[ntp:questions] How do I validate my PPS clocks?

David Taylor david-taylor at blueyonder.co.uk.invalid
Mon Feb 25 10:57:13 UTC 2013


On 24/02/2013 23:11, bodosom at gmail.com wrote:
> I can't find or figure out how to validate my ntp results.
>
> I (currently) have two Linux boxes with PPS via the NMEA driver from
> Garmins (18 & 18x) and a Sure board connected to a purpose built
> NTP server.  When I set up my first Garmin all of my remote offsets
> were negative which seemed unlikely so I set time1 to 5.3ms which
> also seems unlikely but  which brought me into congruence with some
> nearby stratum 1 servers.
>
> Below is the output* from an intermediate machine on my home network
> (3Mb ADSL) which is temporarily connected to several servers.
>
> 192.168.0.2 is running with time1 5.3ms.
> 192.168.0.244 is running with time1 132us.
> 192.168.0.210 is the appliance and has currently has no adjustments.**
>
> Should I expect /all/ (ignoring the high jitter ones) of the remote
> clocks to have negative offsets?
>
> Is there a sensible to way to make this assessment purely within the
> NTP system?
>
>       remote           refid      st t when poll reach   delay   offset  jitter
> ==============================================================================
> +192.168.0.2     .GPS.            1 u   14   16  377    0.109   -5.429   0.006
> *192.168.0.244   .GPS.            1 u    1   16  377    0.112    0.000   0.008
> +192.168.0.210   .GPS.            1 u   15   16  377    0.567   -0.001   0.007
> -216.66.0.142    130.207.244.240  2 u   54   64  377   63.526   -7.131   0.192
> -38.229.71.1     204.123.2.72     2 u   25   64  377   67.175   -4.368   0.472
> -63.240.161.99   72.26.198.233    3 u   19   64  377   70.797   -3.284   0.320
> #204.235.61.9    128.105.201.11   2 u   17   64  177   73.331   -9.732   0.426
> -216.229.4.69    216.229.0.179    2 u    -   64  377   78.848   -4.987   1.337
> -199.7.177.206   64.147.116.229   2 u   13   64  377   86.072   -7.451   0.235
> #2604:180::8138: 164.244.221.197  2 u   56   64  377   91.521  -33.681  25.573
> -198.101.234.139 128.138.140.44   2 u   35   64  377   97.360   -2.065   0.584
> #2604:2880::870: 139.78.135.14    2 u   23   64  377  115.057  -12.785  22.738
> -207.7.148.214   204.123.2.5      2 u    2   64  377  123.982   -8.091   0.747
>
>
> *This billboard is sorted by delay.
> **You'd have to recompile the control software.

For comparison, my results with a number of different stratum-1 servers 
are here:

   http://www.satsignal.eu/ntp/ntp-offsets.txt

This includes FreeBSD, Linux, and Windows stratum-1 servers.

Unless those PCs are very slow, I would suggest that the time1 value 
should be well under 1 millisecond, and likely under 50 microseconds. 
My own results mostly show a positive offset for remote servers, which I 
put down to asymmetry on my consumer Cable Modem connection (30 Mb/s 
down, 3 Mb/s up).

The remote servers are a mixture of pool and other servers, some 
stratum-2 and some stratum-3.  Be aware that some of the "national" 
stratum-1 servers can be rather overloaded, producing poorer performance 
than the better stratum-2 servers.  Equally, servers in the pool may 
well be not as good as the best stratum-2 servers.  So it's a little pot 
luck!

Does that help?
-- 
Cheers,
David
Web: http://www.satsignal.eu



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