John Navas <spamfilter0@navasgroup.com> hath wroth:

>>Correct. Those are the non-overlapping channels. Each channel number
>>is 5MHz wide. However, the 802.11b/g signal is 22MHz wide. If you
>>grind the numbers, that leaves you 1, 6, and 11 as the only
>>non-overlapping channels.

>Actually minimally overlapping channels -- there's no sharp cutoff at 22
>MHz -- significant energy goes beyond those boundaries. Worse, many
>wireless products now use various forms of multiple-channel transmission
>that pollutes much more than a normal channel.

See the Cisco article:
<http://www.cisco.com/en/US/products/hw/wireless/ps430/prod_technical_reference09186a00802846a2.html#wp13 4354>
at Fig 7, which shows the spectral mask for 802.11g. Note that the
signal level is -20dB down at about 10Mhz (2 channels) away. If I
assume that the garbage is a constant -20dB down at any channel that's
more than 10MHz away, I can assume 20dB of isolation.

So, how far away must two radios be isolated before the 20dB isolation
becomes a problem? Let's make it really easy (because I'm lazy) and
say the antennas have +2dB gain, are prefectly aligned with each
other, the xmitter belches +15dBm and the receiver has a sensitivity
of about -85dBm. The required path isolation for the interfering
signal to be equal to a minimal receive signal is:
15dBm + 2dB + 2dB - -85dBm = 104dB isolation
The noise level is -20dB below the signal so we only need:
104 - 20 = 84dB of path isolation
Plugging into:
<http://www.terabeam.com/support/calculations/som.php>
I find that the for 84dB of free space loss, the antennas are 0.1 mile
or 520ft apart.

So, if your neighbor and your access points are stareing at each other
(i.e. line of sight), and you're using the stock rubber ducky
antennas, and the neighbor is on Ch 3 while you're on Ch 6, you can be
528ft apart before there will be any signifigant interference.
Actually, it might a considerably closer because the receive reference
level did not include any fade margin.

Just for fun, the numbers for Ch 1 to Ch 6 show about -30dB isolation.
Using the same assumptions as before, the required path isolation is
74dB, which works out to 0.03 miles or 158ft. Again, if you are
furthur than 158 from the source, there won't be much interference.

>I've seen that behavior with Panasonic Gigarange phones. Others were
>all over the place.

I took my Wi-Spy spectrum analyzer to Office Max and tested a few
phones (before they threw me out). You're right. They do vary. A
few even hogged the entire 2.4GHz band. Unless there's a predominance
of Panasonic phones in the neighborhood, everything I can see from my
house seems to favor the bottom of the band.

>I'd say try 1, 6, and 11 first, but if results aren't satisfactory, also
>try 3, 4, 8, and 9.

Sure, try other channels.

>My speculation(sm) is that the antenna change and repositioning would
>have been enough without a channel change.

Possibly. However, I could have left the customer with a working
system simply by changing channels. Frankly, I was rather suprised at
how well it mostly fixed the problem. However, my ping tests showed
that there was still some interference so I decided to pump up my
exhorbitant charges and try to eliminate the interference completely.
Besides, I didn't want to return the antennas and pigtails I had
bought. So, the new antennas and new location finished the job. I
probably could have put the system back on the original channels, but
didn't see any reason to bother.

--
Jeff Liebermann jeffl@comix.santa-cruz.ca.us
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558