[time-nuts] time transfer over wifi
jimlux at earthlink.net
Sun Jan 15 10:06:32 EST 2017
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
so this "new thing" will be hardware of some TBD form.
But more interesting to time-nuts, I think, is how do you do it without
the new hardware.
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
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
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
More information about the time-nuts