[time-nuts] Cell timing error

Greg Troxel gdt at ir.bbn.com
Tue Dec 18 23:55:44 UTC 2012

Greg Troxel <gdt at ir.bbn.com> writes:

> Date: Sat, 15 Dec 2012 13:51:45 -0500
> In central mass, AT&T and tracfone (? carrier) are showing phone times
> very close to 1 min slow.  Virgin/sprint is ok.   I've never seen this
> before - usually it's a few s slow.

Thanks for all the comments.  Following up from a real keyboard:

  The tracfrone was on the AT&T network, per the code on the SIM.  So I
  think this was just AT&T.

  The timing error was resolved sometime later on Saturday.

  I realize the phone display time and GSM time are not well connected.
  Emerald Time (iPhone program) does NTP and shows the difference
  between the time on the unix computer in the phone and NTP peers.
  Typically I see values from -0.5s to about 4.5s.  Right now it shows
  +4.348s (phone computer is 4.348s slow).  The phone display switched
  to 1849 at about 184907, which I attribute to 4s slow and a slow
  update rate of the display.

  Based on observing Emerald Time's offset over long periods, I believe
  that there is some loose synchronization of the phone cpu to the cell
  network, and that the phone free runs until it gets too far off and
  then it is stepped to be closer.

  On Saturday, Emerald Time was reporting +60.580.  The time on my phone
  was 1 minute slow and updated around minute boundaries, just one
  minute behind.

  A tracphone (LG featurephone) showed exactly the same one-minute slow
  behavior.  So I suspect that something was off in the GSM time
  reference.  I am not sure if more than one cell site was affected.
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