I'm using mac80211 for Intel(R) Pro Wireless 3945 in order to capture Received Signal Strength from Radiotap headers.

I've created two interface (wlan0 and mon0) up to the same Intel card and configured mon0 in monitor-mode.
Radiotap shows SSI Signal (dB) and SSI Noise (dB) but I've some doubts about their values.

I read that SSI is related to a not specified dB value of the card but I can't find more information about it and about relation between RSSI and RSS.

Noise is always 0dB and I read that signal-to-noise ratio (SNR) cannot be computed by ``SSI Signal - SSI Noise'' because the noise in Radiotap Header usually is captured as the background noise level when no packet is transmitted. Thus, the SSI Noise can be interpreted as link quality but cannot be used to properly compute the SNR.

Some of you know if I can trust in Radiotap SSI Signal dB value?
I need very reliable SSI values!