channel separation) rule, one can alternatively find a pair of orthogonal channels 2 and 7, 3 and 8, 4 and 9 and 5 and 10. However, 3 channels are better than 2 channels. If you are a realtor, you want to have maximum occupancy of your building. So majority of internet talks about 1, 6 and 11 as 2.4 GHz band as the golden rule, non-overlap (orthogonal) channels.
majority of internet talks about 1, 6 and 11 as 2.4 GHz band non-overlap (orthogonal) channels.
5 GHz channels are all non-overlap channels.