Atheros driver and radiotap reliability

From: Sam Pierson (samuel.pierson_at_gmail.com)
Date: 08/29/05

  • Next message: Steven Hartland: "Re: bge driver internal routing issues?" 
    Date: Mon, 29 Aug 2005 14:07:46 -0500
To: FreeBSD Hackers <freebsd-hackers@freebsd.org>

    Hi guys,

    I'm trying to get an accurate measurement of signal strength (preferably
    in dBm) on a per-packet basis between two atheros cards that I have. I
    had some correspondence with the ethereal developers and David Young
    and apparently there is a bug in how ethereal handles the radiotap header.
    David told me that tcpdump will correctly report whatever the device driver
    tells it is the correct signal signal strength, but not to trust it until the
    devices have been calibrated. How does the ath driver report the signal
    strength in the radiotap header? From tcpdump, it's giving me this value:

    /* taken from /sys/net80211/ieee80211_radiotap.h */
     * IEEE80211_RADIOTAP_DB_ANTSIGNAL u_int8_t decibel (dB)
     *
     * RF signal power at the antenna, decibel difference from an
     * arbitrary, fixed reference.
    ...

    In this same file, there is a u_int8_t ANTSIGNAL reported in dBm. It appears
    as though everything is driver (and device, probably) dependent, so I'd like
    to know how the driver computes this value.

    As a side question, does anyone have an easier way to reliably measure
    per-packet signal strength? The area has a decent amount of traffic and
    I have to be able to analyze the packets themselves, so a plain hardware
    solution will not do. Thanks,

    Sam
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