Wireless bridge

Thu Jun 18 16:38:00 UTC 2009

> -----Original Message-----
> From: Lyndon Nerenberg [mailto:lyndon at orthanc.ca]
> Sent: June 18, 2009 12:11 PM
> To: Peter Boone
> Cc: nanog at nanog.org
> Subject: RE: Wireless bridge
> 
> On Thu, 2009-06-18 at 11:54 -0400, Peter Boone wrote:
> > Oh I know. Luckily it's located in an industrial area just on the
> > outskirts
> > of the city. There isn't a lot of other WiFi (in my opinion); 3-5
> > total
> > SSIDs spread across 2 of the 3 physical channels (1,6,11) depending
> on
> > which
> > rooftop you measure from.
> 
> 2.4 and 5GHz license-free Wifi is license free because the frequencies
> are shared with the ISM (Industrial/Scientific/Medical) services. In an
> industrial area, competing WiFi is the least of your worries. These
> frequencies are also used by industrial grade heating units. Got anyone
> in the neighbourhood running a large plastic shrink wrap machine, for
> example?

Within range of the beam, not that I know of. The biggest building is just a
supplier, there's 2 other small buildings, not 100% sure what they do
though.

> You can't directly detect these other users with a Wifi transceiver.
> Depending on the nature of the interference you *might* be able to hear
> it directly on a scanner (if you can find one that covers those
> frequencies), but you really need a good spectrum analyzer to tell
> what's going on.
> 
> Anyway, don't assume the competition for spectrum is only other Wifi
> units.
> 
> --lyndon

I don't have a spectrum analyzer available to me (I've found a USB one for
$200 designed for WiFi that will pick up any non-wifi noise around the
frequency range too). Each access point reports a good signal. From what I
recall (not on site today) the noise is very minimal. Noise anywhere from
-98 to -85 with the signal at -20 to -40. The SNR is 30+, even when the
connection isn't working. The DDWRT firmware reports a Signal Quality as a
percentage as well: it's generally high, 80%+ (not sure exactly how it's
calculated though, I've seen it fluctuate while the Signal and Noise remain
about the same). These readings are consistent at both access points, and
remain about the same on each of the 3 physical channels. Hard to tell for
sure since the firmware doesn't keep any averages or historical statistics
on the signals, and no one has the time to sit around and take a reading
every few minutes.

Peter