Clocking Sources (was NTp sources that work in a datacenter (was Re: Is latency equivalentto RTT?))
Stephen J. Wilcox
steve at telecomplete.co.uk
Fri May 16 20:22:15 UTC 2003
On Fri, 16 May 2003, N. Richard Solis wrote:
> You are correct with one notable exception. The presence of a cesium clock is
> considered a de facto timing source. I've never seen a BITS system that
> actively disciplines a Cesium Source. What they will do is take many Cesium
> (or Primary Reference Sources, PRS) and verify them against each other. The
> output from that mathematical function is then used to discipline a Rubidium
> or Ovenized Crystal Oscillator. The output from THAT is then used to drive
> BITS feeds to telecom equipment.
> The desire for everyone to have a timing source that is tracable to a Cesium
> clock comes from the SONET standard. If you tie two SONET networks together,
> if they both don't have timing that's tracable to a Stratum 1 (PRS) source,
A Stratum 1 source will slip but thats ok if its within tolerance
If you tie two networks together that both have their own PRS source they will
still slip because of these in tolerance variances thats why its ideal to have
only one PRS.
> they'll drift at the points where they interconnect and PSE (Positive Stuff
> Event) and NSE (Negative Stuff Event) errors will be the result. This is BAD
> BAD BAD for the voice networks that are provisioned over SONET.
Shouldnt be too bad, the MUXs that do the stuffing have their own clock
independent of the circuits so the data goes out how it comes in.
End devices connected to a single network will derive clocking from that network
and be happy with whatever that clocking does within reason (course this doesnt
work when you connect to two with their own PRS!)
> Spinka, Kristofer wrote:
> > 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.
> > /kristofer
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