[ntp:questions] Re: ref clock for network POP locations
Richard B. Gilbert
rgilbert88 at comcast.net
Fri Sep 29 21:42:49 UTC 2006
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.
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,
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.
It's a very nice toy and, if I should hit the lottery for a couple of
million, I'll probably buy one.
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.
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