[MLB-WIRELESS] Amusing high-power wifi story

Zik Saleeba zik at zikzak.net
Tue Aug 14 12:54:11 EST 2007

This story appeared on the OzAPRS (ham radio) list today and I thought
it might interest you guys.


---------- Forwarded message ----------
From: Damien Gardner Jnr <rendrag at rendrag.net>
Date: Aug 14, 2007 12:04 PM
Subject: Re: [OZAPRS] [aprssig] WiFi for tracking ?
To: ozaprs <ozaprs at aprs.net.au>, TAPR APRS Mailing List <aprssig at lists.tapr.org>

I remember a couple of years ago (a few on this list know this story) -
my work had a 666m (no shit :)  Having a sister company that does GIS
for a living made setting up the link (and choosing the location of our
new premises to ensure that the link WOULD work) a LOT easier, with them
able to do SHED plots for us..) wifi link between our office and a
client site..  There was 15m of the top of a hill between us, so we were
only pushing ~9dB S/N even with 4W out one end, 3.5W out the other.

Over a 2 year period, I'd occasionally notice a 6dB increase in noise,
which was usually 9-5 mon-fri, and go sniffing for it, and find it at
the hospital in between us.  Wrote them a few letters expressing my
concern over the amount of microwave emissions leaking from somewhere on
their campus, never got a reply, and the interference continued, so
started reporting it each time to the ACA.. (to be honest, I was more
concerned about the fact that me seeing 6dB of noise, 300m from the
hospital said to me that there was ~4W of microwave emissions AT the
hospital, unshielded..

About 2 years after we first setup the link (well, shifted from
next-door to the client (everyone laughed at my 1/4 wave whips made out
of a length of copper wire sticking out the top of an N connector, but
we had a 25dB S/N ratio ;) ), to a few blocks away), all of a sudden,
our link dropped, and nothing I could do would bring it back up..  We
quickly brought up an ipsec tunnel across the 'net, though the client
was a little touchy on that (why they thought a 4096-bit encrypted ipsec
tunnel with keys renegotiated every 10 mins was any less secure over the
'net, than it was on a non-WEP'd (though using a custom firmware, which
didn't send node broadcasts) WIFI link, I don't know).  And then set to
work trying to figure out WTF was going on..
</pre story>

I ended up 'hiring' the gear from work for a ham 'experiment', promptly
changed the SSID's to my callsign, took laptops onto the roofs of both
buildings, along with the 14dB amps (bypassing the 20M of LMR400 in one
building, and the 35M in the other), and fired her up..

With high-output wifi cards (150mW), directly into the 14dB amps, we
were pushing close to the amps' full 4W output into the 24dBi dishes at
each end.. - with a theoretical resulting EIRP somewhere around 800W
(yes, it's probably a little lower, I'm doing this in my head ;) ).  And
*still* couldn't bring the link up..  Yet taking another notebook to the
roof of the building next door, I could link up perfectly (and see both
nodes) with the S/N ratio topping out at the +50dB that the dodgy old
Enterasys roamabout maxxed out at.  We gave the link up for dead, and
disassembled it at that point.  We learned later (through an ex-employee
who ended up as a sysadmin at Canberra Imaging Group), that our problem
was that the cause of all that noise I'd been reporting to the ACA over
the years, was CIG imaging staff firing up new equipment outside of
shielded rooms while testing them.  After being harassed and audited
multiple times by the ACA for 'safety violations', they built a new
building.. - and promptly lined all outside walls with an inch of lead
sheeting.  DIRECTLY in the path of our WIFI link.

<the point>
Apparently, that Saturday morning, every WIFI network north of lake
burley griffin (note that both my antennas were 6km SOUTH of the south
shore of LBG, which is a good 6-10km across in itself) went down for the
hour I spent testing, with the cisco AP's reporting an 'insanely' high
noise level, according to the admins at ANU..  They *weren't* happy, and
it took a lot of pointing out that hams are the primary user on the band
and we CAN pump up to 150W (iirc, check the determination if you want to
know for sure :) ) *into* an antenna, without any limit on our EIRP, and
there was nothing they could do really about it, before they stopped
arguing about it...

So while amped-up WIFI would be really cool for passing around APRS
(especially for backhauls between digis in remote areas provided
line-of-sight between towers), I don't think it would be taken very well
by the plebs :)
</the point>



Damien Gardner Jnr
VK2TDG. Dip EE. StudIEAust
rendrag at rendrag.net -  http://www.rendrag.net/
We rode on the winds of the rising storm,
  We ran to the sounds of thunder.
We danced among the lightning bolts,
  and tore the world asunder

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