Re: high precision tracking: trying to understand sudden jumps

Unruh wrote:
"Richard B. Gilbert" <rgilbert88@xxxxxxxxxxx> writes:


starlight@xxxxxxxxxxx wrote:

Hello,

I'm trying to configure a small network for high precision time. Recently acquired an Endrun CDMA time server that runs like a dream, tracking CDMA time to about +/- 5 microseconds.

The clients are a rag-tag assembly of diverse systems including a Centos 4.5 Linux i686, Linux x86_64, Sun Ultra 10, Sun Ultra 80, IBM RS/6000 44p, Windows 2003 X64, and a Windows XP laptop.

All are configured to prefer the Endrun clock and poll it on a 16 second interval. All are attached to a single SMC gigabit Ethernet switch with only the Endrun and two Sun systems running at a lower speed of 100 MBPS. Close to zero network traffic
and system loads.

All systems are running 'ntpd' 4.2.4p4. Compiled NTP native 64-bit for the Windows X64 system. [A #ifdef tweak to 'intptr_t' and 'uintptr_t' is required, will provide patch if desired].

It generally is working well, with the systems tracking anywhere from +/- 100 microseconds to +/- 500 microseconds most of the time.

However once or twice a day, all the systems experience a random, uncorrelated time shift of from one to several milliseconds.

<snip>


Forcing the poll interval to 16 seconds is not always a good idea!
Ntpd will select a poll interval, generally starting at 64 seconds, and ramping up to as long as 1024 seconds as the clock is beaten into submission!


It is his network, he is not going to overload it. So, if he wants a 16 sec
poll interval that is up to him. I agree it is not a good idea for remote servers, but on his own system it
is fine.



Directly connected refclocks are frequently polled at shorter intervals
but I don't think your refclock is "directly connected" in the same sense that a clock working through a serial or parallel port is directly
connected!


A clock connected via ethernet with all the latencies and jitter thereunto appertaining is no different than any other network server and should be polled in the same manner!


??? The longer polls are in order not to swamp the remote server whith
10000 people all polling every 16 sec ( or 1 sec) There is nothing in ntp
itself that mandates a longer poll interval. In fact a shorter poll
interval makes ntp much more responsive to changes ( clock drifts, etc)




The very short poll intervals correct large errors quickly and the very long intervals correct small errors very accurately!


No for a properly designed system both should be corrected.



If you don't measure across a long interval, you will never see some of those small errors. When you measure across 1024 seconds you overwhelm the network jitter. The long interval is part of the design for just that reason.

Suppose your frequency error is 5 PPM or 0.43 seconds per day. Do you think you can measure that error accurately with a 64 second poll interval? If you are working over the internet, an error that small is going to disappear in the jitter. It will be sixteen times more obvious at the longer interval.

You can poll a hardware reference clock at 16 second intervals because the network is not involved! The latency and jitter a PPS signal over a serial port are an order or two of magnitiude less than what you get over a busy network.

.

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