QUESTION: Most network elements have internal stratum 3 clocks with 4.6ppm accuracy, so why does the network master clock need to be as accurate as 1 part in 10^11?
ANSWER: Although the requirements for a stratum 3 clock specify a free-run accuracy (also pull-in range) of 4.6ppm, a network element (NE) operating in a synchronous environment is never in free-run mode! Under normal conditions, the NE internal clock is tracking (and is described as being a traceable to) a Primary Reference Source that meets stratum 1 long-term accuracy of 1 part in 1011.
This accuracy was originally chosen because it was available as a national primary reference source from a cesium-beam oscillator, and it ensured adequately low slip-rate at international gateways.
Note that if PRS traceability is lost by the NE, it enters "holdover" mode. In this mode the NE clock's tracking PLL does not revert to its free-run state, it freezes its control point at the last valid tracking value. The clock accuracy then drifts elegantly away from the desired traceable value, until the fault is repaired and traceability is restored.
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