[ntp:questions] frequency adjusting only

maxime louvel m.louvel at gmail.com
Wed Apr 23 12:05:06 UTC 2008


- The decision of buying a GPS is not mine ...
So let's say for now that's still not possible, if no other solution is
found I'll conclude that a GPS has to be bought.

- Danny, you're talking about the delay I got with ntpq -p ?

- I have chosen a broadcast configuration for two reasons :
  - easier to install
  - I am doing the tests on only 2 nodes, thus with broadcast I assumed that
more nodes won't change the accuracy

I though it was possible to achieve the same precision with broadcast than
with C/S, that's erroneous ?

maxime

On Wed, Apr 23, 2008 at 12:42 PM, Danny Mayer <mayer at ntp.isc.org> wrote:

> maxime louvel wrote:
>
> > Thanks,
> >
> > I am actually using 4 public NTP server, which whom my node-1
> > synchronises to.
> > Then node-1 broadcasts the time to all the other nodes in the subnet.
> >
> >
> If you are using broadcast mode then you have two additional issues that
> you need to deal with:
>
> 1) authentication is required by default
> 2) you are not measuring the delay for every packet
>
> With autokey, the delay gets measured during the autokey dance which
> happens in client/server mode and then it switches to broadcast when it
> completes. You cannot measure the delay in broadcast mode since the server
> sends out the packets and the client just receives them. Broadcast can only
> apply the delay estimates that it got when in the autokey dance and cannot
> account for changes until the next autokey dance.
>
> If accuracy is critical in this environment, don't use broadcast. At least
> in client/server mode you get to measure the delay with each packet.
>
> Danny
>
>
>
>  My goal is to achieve a synchronisation between the nodes (not with the
> > public NTP server) within 50 usec.
> > I don't care if the synchronisation to the public NTP is accurate around
> > half a second.
> > I think it's possible, because the nodes are close to each other and
> > every communication is using gigabit ethernet (cards + switch).
> >
> > I achieve to get a small offset when nothing else than linux and ntpd is
> > running on the nodes
> > (offset arount 10 usec).
> >
> > I have a program which sends data from one node to another.
> > The sending/reception are timestamped and the delay is then computed.
> >
> > I would like the measured delay to be constrained in a 50usec wide
> > range.
> >
> > Do you think it should be possible ?
> >
> > I have tried to run the test for 24h and didn't get the expected
> > results.
> > The range of the measured delay is 2 milliseconds (between -1ms and
> > +1ms) with some spikes (ten or so for a milion measurments). The mean delay
> > is -0.066087 (ms) and the std dev is 0.22849
> > Do you think the spikes can influence ntp  ?
> >
> > thanks very much
> >
> > Maxime
> >
>


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



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