On Tuesday 12 February 2008 18:50:19 Johannes Berg wrote:
> > thats a very good point! if everything was calibrated properly and all
> > delays taken into account *exactly* that would cause us to go thru a
> > merge all the time, on every received beacon. i think we have to take
> > that into account, for different bitrates. what about something like:
> >
> > timestamp >= ( mactime + (24 * 8 / beacon_phy_rate_in_mbps ))
>
> Looks reasonable, though I'm not entirely sure how we defined 'mactime'
> and whether it is at the first data or the first phy symbol. I think
> it's at the first data symbol which makes this correct.
hmm, it seems we don't have a strict definition of mactime... the kerneldoc of
struct ieee80211_rx_status says:
* @mactime: MAC timestamp as defined by 802.11
but as far as i understand 802.11, it only defines "timestamp" for beacon and
probe response frames, where it actually means the timestamp field of the
frame.
"Local Time" is defined as: "The value of the STA’s TSF timer at the start of
reception of the first octet of the timestamp field of the received frame
(probe response or beacon) from the found BSS." [802.11 p. 103]
also in "Process Validate_MPDU: it says:
"Save arrival time of first octet of {what may be a} timestamp field."
[802.11 p 391 and 460]
which could imply that this value is given as a rx timestamp (mactime). in
that case we could directly compare timestamp and mactime.
radiotap on the other hand defines it as the first bit of the data (MPDU):
* IEEE80211_RADIOTAP_TSFT u_int64_t microseconds
*
* Value in microseconds of the MAC's 64-bit 802.11 Time
* Synchronization Function timer when the first bit of the
* MPDU arrived at the MAC. For received frames, only.
so i guess right now it depends on the hardware what we actually get as
mactime...
i could not find any definition when the rx timestamp of atheros hardware is
taken.
do we have that information for other hardware? is mactime
*) the TSF at the reception of the first phy symbol?
*) the TSF at the reception of the first data (MPDU) symbol
*) the TSF at the reception of the byte 24 (where the timestamp field of
beacon and probe response frames is)?
bruno
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