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

Heiko Gerstung heiko.gerstung_removeme_ at meinberg.de
Thu Sep 28 15:06:10 UTC 2006


Richard B. Gilbert schrieb:
> Heiko Gerstung wrote:
> 
>> 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.)
> <snip>
>> 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)?
>>
> 
> That is not true.  You can get microsecond accuracy or better using a 
> reference clock.  What you can't do is get that kind of accuracy over 
> the internet.

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..

> <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 :-)

Best regards,
Heiko


-- 
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