raw to dBm conversion

Pavel Roskin proski
Tue Jun 27 22:28:07 PDT 2006


On Tue, 2006-06-27 at 13:20 +0200, Valerio, Danilo wrote:
> Hi everybody,
> I searched in the mailing list archive for information about the 
> conversion from the raw value of Signal and Noise to a dBm value but i 
> didn't find anything really considerable...

There are FIDs 0xFD43 (CommsQuality) and 0xFD51 (dBmCommsQuality), which
are both supported on newer (post-1.3.x) firmwares.  I checked both for
the same connections with different signal strength and came to the
following formula, which is used in the Orinoco driver for Intersil

#define RSSI_TO_DBM(x) (10 * ((x) - 317) / 33)

> I'm sniffing via TCPdump, and i need to convert that raw values of the 
> prismII header (or AVS header) to dBm values...

I think header RSSI is the same as the one for the connection quality.

> After having a look into the driver code and making some measurement, i 
> realized that a correct formula seems to be:
> ((raw_value) - dBmAdjust) * (100/255) - 100

RSSI            your formula       my formula
27              -89                -88
155             -39                -49

The results are pretty close.  But my formula is based on real
measurements with 1.7.4 firmware.  The only problem with my approach is
that I don't know if dBmCommsQuality can be trusted.  It's possible that
hostap uses something based on closed Intersil documentation.

Pavel Roskin

More information about the Hostap mailing list