Re: CVS-2-2 NMApplet empty bar explained

[Sven <sr12 duke edu>]

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) .

Sven

summary/conclusions of the paper mentioned:
"  Signal strength  is defined in 802.11 as Received Signal Strength
Indicator (RSSI). RSSI is intended to be used as a relative value within
the chipset. It is not associated with any particular mW scale and is
not required to be of any particular accuracy or precision. Therefore,
the signal strength numbers reported by an 802.11 card will probably not
be consistent between two vendors, and should not be assumed to be
particularly accurate or precise. 
"  Signal quality  is defined very briefly in 802.11. Common definitions
have arisen, but they are usually incorrect. The correct definition
hinges on the term,  PN code correlation strength,  which is a measure
of the match (correlation) between the incoming DSSS signal and an ideal
DSSS signal. The common equation of  signal quality  and  signal to
noise ratio  is incorrect. 
"  Signal to noise ratio  is a general term that is used in a novel way
by 802.11 administrators. Most usages of the term refer to the strength
of the signal relative to thermal noise within a circuit, but 802.11
administrators use the term to refer to the strength of the signal at
the receive antenna relative to the ambient, non-802.11 RF energy at the
same frequency as the signal. While this definition isn t wrong, per se,
it may lead to confusion when 802.11 administrators communicate with
engineers who are using the more traditional definition.

On Fri, 2005-02-04 at 09:23 -0800, Jean Tourrilhes wrote:
> On Fri, Feb 04, 2005 at 11:10:46AM -0500, Dan Williams wrote:
> > On Fri, 2005-02-04 at 10:53 -0500, Sven wrote:
> > > iwlib.c in WEXT says (in the example) 
> > >    * 2) value is -54dBm. noise floor of the radio is -104dBm.
> > >    *    qual->value = -54 = 202 ; range->max_qual.value = -104 = 152
> > > i'm confused. why is the max value the noise floor??? in Atheros chips,
> > > given the value in dBm comes from RSSI, should max_qual.value not be
> > > MAX_RSSI (converted into dBm)?
> > 
> > I think this is actually wrong...  What I _think_ it should say is:
> > 
> >    * 2) level is -54dBm. noise floor of the radio is -104dBm.
> >    *    qual->level = -54 = 202 ; range->max_qual.noise = -104 = 152
> 
> 	Doh ! Stupid bug ! You are right.
> 
> > Noise levels _do_ change dynamically, which is something else that the
> > drivers don't do (ahem, atmel, madwifi, and airo for starters).  When
> > you turn on your microwave, that totally screws the 2.4GHz frequency
> > range and impacts 802.11 communications.  Since the microwave is random
> > energy, it is extra noise and therefore decreases the Signal to Noise
> > ratio (ie, the noise value increases, say from -95dBm -> -85dBm, due to
> > the extra energy from the microwave, while the signal may stay the
> > same).
> 
> 	Yes.
> 
> > You _always_ have a noise floor, which is the normal value where in good
> > conditions the card can no longer distinguish the usable radio energy
> > from background energy, but most drivers at this time use that noise
> > floor level in the "qual.noise" field and not the "max_qual.noise"
> > field, because they evidentally don't sample noise on each channel
> > dynamically, or don't know how to pull that value off the card.
> 
> 	You are correct about the definition of noise floor. This is a
> characteristic of the radio and the frequency band, and usually in the
> spec (you can't read it from the hw). I don't believe any driver use
> the noise floor in qual.noise, that would not make sense.
> 
> > Jean: can you give some clarification on that statement in iwlib.c?
> 
> 	MAX_RSSI converted to dBm is some totally uninteresting
> value. It's going to be close to 0 dBm, and a value that has nothing
> much to do with real operation but just an arbitrary limit on the A->D
> converter used for RSSI sampling. On the other hand, MIN_RSSI is very
> relevant, so that's why we use that.
> 
> > Dan
> 
> 	Jean