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 one part in 10^11?
A. 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 tracks (and is described as being a traceable to) a Primary Reference Source that meets stratum 1 long-term accuracy of one part in 10^11. 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: If primary reference source (PRS) traceability is lost by the NE, it enters holdover mode. In this mode, the NE clock’s tracking phase lock loop (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.
Related Questions
- 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 one part in 10^11?
- What kind of cable can be used to distribute time code from a master clock to several slave clocks?
- wireless clocks be used with any other master clock system?
