[ntp:questions] Seeking help configuring GPS refclock

Kevin Oberman oberman at es.net
Mon Aug 3 15:43:57 UTC 2009


> Date: Mon, 03 Aug 2009 07:52:39 -0700
> From: Rich Wales <richw at richw.org>
> Sender: questions-bounces+oberman=es.net at lists.ntp.org
> 
> My experimental stratum-1 server (using a Garmin GPS as its reference clock)
> continues to run stably -- albeit with an offset several msec different from
> the rest of my home LAN (which is synced to Stanford University's time server
> infrastructure).
> 
>      remote           refid      st t when poll reach   delay   offset  jitter
> ==============================================================================
> oGPS_NMEA(0)     .GPS.            0 l    2    8  377    0.000   -0.001   0.002
>  PPS(0)          .PPS.            9 l    6    8  377    0.000    0.000   0.002
> *iknow.richw.org 171.64.7.115     3 u   10   16  376   10.058   -9.249   3.871

I see a couple of things here. First, almost 4 ms. of jitter is
excessive for a stable, uncongested network link. I don't know what the
path is from your location to Stanford's NTP server, but at 10
ms. delay, it's a few hundred miles. I suspect that the link is
congested at some point and, more significantly, I suspect that it is
asymmetric. 

NTP assumes the same delay for the return packet that it had for the
request packet. If that is not the case, you will see a fixed
offset. The only fix is a 'fudge' of the time for that server. If the
path is unstable, even that will not work.

If the congestion that is producing the jitter is pretty consistent, it
will null out eventually, with a small offset it the congestion is only
in one direction (which it usually is). From where I sit in Berkeley, I
see the Stanford system 4.5 ms. away and a std. deviation of only .168
ms., so I suspect the congestion may be closer to you.
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at es.net			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751



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