[RFC] libertas: change quality calculation

[Dan Williams]

On Wed, 2008-03-19 at 11:38 -0400, Dan Williams wrote:
> On Wed, 2008-03-19 at 16:25 +0100, Holger Schurig wrote:
> > > > I also found that the average noise floor is -86 dBm, not
> > > > -96 dBm. So I changed this constant. Maybe this is wrong,
> > > > what values do your cards report?
> > >
> > > When possible, we should use a window-ed average of the cards
> > > reported noise from beacons or received frames instead of a
> > > constant, that's a further improvement though.
> > 
> > The changed value of MRVDRV_NF_DEFAULT_SCAN_VALUE is only used to 
> > magically conjure the missing noise value for scan results, see 
> > my other mail about this.
> > 
> > Okay, it's also used in the case of a failed GET_RSSI command :-)
> > 
> > > Well, when we had the (badly) averaged array values in there,
> > > the noise floor was available to the scan reporting.
> > 
> > Something might have been available for scan reporting, but if 
> > this "something" made sense is a different question. See my 
> > other mail :-)
> > 
> > > There are tables to map RSSI -> dBm for each part which should
> > > be in the firmware spec or seen in the driver code.
> > 
> > I didn't see such a table so far, where is it?
> 
> See section 7.3 of the 5.1 spec:
> 
> 7.3         RSSI and Noise Floor Support
> The firmware supports the reporting of RSSI and Noise Floor values:
> •   RSSI can be derived by adding Noise Floor to SNR value
> •   Noise Floor is returned in each received packet through the NF field
> in the Rx Descriptor (dBm).
> •   Signal Quality is determined by RSSI and SNR.
> 
> Section 3.1 as well (Receive Packet Descriptor):
> 
> "RSSI should be calculated in the driver as a summation of the SNR and
> NF"
> 
> So Marvell doesn't really provide a table, but we'll have to do some
> investigation here.  The RSSI is almost always a random value defined by
> the manufacturer [1] or in dBm.  We're not 100% sure what Marvell is
> using here, but some investigation will show whether the value is
> actually in dBm or in some other units.  A quick check of the RSSI
> reported by the current BSS in scan results versus the output of
> GET_RSSI might show something interesting.

For further information, see something like:

http://www.wildpackets.com/elements/whitepapers/Converting_Signal_Strength.pdf

Dan