[time-nuts] time transfer over wifi
Bob Camp
kb8tq at n1k.org
Sun Jan 15 11:26:04 EST 2017
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.stobbe at 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 <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
>>
>>
>>
>> _______________________________________________
>>
>> time-nuts mailing list -- time-nuts at 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 at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
More information about the time-nuts
mailing list