[ntp:questions] frequency adjusting only

David Woolley david at ex.djwhome.demon.co.uk.invalid
Thu Apr 24 22:27:13 UTC 2008


maxime louvel wrote:
> 
> - I can't use a GPS or other material. I though I wouldn't need any because
> I don't won't a great accuracy with the real time, just between my nodes. I
> also think it would have been easier and better to have a common high
> precision time source (GPS)... But I can't afford that ;)

As others have pointed out, your timing requirements are rather more 
than run of the mill, so you should expect to have to pay to meet them. 
  You may need to take special consideration of not just the time 
distribution network, but also the time characteristics of the 
application and the application hardware.

> 
> Do you have an explanation of the so different performances observed in your
> NTP synchronisations ?

Your biggest problem is that the DCF machine isn't synchronised; it is 
free running, probably with a frequency error of 10s of parts per 
million and is occasionally violently reset.  On the other hand, it is 
claiming to be synchronised to quite a high level of accuracy.

If you can't use GPS and PPS to get a stable master server, you should 
not use any source of UTC at all.  Simply calibrate the drift file on 
the master (so that you get a full +/- 500ppm tolerance for the other 
machines, and run it with only the local clock as source.

However, depending on the quality of your network, you may need to 
provide a common 1PPS feed to all the machines.  I'm not sure it is 
possible to do this with the local clock as the master clock on one 
machine, so you may need to make some code changes.

You should allow time steps, as not allowing them tends to disable the 
kernel time discipline, which you will need for very accurate clock 
tracking.




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