question

[David A. Bandel]

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On Tue, 13 Jan 2004 01:27:49 -0500
Matthew Carpenter <matt at eisgr.com> wrote:

[snip of lucid wireless info]
> 
> Channel choice is also made when configuring the AP.  Most common is
> 6, although there are 11 choices, each using a combination of
> frequencies in the 2.4GHz range.  Channels 1, 6, and 11 use
> frequencies which do not overlap, whereas the others do.  So these
> channels are the most common, and are used to design wireless networks
> that can share the airwaves overlapping without interfering with each
> other.
> 

Just a note so someone doesn't assume two APs will interfere with each
other if they're not on 1,6,11.  Mutual interference (called isolation)
is only required under certain circumstances.  A general rule of thumb
is, if you want to run multiple APs where one can see the other and they
are not going to run WBS (wireless backbone system), you want 3 channels
of separation where possible.

Factors that mitigate the frequency separation need:
power output (effective radiated power - ERP) at the antenna
polarization (omnis will always be vertical, so if you have a
point-to-point, go horizontal)
for directional antennas, parasitic lobes (lobes other than the main
lobe, like the side lobes and back lobe)
vertical separation of antennas on a tower

The overlap happens because the frequencies start at 2.412GHz, are
separated by 5MHz (so are found at 2.417, 2.422, 2.427, ... ), but are
22MHz wide (11 MHz each side of the freq, so 2.412 runs from 2.401 to
2.423).  Some overlap (up to 50%) causes only minimal signal
degradation).

Other things not covered by Matt include:  speeds:
on 802.11b only cards, you have 1, 2, 5.5, and 11Mb.  At 11Mb you have
up to 4Mb throughput.  All cards b cards and b-g cards in b mode run the
management frames at 1Mb contributing to the low throughput.  b-g cards
in g only mode run management frames at a much higher rate, so
throughput is significantly higher percentage than with b cards.  Lower
speeds also improve isolation.

Even more factors contributing to performance:  size and location of
ground plane, fresnel zone encroachment, amplification (don't over-amp,
an SNR over 50 usually indicates you're overdriving your receiver
degradating performance), height of the antenna, use of external
antennas, etc.

I've successfully run 25 mile lashups w/ 24dbi point to point antennas.

To give you an idea of how complicated this can all get, the ARRL
Antenna Book is 944 pages annd makes a helluva doorstop.  But the basics
can be easily mastered.

[snip of more lucid wireless info]

Ciao,

David A. Bandel
- -- 
Focus on the dream, not the competition.
		Nemesis Racing Team motto
GPG key autoresponder:  mailto:david_key at pananix.com
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