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Wifi channels 1, 6 and 11 do not overlap.

However, any channel in between them does.

http://i.stack.imgur.com/GVjVb.jpg

e.g. channel 3 would use some of the frequency band of channel 1 & 6, and channel 9 would use some of the frequency band of channel 6 & 11.

Why would one choose to use channel's other than 1, 6 or 11 if that is the case.

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3 Answers 3

up vote 5 down vote accepted

Cisco has a deployment page that illustrates this. The problem comes from having the center frequencies on 5kHz separation, but with 22MHz wide passbands. Normally, in a radio frequency assignment plan, you have for example a 12.5kHz passband and channels on center frequencies every 12.5kHz. Adjacent channel interference usually means you assign out every other channel in a local area, unless the spectrum starts getting crowded.

Because of the insane amount of overlap on 802.11, in a close area, say a warehouse, you can only use 1, 6, 11 without adjacent channel interference. Down the street where the signal falls off, someone else could use channels 2 & 7 simultaneously, a little further on, 3 and 8, and so forth.

As to the reason for the overlap, I'm guessing that they had too much faith in their spread-spectrum modulation scheme they were using when the specs were created.

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4  
That paper is for setting up multiple APs near each other, not for setting up your single AP to avoid colliding with the transmissions of your relatively quiet neighbors. –  endolith Oct 13 '12 at 1:34
    
And was very useful in our rather non-quiet neighborhood for cutting down on interference. Ranch-style houses are very RF transparent, 5/8 plywood and sheetrock let it through pretty readily. –  Fiasco Labs Oct 13 '12 at 2:26
    
@endolith - And in the time since this was posted, I now have 19 access points popping up in the listing. Mobile devices have converted neighborhoods into the density the paper was intended to alleviate. Not that relatively quiet anymore. "Sitting in my back yard on my Android Device with WiFi Spectrum Analyzer app." –  Fiasco Labs Jul 8 at 19:05

IEEE 802.11 signals are designed to partially overlap!

So, go ahead and use those other channels!

A lot of people mistakenly consider IEEE 802.11 signals like solid cars on a multi-lane highway. They frown upon people driving over the lines, partially occupying more than one lane.

However, Wifi signals are rather like coloured plumes of smoke. Along the open lanes, the colour plumes are allowed to intermingle. As long as I can still tell the colour of my plume of smoke at the end of the road, all is fine. The partial overlap of differently coloured plumes is then like a grey mist of noise to my signal. This is the principle of spread spectrum communication.

For this very same reason, in moderately congested neighbourhoods, one stands a very good chance to benefit from not sticking to the proposed 1-6-11 channel scheme.

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And yet the throughput degrades very heavily when you have 6 stations all on the same channel with a couple in the distance. Moving off that channel does wonders for getting your channel throughput back. –  Fiasco Labs Jul 8 at 19:07
    
@FiascoLabs I completely agree! Following your rightful remark, I expanded a bit more about the benefits of not sticking to the 1-6-11 channel scheme. Check out that link! –  Serge Stroobandt Jul 9 at 10:26

It is because other people use those channels, and as such, having a overlapping but less crowded channel is better than having the same channel as someone else. It would have some contention, but not as much

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Are you sure? Wouldn't using, say channel 4 just mean you conflict with both channels 1 and 6, rather than just conflicting with one of them? (And Cisco's test data confirms this.) –  David Schwartz Jan 25 '12 at 2:48
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as opposed to totally conflicting with everyone on the exact same channel? –  Journeyman Geek Jan 25 '12 at 3:33
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The WiFi system has logic to handle total conflicts and that logic works well. Partial conflicts, and twice as many of them, have a worse impact on throughput. And, again, Cisco's test data confirms this. –  David Schwartz Jan 25 '12 at 4:01
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@DavidSchwartz - important point. The reference by Fiasco Labs seems to contain the results by Cisco you mention. See also superuser.com/questions/443178/…. –  lxgr Jan 15 '13 at 13:17

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