Background Noise

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.

Is -75 db unusally high or is it fairly common to see these kind of noise
levels in rural areas?

Ian

Wished I lived there. Here on Long Island, NY we are tripping over
ourselves, traffic jams..etc


"Ian" <(E-Mail Removed)> wrote in message
newsbBKe.216247$on1.125316@clgrps13...
>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.
>
> Is -75 db unusally high or is it fairly common to see these kind of noise
> levels in rural areas?
>
> Ian
>

On Thu, 11 Aug 2005 05:17:08 GMT, "Ian" <(E-Mail Removed)>
wrote:

>Is -75 db unusally high or is it fairly common to see these kind of noise
>levels in rural areas?

-90 to -100 dB here in Rural England

as wifi is supposed to work down to -82 dB or so of signal I would say
-75 dB of noise is high.


Phil
--
Remember - Global Warming is only a weather forecast :-)

"Ian" <(E-Mail Removed)> 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) (E-Mail Removed)

On Thu, 11 Aug 2005 19:40:14 -0800, (E-Mail Removed) (Floyd L.
Davidson) wrote:

>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.

Why me? Brace yourself for some muddle.

Well, -75dBm seems way too high. A quick check of two laptops running
Netstumbler show about -90dBm when I turn off all the local wireless
junk. I've seen this before with laptops that generate considerable
CPU and CCFL power supply noise. However, I've never investigated the
exact cause or if it's really a problem.

The Prism 2 chipset gets its signal strength numbers by calculating a
conversion table from the RSSI value. The noise numbers are
apparently (not sure) from the RSSI heard between symbols. The access
point will adjust the speed of the connection based upon the bit error
rate (BER). 1 bit error in 10E6 is common. The BER is totally
dependent on the S/N ratio. Therefore, the access point adjusts the
speed for a constant S/N ratio. The required S/N ratio varies
radically with the speed. High speeds need a cleaner signal than
slower speed. Therefore, the S/N observed with Netstumbler should
vary with connection speed. However, it doesn't.

What happens is that with 802.11b and compatible 802.11g, all
management frames are sent at the slowest 802.11 speed of 1Mbit/sec.
The Prism chipset uses only these management frames to determine
signal strength, noise, and S/N ratio. In fact, the S/N ratio is
broadcast in some management frames. That's why these chips are so
slow in showing changes in signal strength. There aren't that many
frames to work with. I don't know how it's done with 802.11g, but I
also suspect it's with management frames at the slowest configured
speed. I'll have to risk turning my brain to mush by reading
802.11g-2003.

Disclaimer: I'm not very confident the above guesswork is correct and
will post a correction if I find time to research how it really works.


--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
AE6KS 831-336-2558

"Jeff Liebermann" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> On Thu, 11 Aug 2005 19:40:14 -0800, (E-Mail Removed) (Floyd L.
> Davidson) wrote:
>
>>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.
>
> Why me? Brace yourself for some muddle.
>
> Well, -75dBm seems way too high. A quick check of two laptops running
> Netstumbler show about -90dBm when I turn off all the local wireless
> junk. I've seen this before with laptops that generate considerable
> CPU and CCFL power supply noise. However, I've never investigated the
> exact cause or if it's really a problem.
>
> The Prism 2 chipset gets its signal strength numbers by calculating a
> conversion table from the RSSI value. The noise numbers are
> apparently (not sure) from the RSSI heard between symbols. The access
> point will adjust the speed of the connection based upon the bit error
> rate (BER). 1 bit error in 10E6 is common. The BER is totally
> dependent on the S/N ratio. Therefore, the access point adjusts the
> speed for a constant S/N ratio. The required S/N ratio varies
> radically with the speed. High speeds need a cleaner signal than
> slower speed. Therefore, the S/N observed with Netstumbler should
> vary with connection speed. However, it doesn't.
>
> What happens is that with 802.11b and compatible 802.11g, all
> management frames are sent at the slowest 802.11 speed of 1Mbit/sec.
> The Prism chipset uses only these management frames to determine
> signal strength, noise, and S/N ratio. In fact, the S/N ratio is
> broadcast in some management frames. That's why these chips are so
> slow in showing changes in signal strength. There aren't that many
> frames to work with. I don't know how it's done with 802.11g, but I
> also suspect it's with management frames at the slowest configured
> speed. I'll have to risk turning my brain to mush by reading
> 802.11g-2003.
>
> Disclaimer: I'm not very confident the above guesswork is correct and
> will post a correction if I find time to research how it really works.
>
>
> --
> Jeff Liebermann (E-Mail Removed)
> 150 Felker St #D http://www.LearnByDestroying.com
> Santa Cruz CA 95060 http://802.11junk.com
> AE6KS 831-336-2558

Thanks for the info. After reading your post I connected the laptop to an
external antenna and the noise dropped to about -84.

Ian