[ntp:questions] Garmin 18 LVC: whether to fudge

Hans Jørgen Jakobsen hjj at wheel.dk
Sun Feb 15 22:44:08 UTC 2009


On Sun, 15 Feb 2009 18:04:36 GMT, Unruh wrote:
> Hans =?iso-8859-1?Q?J=F8rgen?= Jakobsen <hjj at wheel.dk> writes:
>
>>For my 14M/1.5M VDSL line I see an offset of 7-800 microseconds:
>> ntpq -p
>>     remote           refid      st t when poll reach   delay   offset  jitter
>>==============================================================================
>>-ns.tele.dk      .GPS.            1 u   48   64  377   22.450    0.891   0.096
>>-tix.ns.tele.dk  .GPS.            1 u    7   64  377   22.338    0.698   0.183
>>-puma.tele.dk    .GPS.            1 u   19   64  377   27.048    0.784   0.145
>>+GPS_HP(0)       .GPS.            0 l   10   16  377    0.000   -0.776   6.230
>>oPPS(0)          .1PPS.           0 l   10   16  377    0.000    0.012   0.015
>>+GPS_ONCORE(0)   .GPS.            0 l    8   16  377    0.000    0.035   0.006
>
> Something is really weird here. .776ms from a GPS refclock is horrible.
> That is about a factor of 300 worse than you would expect. In fact all of
> your GPS refclocks show bad time What are you doing to your clocks?
>
Dave Hart has given the answer why they behave as they do.
>
>>20.6 ms of the roundtrip time are spend on the DSL line.
>
> How do you know it is exactly 20.6ms?
>
I left out that I have acces to above ntpservers and have done a ping to the
DSLAM. Ping ns.tele.dk <-> DSLAM are ~1.85ms. Symmetric delay can be assummed.
/hjj




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