[ntp:questions] Using Trimble TSIP under Linux

David Taylor david-taylor at blueyonder.co.uk.invalid
Tue Nov 27 08:10:46 UTC 2012

On 26/11/2012 23:59, David Woolley wrote:
> David Taylor wrote:
>> server reached over the LAN to have an offset of -0.028 milliseconds,
>> whereas its own PPS source is reported to have an offset of -0.001
>> milliseconds.  Is that the right sign and amount for the offsets you
>> might expect comparing the LAN input and the GPIO pin interrupt for
>> the PPS source?
> Offsets should be randomly distributed between + and - signs.  If they
> are all of one sign, you are not locked up or there is a bug.  They
> should be of s similar magnitude to the jitter.
> They may not be evenly distributed, because of limitations of the
> network, but they must be spread across zero.

David, for clarification:

On the card which is synced purely to LAN servers, over a day the mean 
offset of the FreeBSD stratum-1 server (the selected sync source) is 
0.000 ms, and that of two Windows stratum-1 servers is 0.014 ms.  The 
mean jitter is 0.041 ms (FreeBSD) and 0.048 and 0.280 ms (Windows).  The 
offset of WAN-based pool servers was in the region of 2 - 8 ms.

On the card with the PPS source, the mean offset of the other servers is 
-0.020 ms (FreeBSD) and -0.008 ms (both Windows).  Mean jitter is 0.230 
ms (FreeBSD) and 0.036 ms and 0.230 and 0.047 ms (Windows).  The offset 
of WAN-based pool servers was in the region of 2 - 14 ms.

The WAN is via a cable modem connection, rather asymmetrical at 30 Mb/s 
down and 3 Mb/s up.

Looking at the graph, the offsets of the LAN servers are very stable, 
and much less variable than the WAN servers.  They also are clustered 
around zero.  I conclude that there's no bug and the NTP is locked. 
What I may well be seeing is the differences in interrupt latency 
between the different hardware and operating systems.  It's not a 
problem for me.
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