Sam Pierson wrote:
I'm trying to get an accurate idea of what signal strength
is being received by a laptop in monitor mode, listening
to packets being sent out by another laptop. With a
big change in distance, the SSI value changes, and with
a change in transmission power, the SSI changes just
slightly.
How exactly is the SSI value calculated and what sort of
"unit" is it? I've heard of RSSI values being unitless in the
past, not sure if this is the same thing or not.
The SSI value shown in Ethereal corresponds to the radiotap values
IEEE80211_RADIOTAP_DBM_ANTSIGNAL and IEEE80211_RADIOTAP_DB_ANTSIGNAL;
unfortunately, it doesn't distinguish between them.
The comments in the 1.12 revision (the current revision) of NetBSD's
"net80211/ieee80211_radiotap.h" header file for those values are:
* IEEE80211_RADIOTAP_DBM_ANTSIGNAL int8_t decibels from
* one milliwatt (dBm)
*
* RF signal power at the antenna, decibel difference from
* one milliwatt.
and
* IEEE80211_RADIOTAP_DB_ANTSIGNAL u_int8_t decibel (dB)
*
* RF signal power at the antenna, decibel difference from an
* arbitrary, fixed reference.
As the two values are displayed in the same fashion by Ethereal, you
can't tell what the baseline value is.
The values aren't calculated by Ethereal, they're just displayed by
Ethereal; they're calculated either by the wireless card or its driver.
You'd probably have to ask the writer of the driver precisely how it's
calculated. I *suspect* IEEE80211_RADIOTAP_DB_ANTSIGNAL exists for the
benefit of wireless cards where the reference value for the signal power
isn't known (perhaps because the card vendor doesn't document it).
Those values could presumably only be used for comparing the signal and
noise values, or for showing the changes in signal value between
packets, not for showing the absolute power at the antenna.
Further questions are probably best sent to David Young:
http://www.ojctech.com/people/dyoung.html
as he's the original inventor of the radiotap header, as I remember, or
to Sam Leffler:
http://www.errno.com/
as he's the original author of the Ethereal dissector for the radiotap
header.