Received signal strength

Pavel Roskin proski at
Mon May 10 17:02:34 EDT 2004

On Mon, 10 May 2004, Christopher Dobbs wrote:

> I dont know about rssi but mabey this
> will help.

No, we are talking about conversion between two logarithmic scales, dBm
and the scale used in the card, known as RSSI (received signal strength
information).  The table you posted the link to converts between dBm and
Watts, a linear scale.

Firmware 1.3.1 and newer has RID 0xfd51 (dBmCommsQuality), which is
supposed to give the information in dBm.  HostAP uses RID 0xfd43
(CommsQuality), which uses its own scale.  That scale was also changed in
1.3.1 firmware.

Also, RID 0xfc46 (dBmAdjust) contains a constant that should be used for
conversion between RSSI and dBm.

If dBm values are needed, I suggest using dBmCommsQuality instead of
CommsQuality.  Even then, I don't expect the data to be very accurate, but
it's probably the best data the card can provide.

I'm working on a patch for Orinoco driver that would use dBmCommsQuality
if possible and provide reliable data in its absence.  Once I'm satisfied
with it I plan to submit similar patch to HostAP.

> > I have a Harris Semiconductor Prism 2.5 Wavelan Chipset.I want to convert the
> > parameter rxdesc->signal into dBm.
> >Is  (rxdesc->signal-100) the right way to do it?.

I don't think so.  You can print dBmCommsQuality and CommsQuality to the
kernel log and try to find the dependency.  I think it should be linear.

> > Will this conversion hold good even if I am using this card in managed
> >mode and use an alternate driver like orinoco?

I suggest that you don't trust drivers too much at this point.  There is
some work that need to be done before you can trust the drivers, both
Orinoco and HostAP.  Sorry for inconvenience.

Pavel Roskin

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