"Ian" <ian_agerskov@hotmail.com> wrote:
>I live in a rural area and when I am half a mile away from any house
>netstumbler (prism2.5 card) reports around -75 db noise.
>
>The signal to noise ratio appears to be accurate beacause as the signal
>level and noise level approach each other the connection drops.
First, that is *not* the Signal-to-Noise Ratio, but a raw number
for "Noise". Second, the dropped connection does not signify
accuracy, but does mean the numbers are at least indicative.
In fact, I doubt the radio actually measures noise at all. I
can't remember the details, but I've seen something suggesting
that it derives the supposed "noise" figure from the Signal
strength by comparing that with the error rate. At any given
signal strength, the bit error rate is, in theory, a function of
the SNR, from which the "noise" can be derived if you measure
the signal strength! Of course they aren't using the bit error
rate either, but the errored packet rate, so it isn't anything
like accurate.
But it does provide a very good *indication* of the conditions
which affect your connection.
>Is -75 db unusally high or is it fairly common to see these kind of noise
>levels in rural areas?
Depends... on what your signal strength is being reported as!
And probably also on what rate the connection is too. I'd guess
that a 20 dB or greater difference between signal and noise
means the connection will be stable. I'd guess that as it gets
down to less than 20 dB difference things begin to deteriorate
rapidly... I'd bet at 10 dB the connection is down as much as
up???
Jeff Liebermann as a trademarked Guess(tm) for these things, so
maybe he'll pitch in with some accurate details and corrections
to my simple guesses at what it all means.
Regardless, keep in mind that "noise" is everything other than
signal. If you point your high gain antenna at the sun, you'll
see more noise... but if you are using channel 9 you will have
a lot of "noise" when the neighbors use channels 6 and 11.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska)
floyd@apaflo.com 
Background Noise 
Linear Mode
