[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
>> 
>> 
>> 
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