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December 2013
January 2014

2013-12-13 »

Oh good.  One more reason not to try to use 40 MHz wide channels on 2.4 GHz wifi: there is no way to do the "1, 6, and 11" trick to prevent partially overlapping channels.  40 MHz mode basically uses two adjacent 20 MHz bands, which means the two channels you eat are exactly 4 channel numbers away from each other:  But anybody using a channel less chan 4 channel numbers away from you will be partially overlapping with you, which is a worst-case scenario for wifi.


If you put 40 MHz activity on channel 1, it also uses channel 5, which interferes with channel 6.  If you use channel 6, it also uses 10 or 2, which interfere with 11 or 1, respectively.  If you use channel 11, it also uses 7, which interferes with 6.

It would be possible to have on 40 MHz channel and one 20 MHz channel in the band (1+5 and 11, for example) if we all agreed to do that.  But of course we didn't all agree to do that.

That said, it explains the strange popularity of channel 5.  It's okay for some people to use channel 1+5 and some to use channel 5; they are still competing for the same channel 5, but in a way that wifi CSMACD is designed to deal with.

So it would appear that 1, 5, and 11 is a better choice than 1, 6, and 11.

Of course there's one more complication: 802.11b carriers apparently take 22 MHz of bandwidth instead of the 20 MHz used by 802.11g and above.  Thus, channels 1 and 5 overlap slightly if you're using 802.11b.  Luckily, nowadays basically nobody is (or if they are, it's because they're super far away from their nearest AP, so probably the extra 2 MHz overlap will have especially low power and not interfere much.)

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