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

Heiko Gerstung heiko.gerstung_removeme_ at meinberg.de
Thu Sep 28 08:20:52 UTC 2006


Jeff W. Boote schrieb:
> I have some network locations where I have no way to access GPS or CDMA. 
> But, I would still like to have a very accurate time source. (I would 
> like sub-microsecond accuracy from NTPD if I can get it.) These 
> locations are in telco-hotels, so they are typically very well shielded 
> making any broadcast technology unworkable. (We are not using sonet - so 
> getting time from there is also not an option.)
> 
> I was thinking that it might be possible to get an oscillator to provide 
> a PPS signal, and then use a very well connected (possibly even a 
> dedicated network link with no router/switch) network peer as the 
> preferred peer. Is this a reasonable solution? Can anyone suggest 
> anything better?
> 
> The PPS would need to be calibrated... Is the normal ntp calibration 
> reasonable for this? I would guess it depends on the oscillator...
> 
> If this is a reasonable solution, does anyone have recommendations for 
> oscillators? Most of the ones I have seen want a reference source of 
> their own. This of course does not work in my case...
> 
> Thanks,
> jeff
> 
> P.S. I will happily add information to the wiki on this after figuring 
> out how to do it. (If?)

Hi Jeff,

as already mentioned, sub-microsecond accuracy is not a possibility with 
NTP these days. Did you check out PTP (IEEE 1588) instead 
(ieee1588.nist.gov)?

We offer a NTP appliance that has its own free-running OCXO-HQ that is 
disciplined by NTP over the network. Using a very smooth filtering and 
applying only very small corrections allows us to get down to 50 to 100 
microsecond accuracy in an "ideal" network environment, i.e. 
crossover-cable.

The model is called LANTIME/NDT (Network Disciplined Timeserver) and you 
can find it on our website (www.meinberg.de, see Products section).

I would say that you have to check out PTP if you really need to sync 
down to the nanoseconds ...

Best regards,
Heiko


-- 
Meinberg radio clocks: 25 years of accurate time worldwide

MEINBERG Radio Clocks
www.meinberg.de

Stand alone ntp time servers and radio clocks based on GPS, DCF77 and 
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