Clocking Sources (was NTp sources that work in a datacenter (was Re: Is latency equivalentto RTT?))

Fri May 16 19:55:51 UTC 2003

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, 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.

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
>
>