Why are there eleven wifi channels at 2.5 gHz when only 1, 6, and 11 won't interfere with one another? We only need three channels, right?

I’m sure we’ve all heard that we should use wifi channels that don’t interfere with one another, and that because wifi signals are broadcast at their base frequency plus or minus 15 mHz, the only way to avoid interference between adjacent units is to space their frequencies as far apart as needed to avoid interference. This is very well illustrated at the very homey web page [Link: wifiyacht.net], along with a bunch of other information.

So, pretty much, everybody recommends only using channels 1, 6, and 11. In reality those channels are 25 mHz apart, so according to the reasons given for choosing channels some distance apart, even these channels don’t meet the “requirement.”

But something is really really wrong with this whole idea.

The other day I was looking at wifi channels on my phone and happened to leave the program on as I drove home. Going along the street, I saw signal after signal on channels 1, 6 and 11. Not only did I see signal after signal, I saw as many as a half dozen signals on the same frequency at the same time.

As I drove, glancing at the phone, I occasionally saw one signal here and there on 2, 4, 5, 7, and 8. As soon as I got into a residential area, I saw many more channels being used, but never as many signals on any individual channel as the pileups I saw in the commercial area.

That makes sense. Residential systems are likely set up by consumers who haven’t gotten into setup far enough to know they can choose a channel. Commercial setups are more likely to have been set up by those very pros who tell us to use only 1, 6, and 11.

But almost every place had three to six signals on 1, and on 6, and on 11! I ask you: if it’s a problem to have possible interference on, say, channel 3 from channels 1, 2, 4, and 6, isn’t it much worse to have interference on channel 1 from other signals on channel 1? And the same for channels 6 and 11?

And what’s so magical about 1, 6, and 11? Channels 2 and 7 have the same separation as 1 and 6, but nobody tells us to use 2 and 7. The same goes for 3 and 8, 4 and 9, etc.; and of course there’s even more separation between 2 and 8, 3 and 9, 4 and 10, etc.  The best juju (magic, you see, not logic based on information) here is that if you use 1, 6, and 11, you can use three different channels in one location and get the best separation.  If you're all alone in the desert (and don't count on that any more, either!), of course, you can do what you choose.

It seems someone made a good observation about channel width and center frequency separation but did not stop to think or observe.

The idea of using only 1, 6, and 11 assumes that there are very few wifi systems in range. False.  As I said, I sometimes saw a half dozen signals on each of those channels.

It assumes that nobody nearby will use the other channels – also false. If I pick channels 6 and the guys next door use 4, 5, 7, and 8, we all will supposedly have horrible problems.

It assumes that if someone is on the same channel as you, that’s better than someone being on an adjacent channel – false.

Worse yet, none of this posing has ever mentioned the selectivity of the wifi receivers. Selectivity is one of the primary technical specs of a receiver: it’s the ability of a receiver to reject signals whose center frequency is not the same as the frequency that’s been tuned in. In truth, do signals have to be further than one channel away from one another for wifi to operate properly?

Here's something parallel that we might have a feel for: FM stations in any given area are never assigned to adjacent channels – you just never see, for instance, 99.3 and 99.5. FM receivers are not selective enough for those stations not to interfere with one another; the FCC knows that; and even in very crowded markets the stations that are that close in frequency are miles and miles apart. An area might have 99.3, but the next station up the dial would be 99.7. (Some huge markets [Los Angeles County, for instance] have so many FM channels that it’s impossible to use an FM modulator in your car to get iPod music into the tuner unless you confine your driving to a small area; go over the hill from LA to the Valley and you have to change frequencies.)

At any rate, stating it a bit differently, I think we now are not using technical information, but assuming that because we can see the overall bandwidth of signals overlapping on our iPhone apps, that the wifi signals WILL interfere with one another. False. (They may, but it’s not true that they will.)

Let’s back up a bit, too: if wifi devices WILL interfere with one another in such cases, why did the FCC set up wifi channels so close to one another? Was the technology somehow better, years ago when this scheme originated, and wifi products have gotten worse in the meantime? I doubt it. Those guys knew some things that we don’t know.  If we insist on only using 1, 6, and 11, we ignore the fact that there must have been acceptable technical reasons for the narrow channel spacing and the resulting number of channels.

Do routers use any kind of intelligence when autoselecting the frequency they use? If so, it seems to me that the best thing to do is to leave the router to select its own frequency based on what it picks up in its environment when it picks a frequency.  And to let it do that again every once in a while.

I’m not sure how this idea is to be applied when you’ve got more than one AP… do they even necessarily use different frequencies? Sorry for my ignorance about that; please fill in the blanks here. It just seems like we're being ignorant by slavishly assigning only channels 1, 6, and 11.