NIST NTP servers

Lamar Owen lowen at pari.edu
Sat May 14 14:27:41 UTC 2016


On 05/13/2016 04:38 PM, Mel Beckman wrote:
> But another key consideration beyond accuracy is the reliability of a server's GPS constellation view. If you can lose GPS sync for an hour or more (not uncommon in terrain-locked locations), the NTP time will go free-running and could drift quite a bit. You need an OCXO to minimize that drift to acceptable levels.
While this is drifting a bit off-topic for NANOG (and drifting into the 
topic range for time-nuts at febo.com), I'll just add one more thing to 
this.  The Hold time (when the oscillator is free-running) is a very 
important consideration, especially, as you say, when terrain is an 
issue. For us it is even more important, as the 10MHz output from the 
timing rack is used as a site-wide frequency standard.  Of course, you 
never discipline a cesium PRS, but the rubidium secondary is disciplined 
by circuitry in the SSU2000.

Back in the days of common backbone delivery over SONET discussion of 
cesium standards would have been on-topic, as some SONET gear (Nortel 
Optera for instance) needs a master clock; especially if you were 
delivering channelized circuits or interfacing with customers and telcos 
with DS3 or even DS1 circuits or DS0 fractions within them.  Ethernet is 
far more forgiving.



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