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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Clocking Sources (was NTp sources that work in a datacenter (wasRe: Is latency equivalentto RTT?))
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 |