[Server-devel] [XSCE] Large groups of XO-1 do not work with access points

[James Cameron]

On Fri, Feb 07, 2014 at 09:29:46AM -0500, Tim Moody wrote:
> [Don't forget that James wrote:]
> >If you choose to deploy with interfering channels for some reason,
> 
> Many routers support narrowing the band around channels.

No, it is more correct to say that many routers support widening the
band around channels, to achieve a higher data rate.

Consider a spectrum; the X axis is frequency, the Y axis is signal
strength.  The bandwidth of the transmitter is a curve shaped like a
hill.  The bandwidth of the receiver is another curve.  The channel
selects the centre X position of the hill.

The certification defines an area as a rectangle, and routers and
devices pass certification based on spectrum analysis of the
transmission, not of reception.

There is always an interference between all channels, but the standard
defines the limits on the interference in quantifiable terms.  But
these can be translated simply as:

- the interference between 1 and 11 is trivial,

- the interference between 1 and 6 is low,

- the interference between 1 and 2 is significant.

- the interference between 1 and 1 is extreme.

The interference causes backoff, where a node that would transmit
waits a bit longer, and collisions, where a node receives a corrupted
transmission.  Both of these act to reduce total system data
bandwidth.

Given fixed channels, the interference increases as the distance
between the interfering devices decreases.

> Are you of the opinion that this is ineffective and that there is
> always a 5 channel overlap?

I'm of the opinion that widening the bandwidth used around channel 6
will impact channel 1 more.

> Of course, we do not always have control over the entire
> environment, so 1, 6, and 11 may already be taken and we have no say
> in the matter.

It is more complex than that.  While you might see another access
point on a channel, you might still be able to deploy your own access
point on the same channel, and the two access points will cooperate in
a limited fashion.  They listen before they transmit.  So too do the
laptops.  The TCP/IP rates are reduced.

If you are faced with 1, 6, and 11 all taken, and you wish to run
one access point, then find which of 1, 6 and 11 have the least
received power, and use that.

If you are faced with 1, 6, and 11 all taken, and you wish to run two
access points, then find which two of 1, 6 and 11 have the least
received power, and use them.

If you are faced with 1, 6, and 11 all taken, and you wish to run
three access points, use 1, 6, and 11.

If you wish to use a channel outside 1, 6, and 11, then you would do a
more complex survey, to find the channel with the least received power
for both your access point and your devices.  Then measure the TCP/IP
rates you get.

That survey is invalidated every time someone walks in with an Android
or iOS phone or a portable hotspot; these act as an access point.

-- 
James Cameron
http://quozl.linux.org.au/