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The general wisdom is that it is better to only ever use channels 1, 6, and 11 as they are the only three non-overlapping channels for 2.4ghz wifi (in the US), and two adjacent networks on the same channel would perform better than two adjacent networks on different (but overlapping) channels.

So then, why would the IEEE even allow for 11 channels in the specification? Is there any possible use case where it would be desirable to use one of the in-between channels? Even if you were completely isolated from other networks, it wouldn't make sense to use other channels because you would then be limited to only two, rather than three non overlapping channels/access points.

I have read many articles explaining why its better to stick with 1, 6, and 11, but I have yet to come across any explanation for why they didn't just make three channels to begin with (1, 2 and 3 mapping to 1, 6, and 11 respectively).

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I was wondering the same once actually. –  Dzarda Jan 31 at 16:28
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I've heard a theory that aliens use those "spare" channels for communicating with each other. I know it's a sensible question but I couldn't help myself, sorry... I have upvoted the question because it would be cool to hear a real reason. –  Andy aka Jan 31 at 17:56

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up vote 4 down vote accepted

802.11 specifies operation in the 2.4 GHz band. This is one of the industrial, scientific and medical (ISM) radio bands, which are used for quite a lot more than Wi-Fi. The ISM bands were in fact allocated for non-telecommunications use, where devices that must necessarily spew RF interference may do so. In the US Wi-Fi gets to operate here under FCC part 15 on the condition that if a Wi-Fi device receives interference, it just has to deal with it. I'm sure other countries likely have similar regulations.

There's no requirement that the other applications in this band (RF process heating, microwave ovens, medical diathermy machines, etc.) will conform to the 802.11 channels. Thus, the ability to move the channel to some degree smaller than the channel width may be useful to avoid interference from or to non-Wi-Fi devices.

It's also worth noting band allocations are not the same in all countries. In some places that aren't North America, it's possible to get three non-overlapping channels on (1, 6, 11), but also (2, 7, 12) or (3, 8, 13).

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The issue isn't just interference from non-WiFi devices, but also interference to them. An analog 2.4GHz video transmitter might "work" even when a WiFi device happens to use a frequency within its bandwidth, but the picture quality would be degraded. If two such transmitters were being used at different frequencies, it might be possible that a WiFi "channel" of e.g. 4 might avoid clobbering the video on either side, even though 3 or lower would clobber the video below, and 5 or higher would clobber the video above. –  supercat Jan 31 at 18:34
    
Great explanation! And thanks @supercat for pointing out that sometimes its the WiFi interfering with other devices (though not a big concern for most home users). –  AndrewH Feb 2 at 4:23

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