Clocking Sources (was NTp sources that work in a datacenter (was Re: Is latency equivalentto RTT?))
kspinka at style.net
Wed May 14 22:06:51 UTC 2003
The actual clock source, the time pulses, produced by BITS (Building
Integrated Timing Systems) are not usually just a "re-feed" of the GPS
source. Instead, contained within the clock system, BITS device, is some
sort of highly resolvable, measurable, predictable, stable, oscillator
such as Cesium, Rubidium or Quartz, depending on your budget.
The GPS signal, being transmitted from orbiting Cesium clocks, is used
to "discipline" or prevent your local oscillator drift from degrading your
time code. This basically means that if the oscillator installed in
the unit is supposed to oscillate exactly 10,255,432 times per
second, and an interal or external condition has now caused it to
oscillate at 10,255,434 times per second, your pulse generator will
account for this and keep your time code stable. These GPS satellites
have Cesium oscillators that drift also, and are in turn disciplined
themselves against the "USNO Master Clock", which is an average time code
calculated from a slew of cesium atomic and hydrogen maser clocks.
So, why can't an entire bulding share the same GPS discipline source?
It would save some roof space and money. If you are worried about someone
kicking the antenna over, just make sure that your oscillator is
reasonable enough to maintain your necessary accuracy without external
discipline while they glue the antenna back on. For most applications a
high quality, stable quartz based oscillator will be more than sufficient
and is the least expensive. If you need to get fancy, and depending on
your internal and external condition variables, the general rule is:
Quartz < Rubidium < Cesium.
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