[ntp:questions] frequency adjusting only

maxime louvel m.louvel at gmail.com
Fri Apr 25 06:28:52 UTC 2008

On Thu, Apr 24, 2008 at 11:27 PM, David Woolley
<david at ex.djwhome.demon.co.uk.invalid> wrote:

> 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.

I actually don't really understand why a GPS would improve my
Indeed I can't have a GPS to all the nodes (no connectivity possible). My
nodes are actually embedded cards. They run linux have have some pretty good
features for embedded platforms. But still the available connectivity is
Thus if I use a GPS it would be connected to a server in the same subnet and
my nodes will then synchronise to it. Thus why should I get a better
accuracy between my node than I already have ? The network and the nodes
latency won't be improve.

Unless if the GPS allow my server to be more stable and thus I can assume to
get a better synchronisation...

> >
> > 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.

- I have to keep my nodes synchronised to real time, for several reason:
they use NFS, they interact with the outside world.
- I can't have any clock step, because I have some application that assume
there is no one. If clock steps occur, my applications will be screwed up.

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Maxime Louvel
0044 7964 5555 80
43 Allen road
Whitemore reans
WV60AW Wolverhampton
United Kingdom

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