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RE: WiFi question
From: "Lachniet, Mark" <mlachniet () sequoianet com>
Date: Thu, 18 Nov 2004 11:42:52 -0500

Okay, enough people commented on this that I had to dig out my
documentation.  FWIW, this is what my co-worked documented.  My previous
summary was not totally accurate.  This was discovered by one of my
co-workers, not myself.


From:  employee () analysts com (name withheld to protect against spam)
Subject: Some Occupancy Sensors May Cause WiFi Interference

So what interferes with 802.11b/g wireless? So far the list seems to be
short; microwaves, 2.4 GHz cordless telephones, existing WiFi or
Bluetooth equipment...... nuclear reactors!?

Now add some occupancy sensors to the list. Specifically, Hubbell MyTech
24KHz ceiling mount sensors, manufactured about ten years ago. I've
attached a picture of a newer model; the one that I had problems with is
shaped more like a square. They're used to turn lights on and off when
people enter large rooms and to regulate heating and air conditioning.

24KHz doesn't sound like WiFi right? Most wireless devices have
emissions at some multiple of their operating frequency, in this case
10x. This is called a harmonic frequency and normally these emissions
are filtered out. Ten years ago there wasn't much going on with the
unlicensed ISM band so my best guess is that the 2.4 GHz harmonic was
not filtered out to save costs. 

I first observed the interference using our Surveyor software although
Surveyor did not detect any wireless devices. Curiously, NetStumbler
detected an infinitely increasing number of wireless MAC address on an
invisible SSID, all operating on channel 10. If I place the NetStumbler
tool next to one of the sensors, the SNR goes off the charts every time
I wave my hand in front of the sensor. A new random MAC address often
times pops up.

The MAC addresses aren't registered with any specific manufacturer. They
start out with 02:00 and are random for the remaining characters. It
might be that NetStumbler is attempting to treat the interference as an
actual WiFi device.

Anyways, it's something to look out for!

Mark Lachniet

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