[ntp:questions] Re: ref clock for network POP locations

Heiko Gerstung hg at heiko-gerstung.de
Wed Oct 4 01:31:07 UTC 2006

Richard B. Gilbert schrieb:
> Jeff W. Boote wrote:
>> Heiko Gerstung wrote:
>>> Richard B. Gilbert schrieb:
>>> I was referring to sub-microsecond accuracy over the network, it
>>> seems that the OP has no chance to use a hardware ref clock.
>>>> My only knowledge of PTP is based on some earlier messages here but
>>>> I believe that it must have the same problems as NTP over the internet.
>>> PTP uses hardware timestamping at the MII/PHY level, this will not
>>> help when you use the Internet, but maybe the OP has his own nice WAN
>>> with low-jitter connections available. You can get in the lower
>>> nanoseconds with PTP over Ethernet, but only in very small networks
>>> or by using PTP-aware infrastructure components like switches with
>>> integrated hardware timestamping..
>> I do have access to a very nice WAN with extremely low-jitter
>> connections. I can likely remove nearly all buffered devices from the
>> path - and get nearly the equivalent of a really long cross-over cable.
>>>> <commercial for $1000+ product snipped>
>>> The mentioned LANTIME/NDT is basically an oscillator that is
>>> disciplined by NTP. And yes, its too expensive for using it at home :-)
>> Right - eventually it all comes down to price. How much accuracy can
>> you afford... But, this at least sounds interesting.
>> Thanks for the pointers.
>> jeff
> Jeff,
> I think the killer keywords here are "disciplined by NTP".  (BTW, I
> thought it was GPS disciplined)  If it is, in fact, disciplined by NTP,
We have several different NTP time servers, synchronized to GPS, NTP,
IRIG B and so on. The NDT is the one that is using another NTP server as
its primary reference.

> it is no better than your NTP time source, be it reference clock, or
> internet server.  If it is, as I thought, disciplined by GPS, then it's
> just a high grade crystal oscillator if you cannot apply the GPS signal
> to it.  I question whether it can maintain "sub microsecond accuracy"
> without the GPS discipline.
Sub-Microsecond accuracy is not possible with it, our measurements in a
separated LAN show an accuracy of +/- 30 microseconds.

> It's a very nice toy and, if I should hit the lottery for a couple of
> million, I'll probably buy one.
I would not call it a toy, there are actually people doing serious
things with it, but we are getting off-topic :-)

> If you really need that sub microsecond accuracy and must live with the
> constraint of "no radio reception" your best bet is a Cesium clock. It's
> expensive, but once calibrated by NIST it should drift no more than  400
> nanoseconds per year.  If that 400 nanoseconds is significant, you'll
> want to have it calibrated annually.

Best regards,

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