[ntp:questions] Re: time

Bjorn Gabrielsson bg at lysator.liu.se
Wed Nov 30 22:35:54 UTC 2005


K,

I think you misread some points.

"K" <spam at nospam.com> writes:

> So to summarize you guys.  I think that you are saying that
> ntpd v3

We are using the current ntpd version, which is v4.

> is accurate on a gigabit network that is congested  to 5-20ms  is 
> the accuracy is a reasonable expectation.

No, Richard gets some 10ms from _Internet_ servers, I get sub
1ms. That is computers on our local lan, getting time from Internet
servers located far away - in my case - 10-20ms 'ping'-distance away,
10-15 router hops and 200 to 600 km physical distance away.

My lan is not gigabit and I _dont_ have gigabit internet
connectivity. In fact its a slow 512kbps ADSL-line.

>  What about IEEE 1588, since this 
> is also ethernet bases can I get any better time accuracy with that?

Yes, but IEEE 1588 uses special ethernet chips that timetag the
arrival time of your time packets already in the ethernet hardware. If
you have switches/routers between your computers, it implies that these
switches/routers are IEEE 1588 compliant.

Look for what Martin from Meinberg, said about 1588 some time ago.

> > Either your internet connection, your internet servers, or both are far 
> > better than mine.  I have broadband cable.  My NTP server, Sunblok, uses 
> > five internet servers in addition to the Motorola GPS reference clock. 
> > The internet servers show systematic offsets with respect to GPS of +1 to 
> > +5 ms.  There are also seemingly random spikes in the time received from 
> > the internet servers of +/- 5 to 20 ms.  When the GPS is not running, ntpd 
> > tends to synchronize with a different server every few minutes which does 
> > nothing for the stability of the clock. 

The servers I use are very good, and have excellent network
connectivity at the Internet exchange points in my country. The
connectivity means their networks will not be congested. As for the
refclocks they use a GPS and quad Rubidiums for holdover. See,

        http://www.netnod.se/internettime.htm

My ntp-performance is very dependant on that I do not fill my DSL line
with heavy downloading. That will easily give jitter in the hundreds
of ms range. I have heard that some DSL modems/DSLAMS will buffer up in the
range of seconds of throuput in the inbound or outbound buffers! Not
strange that our ntp performance goes away... 

--

    Björn




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