Re: CVS-2-2 NMApplet empty bar explained

[Jean Tourrilhes <jt hpl hp com>]

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