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 From:  Douglas Stringer <dougstringer at mac dot com>
 To:  Jim Thompson <jim at netgate 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 22:02:01 -0500
Yeah.. I actually got several indoor B's back in 2003 and have them  
at my shop and have deployed several outdoor B's in NYC (through  
Tanis) ---and had to keep them all on one channel per switch, btw :).

Anyway, what causes the interference if you're using non-overlapping  
channels on a WRAP with 2 wireless cards with m0n0?... and how is it  
noticed? Lower data rate?
doug





On Nov 13, 2005, at 9:40 PM, Jim Thompson wrote:

> 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  

> MHz from the center frequency, and attenuated by at least 50 dB  

>
> 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
>