How is wireless signal strength for NetworkManager calculated

[Tim]

Allegedly, on or about 19 January 2014, Ranjan Maitra sent:
> I was wondering how wireless signal strength (as displayed by the
> network-manager applet) is calculated. There is a percentage reported:
> what does this percent mean and where does NetworkManager get its
> values from? 

For what it's worth, wireless signal indicators are rarely simply just
"signal strength" indicators, but are usually a combination of things
that represent "signal goodness."  

While strength is part of the equation, there's also a quality aspect,
which may take into account - noise, transmission/reception errors,
connection speed, etc.  So, what might appear to be the best access
point, out of a selection of access points, where you have a little bar
graph on each, and one has more bars than another.  It *may* not
necessarily indicate which will work best for you (were you in a
position of having to choose one out of many that you could use, rather
than the usually simply having to choose the one that is for your
network versus someone else's).

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