Re: TSFT

Hi Simon,

On Fri, Nov 16, 2012 at 10:26 PM, Simon Barber <[email protected]> wrote:
> This is the current definition of TSFT:
>
> "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."
>
> My experience with TSFT is with Atheros cards, which record the timestamp at
> the end of the frame, not the start.

There is already a patch in mac80211-next which accounts for this and
normalizes frame TSFT to beginning of frame.

> Do you know the behaivour of any particular capture systems re: TSFT - where
> do they timestamp the frame?
>
> Furthermore for DSSS/CCK the definition above is reasonable, but for OFDM
> and HT (802.11n) the SIGNAL field (part of the PLCP header, not part of the
> MPDU) is part of the first data symbol. It would be much clearer to change
> the definition to state that the timestamp is at the start of this first
> SIGNAL/data symbol. Any objections to doing this, or preference to change
> the reference point to a different part of the frame?

Keeping the TSFT at start of MPDU is cleaner from the user's point of view.

> In my wireshark patch I've added options to interpret TSFT as the start of
> end of the frame.

I'm not sure which patch you're referring to, but it seems cleaner to
handle normalizing TSFT to a single reference in the kernel.

Thomas