First, I wasn't flaming, though I admit it was a "trick question".

Its true that the transmit masks of the "non-overlapping channels" (1, 
6, 11) don't overlap.  But none of the radios on the market have enough 
selectivity to eliminate in-band signals from an adjacent (25Mhz away) 
or alternate (50MHz away) channel.

Minimum IEEE specs for a 11b receiver is 35dB of adjacent (25MHz away) 
channel rejection.  IEEE doesn't publish specs for alternate channel 
rejection, but I can tell you that the best designs (in terms of ACR) 
are the "old" super-het receivers.  Intersil (now Connextant)'s 
Prism2/2.5 is good for about 41dB of ACR.   Alternate channel rejection 
is perhaps 20dB more with this chipset.

That means that if you're operating (your Prism 2/2.5 chipset radio) on 
channel 11, a signal on channel 1 will be 60dB down from where it would 
be if your radio was on channel 1.

Path loss in the first meter @ 2.4GHz is 41dB.   Lets say you've got a 
garden-variety radio that puts up 32mW (15dBm) of tx power, and ignore 
antenna gain for now (so 0 dBi antennas on both radios).

15dBm - 41dB - 60dB = -86dBm   This is the in-channel 'noise power' of 
the alternate channel radio.  Notice that it is at least 15dB above the 
thermal noise floor.  Translated:  you've lowered your SINR.

Thus, even if you pick "1 and 11", you're going to end up with 
significant in-channel power from any operate on the alternate channel 
by a close-by transmitter.

Now, if you add any antenna gain to the equation, then, unless you 
manage to find really special antennas, (or use channel filters, etc) 
then the gain of both antennas affects the above in a negative way.

The situation is (much) worse with 11g.   Due to their (quite common, 
but non universal) direct conversion archectures, and some interesting 
properties of OFDM, most 802.11g receivers  can't muster even the 35dB 
of ACR that the IEEE specifies when operating in the 802.11g modes.

It varies by modulation rate, but the minimum ACR for 802.11g at 6Mbps 
is 16dB (alternate is 16dB more, for 32dB).  This goes down to -1dB 
(yes, -1dB) ACR @ 54Mbps (with alternate channel rejection at 54Mbps 
down to 15dB.)

If you try the obvious experiment, be sure to make sure that you've got 
the same "range" from both cards operating as from one card operating.

1) establish range for first card with no second card operations
2) enable 2nd card
3) re-check range of 1st card while 2nd card is under full operation 
(passing a lot of traffic)

You'll find it appears to work if the clients are at close range.

11a isn't immune either.

Jim

> Jim,
> 
> Assuming it's 802.11b and you're in the U.S., why couldn't you have three
> separate WLANs on channels 1, 6, and 11?
> 
> Thanks,
> Christian
> 
> 
> On 10/8/04 2:02 PM, "Jim Thompson" <jim@netgate.com> wrote:
> 
> 
>>Christian Kuhtz wrote:
>>
>>>Why?  There's nothing preventing you from assembling a system, with several
>>>802.11b or 802.11a cards, for example, and using them simultaneously (as
>>>long as you use channels which don't interfere).
>>>
>>>I think it all depends on the specifics of what is attempted.
>>
>>OK, just for the sake of discussion.  Lets put two 802.11b or 802.11g
>>cards in the system, fit them both with the antenna(s) of your choosing,
>>and locate those antennas 1m apart.
>>
>>Please let me know which two channels you pick.  Be sure to also specify
>>the transmit power of the cards, your intended modulation rate, etc.
>>
>>Jim
>>
>>
>>>But thanks for the flame anyway.
>>>
>>>Cheers,
>>>Christian
>>>
>>>On 10/6/04 3:59 PM, "Jim Thompson" <jim@netgate.com> wrote:
>>>
>>>
>>>
>>>>Christian Kuhtz wrote:
>>>>
>>>>
>>>>>On 10/6/04 11:12 AM, "John Mullen" <jomullen@nmsu.edu> wrote:
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>If you want to receive on two or more channels that are simultaneously
>>>>>>active, you must have dedicated circuitry for each channel.  Otherwise,
>>>>>>the RF signals will mix and interfere with each other, causing data
>>>>>>collisions, rather than distinct data streams.
>>>>>
>>>>>
>>>>>Seems that's a fairly easy to solve problem.  Anyone could cobble this
>>>>>together today, and there's no real reason why you couldn't do that in a
>>>>>more integrated fashion if there is market demand.
>>>>
>>>>Obviously, you should file the patents now.  You'll shortly be proven
>>>>wrong, or become a very rich, and very famous man.
>>>
>>>
>>>
>>>*****
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>>>this in error, please contact the sender and delete the material from all
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>>
> 
> 
> *****
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