[Server-devel] Technical questions

[akleider at sonic.net]

Thank you, James, very much for breaking down the issues and warning of
the pitfalls.
I for one would be interested in knowing if you can comment about 
specific hardware (hopefully things currently available) that might serve
as examples of some of the things you've discussed:
i.e.: access points, improved antenna systems, larger omni directional
antennas
Thanks again.
Alex



> On Wed, Jan 20, 2010 at 05:43:31AM -0500, Reuben K. Caron wrote:
>> And by non-overlapping ranges he means you can ONLY USE channels 1, 6,
>> and 11, spaced out so that two APs with channel 1 do not overlap.
>
> For collective interest ... the air is a commons, divided by
> frequencies.
>
> The radios in the access points and the laptops switch rapidly between
> transmitting and receiving, and they do this using a timing beacon
> provided by the access point.  They share in time.
>
> Access points on separate channels (frequencies) do not hear each other
> directly, and so they are unable to collaborate well on sharing the
> commons.  Often one receiver may be trying to receive from a distant
> transmitter while a nearby transmitter on a separate channel transmits.
>
> A radio receiver is less sensitive if there is a nearby transmitter.
> This is called desensitisation, and it manifests as higher noise or
> lower signal level in the receiver.  If the noise is high enough, or the
> signal low enough, then the packet is not decoded and a retry must
> happen.  Retries pollute time commons.
>
> The amount of desensitisation depends on the difference in frequency;
> and falls off more sharply the greater the difference.
>
> For typical access point hardware, the desensitisation caused by five
> channel spaces is sufficiently low to provide reasonable range; hence 1
> vs 6 vs 11.
>
> Slightly more range can be had by reducing the number of access points
> and improving the antenna systems.
>
> To cover a larger open area you may gain greater overall performance by
> restricting the number of access points to 3, using a larger
> omnidirectional antenna and increasing their height above ground.
>
> To cover a larger closed area you may gain performance by using
> directional panel antennas that face all in the same direction ...
> creating a series of cells.
>
> Above all you should test performance before and after making a change;
> with all radios fully active and operating.  Theory is helpful, but
> reflections from metal surfaces in a built environment can mock theory
> badly.
>
> --
> James Cameron
> http://quozl.linux.org.au/
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