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 From:  Jim Thompson <jim at netgate dot com>
 To:  Douglas Stringer <dougstringer at mac dot com>
 Cc:  Administrator at mac dot com, Justin Wilson <j2sw at mtin dot net>, m0n0wall at lists dot m0n0 dot ch
 Subject:  Re: [m0n0wall] Wireless Questions
 Date:  Sun, 13 Nov 2005 16:40:07 -1000
Douglas Stringer wrote:

> I agree, however it doesn't stop lots of vendors from doing it  
> (besides Vivato) i.e. Tropos!

Um, what Tropos is trying and what Vivato did are two *very* different 
things.  Vivato can actually make multiple radios work via hardware mods 
that sample the baseband's CCA line (or its equivalent).  Tropos ... 
can't, and as such, their performance stinks.   They are good at 
marketing, however.  :-)

> Even with 22+ MHZ separation between channels (1 and 11), there are  
> issues that I've seen... though I'm not sure what they are.

In the 2.4 GHz ISM band, the IEEE defines 14 channel centers 5 MHz 
apart.   In the US 11 of these can be used.

The center frequency for ch1 is 2.412GHz, and the center frequency for 
ch11 is 2.462GHz.   This is 50MHz, not 22MHz.

The spectral mask requirements for 802.11b state that the signal must be 

center frequency, and attenuated by at least 50 dB from its peak energy 


The issue is that the radios on the market don't do much better than the 
minimum adjacent channel rejection required by the IEEE standard.  For 
802.11b the required ACR is 35dB.   You can probably google my name and 
"ACR" to get the rest of the math, since its not appropriate on the 
m0n0wall list.

> Is this a case of harmonic frequencies.... or is it the radiation  
> mask extending down and beyond the 22MHz channel (and across the  
> band), or something else??

The second, essentially.

Jim