While I
was looking up how to spell "Johnson" I discovered that the
Johnson Nyquist thermal noise is:
Is indeed ~100dBm for a
20MHz channel. Which makes the Linux limit of -110dBm for 'no
bars' kinda dumb: the signal is 1/10th of the thermal noise
level. As I have commented before, displaying 'signal strength'
as a linear representation of the power in dBm also makes no
sense. A signal of -40dBm is about as good as it gets with the
radio within a foot or so of the AP transmitter so that is OK as
the 100% level. At -70dBm you are down to 1/1000th of that level
and that ignores whatever noise is around.
Red line: "signal strength
percent, as reported by Linux" versus dBm
On 11/9/19 1:45 PM, Bill Riemers wrote:
I will bite. How can a signal be too small to
be considered a no signal? You need a certain minimum to
receive data over the background noise. Anything smaller than
that you want to consider no signal.
I haven't given up in
the hope that NM will make a better WiFi AP selection.
From what you have told me, a "supplicant", requests the
NM to open a network connection. The NM manager then
selects something from the various WiFi APs or wired
connections. From what you have said so far, the NM
manager prioritizes potential
data bandwidth over signal strength. As a
consequence, a 5GHz signal at -100dBm wins out over a
-40dBm 2.4GHz signal, despite the fact that the 5GHz
signal one millionth
the power of the 2.4GHz signal.
So I began to research how much power -100dBm represents.
My first thought was to compare the power to an ant. Not a
good choice since an ant can put out 50mW, according to
some people. A more useful comparison turns out to be a
bacterial flagellar motor (E. coli has them, for example).
They crank out 10⁻¹² mW which is -120dBm. So for -100dBm
you need 100 E. coli.
So the point of this? -100dBm is way too small to be
considered no signal. The edge of an acceptable signal is
around the power output 100,000 E. coli.
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--
Bill
C Riemers, PhD,
CSM, CSD, SALESFORCE CPD I
Senior
Software Engineer
Red Hat
Canada Ltd
Enterprise
Sales + Services (ESSA)
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