Adjacent-Channel Interference

Author: James Rugh | Senior Data Network Engineer Network Operations

Selecting appropriate channels for your enterprise WiFi system is critical to ensure the best performance and to avoid adjacent-channel interference.

The designers of the 2.4 MHz WiFi standard allocated 11 channels in the US.  The problem is that they put them too close together. WiFi spread-spectrum technology uses not only the channel you select, but also the two channels above and below.  Imagine a bell curve with the center on channel 6.  The tails of the bell curve occupy channels 5 and 4 on the low side, and 7 and 8 on the high side.  This means your AP that is “on” channel 6 is actually using the entire spectrum from channels 4 through 8.

The only three channels that do not overlap are 1, 6, and 11.  These are the only channels you or anyone should ever use.  If you have a close neighbor using anything else, they are not being very neighborly.

What happens if you assign your AP a channel other than 1, 6, or 11?  This is where the concepts of co-channel interference vs adjacent-channel interference come into play.

Co-channel interference is another WiFi device using the same channel as you.  WiFi is designed to take that into account.  It knows the interfering device is a WiFi AP and the protocol allows your AP to back off and compete for airtime gracefully.  You might suffer a little bandwidth degradation during the time that another AP is transmitting data on that channel, but it’s not a show-stopper.

Adjacent-channel Interference
2.4 GHz WiFi environment in an office building.

Adjacent-channel interference is much worse.  Another device using a channel adjacent to yours will bleed over into yours.  Your AP will see that as random RF noise, just as if you were running a leaky microwave oven.  This can kill your bandwidth, as well as that of your neighbor.

To the right, you’ll see a screen shot of the 2.4 GHz WiFi environment in an office building.

Most of the access points in this area have been set up correctly.  But one SSID “wwssrights” is on Channel 4.  This will cause adjacent-channel interference with the APs on channels 1 and 6.  In the location where this reading was taken, its signal strength is low (about -79 dBm), but if your users were closer to it, it could be a problem.

Many WiFi systems have the ability to scan their environment at boot time or at certain intervals, and automatically select the best operating channel based on the result of the scan.  If yours allows it, you should exclude all channels except 1, 6, and 11 from the choices of candidate channels.

Here is how Arista (formerly Mojo) enables this sort of granularity in their CloudVision WiFi cloud management portal tool.

Adjacent-channel Interference
CloudVision WiFi cloud management portal tool.

The overlapping channel problem inherent to the 2.4 GHz WiFi standard is one of many reasons why the 5 GHz band is better if you have a choice.  You also have more channels to choose from, and there will be less interference from non-WiFi sources of RF noise such as wireless baby monitors and microwave ovens.