Cost-effectivenesss of highly-accurate clocks for NTP

Eric S. Raymond esr at thyrsus.com
Sun May 15 10:21:33 UTC 2016


Baldur Norddahl <baldur.norddahl at gmail.com>:
> Ok how many hours or days of holdover can you expect from quartz,
> temperature compensated quartz or Rubidium?

Sorry, I don't have those drift figures handy.  I'm a programmer, not
a large-site sysadmin - I've never had purchase authority with a
budget large enough to buy a rubidium-oscillator GPSDO or any other
device with holdover.  Better to ask Mel Beckman or someone else
with field experience.

>                                 Should we calculate holdover as
> time until drift is more than 1 millisecond, 10 ms or more for NTP
> applications?

If you want to go super-accurate, 1ms.  If you want to go cheap, on
sampling-theory grounds I'd say you want to vary your drift threshold
from 1 to 5ms (half the expected precision of WAN time, think of it as
the Nyquist rate) and look for a knee in the cost curve.

> I am thinking that many available datacenter locations will have poor GPS
> signal so we can expect signal loss to be common. Some weather patterns
> might even cause extended GPS signal loss.

Weather won't do it, usually. Rain and fog and clouds are transparent
to GPS frequencies. Standing water directly on an antenna can cause
some attenuation, but with any serious GPS engine made more recently
than 5 years ago I would be extremely surprised if that lost it
lock.  The newer ones handle down to 30 feet in ocean water on robot
submarines.
-- 
		<a href="http://www.catb.org/~esr/">Eric S. Raymond</a>


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