[time-nuts] time transfer over wifi
Scott Stobbe
scott.j.stobbe at gmail.com
Sun Jan 15 11:10:30 EST 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
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> semiconductor firms will have to create new Wi-Fi chips including the
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> feature."
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>
>
> so this "new thing" will be hardware of some TBD form.
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> https://www.wi-fi.org/discover-wi-fi/wi-fi-timesync
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> But more interesting to time-nuts, I think, is how do you do it without
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> the new hardware.
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>
>
> http://www.cse.msu.edu/~glxing/docs/WizSync.pdf
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> says, in part:
>
>
>
> 802.11 requires all APs to broadcast periodic beacon frames that
>
> carry important management information (e.g., supported rates and
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> security settings). The default beacon period is 102.4 ms, which is
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> rarely changed on production APs. ...However, as defined in 802.11,
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> whether a beacon frame is delayed or not, the subsequent beacon
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> frame shall always be scheduled at the undelayed nominal beacon interval.
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>
>
> so this is the "use a 1pps, but throw out outliers" kind of strategy...
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>
>
> And there would need to be some sort of measurement of the AP's timing
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> error - they make the assumption that the timing of the beacons is
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> driven by a clock with max 25ppm error (as required by the 802.11 std),
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> although they've measured <5ppm normally
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>
>
> Ultimately, they got on the order of 0.1 0.2 ms.
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>
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> That's a few orders of magnitude worse than "microsecond", but it's also
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> an interesting read.
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> an older presentation (2006) might be useful
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> http://www.ieee802.org/1/files/public/docs2006/avb-stanton-wifi-timesync-intro-060613.pdf
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>
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> discusses 802.11v
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>
>
> there's been a lot of stuff on time sync/distribution over 802.11 links
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> for the last decade.. maybe this CES announcement is more about "we at
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> WiFi alliance are ready to market it". Has anyone gone through the
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> 802.11 standards list recently? It might well be that the standard is
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> already there.
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>
>
> 802.11aa says "Amendment 2: MAC Enhancements for Robust Audio Video
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> Streaming" in the description... although that might just be things
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> like QoS and access control-digital rights management
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>
>
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