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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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2 Answers

up vote 6 down vote accepted

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
    
@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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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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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
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