Re: CVS-2-2 NMApplet empty bar explained

[Dan Williams <dcbw redhat com>]

On Fri, 2005-02-04 at 13:25 -0500, Sven wrote:
> thanks for the info - i have a few more questions/suggestions:
> 
> if the card reports RSSI and i know MAX_RSSI, is the relative "signal
> strength" then  RSSI / MAX_RSSI (* 100)? if one wants to use that as the
> reported "Link Quality" but for some reason do not want to use %, what
> should one do? converting the RSSI #'s to dBm and using that to compute
> a % is clearly wrong. IMHO some better guidelines as to what "Link
> Quality" in WEXT should mean is desirable. borrowing definitions from
> Joshua Bardwell in
> http://www.connect802.com/download/techpubs/2004/you_believe_D100201.pdf,  
> from a practical point of view "Signal Quality," though desirable as
> link quality, is probably not feasible to get a handle on with the
> (current and future) drivers. next best is probably "Signal Strength" -
> from the RSSI values. Or is SNR better as a measurement of link quality?
> but that would require a better reporting of noise by the drivers (and
> not just a hardcoding) .

RSSI is totally manufacturer dependent.  AFAIK, Cisco uses a MAX_RSSI of
63 for the 340/350, Atheros uses something like 30, etc.  It depends on
how many voltage values the hardware can physically measure.  So yes,
you do get a sort of Link Quality % when you take RSSI / MAX_RSSI * 100.
You can (and should) augment this value with things like the ipw2200
driver does, ie receive packet errors, link speed, etc.

Converting RSSI to dBm and using that for link quality is actually
pretty wrong.  dBm is actually useful though, you can do some
interesting things with it like 1) distance from transmitter (if you
know detailed antenna and card characteristics), 2) signal power levels
and noise levels, 3) more accurately test different antennas, etc.  Its
just not useful for getting a Link Quality % of any accuracy whatsoever,
except when the Signal approaches the Noise you know your reception is
starting to suck.

So 4 points to take out of this:
1) Drivers SHOULD use subjective values for calculating Quality, but
that value SHOULD include some sort of RSSI measurement in addition to
whatever else (ie invalid packets, retransmit count, link speed, etc)
2) Drivers SHOULD set both current level (ie qual.qual, qual.level) and
max level (max_qual.qual, max_qual.level)
3) Drivers SHOULD use same units for level & noise (ie, either RSSI or
dBm)
4) Drivers SHOULD use dBm wherever they can, if they can.

Dan