[time-nuts] time transfer over wifi

Scott Stobbe scott.j.stobbe at gmail.com
Sun Jan 15 16:10:30 UTC 2017


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 <jimlux at 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
>
>
>
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