[ntp:questions] RE: NTP precision

Eric Liu EricLiu at moxrd.com
Fri Oct 21 08:17:59 UTC 2005


> Message: 1
> Date: Fri, 23 Sep 2005 16:03:13 GMT
> From: Tom Smith <smith at cag.zko.hp.com>
> Subject: [ntp:questions] Re: NTP precision
> To: questions at lists.ntp.isc.org
> Message-ID: <43341FD3.5080404 at cag.zko.hp.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> Leandro Pfleger de Aguiar wrote:
> > Hi
> >
> >     I4m using NTP to timestamp digital events in a specifit
> application. On my application i need to know what is the exact
> estimated error that my timestamp marks should consider. Some
> peaple frequently use offset to mean what is the real estimated
> difference between local and ref clock. Should i consider other
> parameters like jitter,  precision and accuracy manually ?
> Remember: i need to say how mutch can my timestamp be wrong.
> >
> > Tanks Again !
>
> Interesting question. I think the data needed, taking into account
> all of the servers, is in the results of "ntpq -c rl 0":
> ^^^^^^^^^^^^                    ^^^^^^^^^^^^

Hello, everyone:

Talking about NTP accuracy, I'd like to say something because I have done
experiments on this.

This is my test environment:
Take one machine as NTP server and use its local clock. There are other two
machines to be synchronized to the server. All machines can be interrupted
by external digital events. As soon as the external event comes, the
interrupt routine will record its own time. Just compare the time stamps of
server and client, I can find out NTP performance.

If a low-level oscillator(such as 100PPM) is installed on NTP client, use
less shorter poll interval, such as 500us accuracy with poll interval
16-32s. If the default poll interval 64-1024s, I've got the accuracy 2ms.

Network delay and network jitter does influence NTP accuracy very much. I
know this by an experiment on LAN.

All machines that you are using are capable of running ntpd in case that you
donnot have 486 or 586 PCs. CPU speed is not a problem. But donnot make your
machine run with very very heavy burden.

Use Linux, FreeBSD, UNIX, never Windows.


FYI
Eric






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