[ntp:questions] high precision tracking: trying to understand sudden jumps

Richard B. Gilbert rgilbert88 at comcast.net
Sun Mar 30 22:30:53 UTC 2008


Unruh wrote:
> "Richard B. Gilbert" <rgilbert88 at comcast.net> writes:
> 
> 
>>starlight at binnacle.cx 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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