Hi Ok, that’s a pretty good paper. At least it shows data and digs into the details. It also would lead one to believe that a “Time Nuts” grade sync system might be a hackable sort of thing …… hmmm…..Given how highly integrated these WiFi chip sets have become, that probably is a fantasy.
Bob > On Jan 15, 2017, at 11:10 AM, Scott Stobbe <scott.j.sto...@gmail.com> wrote: > > Here is a ti app note with timestamping hardware wl8 but ordinary ap's with > no special protocol just timestamping the beacon frame. > > http://www.ti.com/lit/an/swaa162a/swaa162a.pdf > > On Sun, Jan 15, 2017 at 10:06 AM jimlux <jim...@earthlink.net> wrote: > >> Returning to the OP >> >> "A TimeSync certification program will appear later this year, but >> >> semiconductor firms will have to create new Wi-Fi chips including the >> >> feature." >> >> >> >> so this "new thing" will be hardware of some TBD form. >> >> https://www.wi-fi.org/discover-wi-fi/wi-fi-timesync >> >> >> >> >> >> >> >> But more interesting to time-nuts, I think, is how do you do it without >> >> the new hardware. >> >> >> >> http://www.cse.msu.edu/~glxing/docs/WizSync.pdf >> >> says, in part: >> >> >> >> 802.11 requires all APs to broadcast periodic beacon frames that >> >> carry important management information (e.g., supported rates and >> >> security settings). The default beacon period is 102.4 ms, which is >> >> rarely changed on production APs. ...However, as defined in 802.11, >> >> whether a beacon frame is delayed or not, the subsequent beacon >> >> frame shall always be scheduled at the undelayed nominal beacon interval. >> >> >> >> so this is the "use a 1pps, but throw out outliers" kind of strategy... >> >> >> >> And there would need to be some sort of measurement of the AP's timing >> >> error - they make the assumption that the timing of the beacons is >> >> driven by a clock with max 25ppm error (as required by the 802.11 std), >> >> although they've measured <5ppm normally >> >> >> >> Ultimately, they got on the order of 0.1 0.2 ms. >> >> >> >> >> >> That's a few orders of magnitude worse than "microsecond", but it's also >> >> an interesting read. >> >> >> >> >> >> >> >> an older presentation (2006) might be useful >> >> >> http://www.ieee802.org/1/files/public/docs2006/avb-stanton-wifi-timesync-intro-060613.pdf >> >> >> >> discusses 802.11v >> >> >> >> there's been a lot of stuff on time sync/distribution over 802.11 links >> >> for the last decade.. maybe this CES announcement is more about "we at >> >> WiFi alliance are ready to market it". Has anyone gone through the >> >> 802.11 standards list recently? It might well be that the standard is >> >> already there. >> >> >> >> 802.11aa says "Amendment 2: MAC Enhancements for Robust Audio Video >> >> Streaming" in the description... although that might just be things >> >> like QoS and access control-digital rights management >> >> >> >> _______________________________________________ >> >> time-nuts mailing list -- time-nuts@febo.com >> >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> >> and follow the instructions there. >> >> > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.