lator using the input timing signal as a reference. When the equipment loses all connections to a reference clock, it moves into the holdover state. The local clock uses the data previously learned during the locked period to try to maintain the alignment of its oscillator with the reference clock. Note the difference compared to free-running state: in holdover state, the clock is trying to maintain clock accuracy by applying historical data to its oscillator, whereas with free-running, no attempt is made to train or guide the oscillator. The accuracy of a clock in holdover will generally be much better than one in free-running, at least for a while after the start of holdover. Clocks generally signal this change in state to downstream clocks SO that those clocks can decide whether to continue to be steered by this holdover clock or try to select another, better source that might be traceable to a valid reference. If the clock in holdover state is measured against a reference clock, the frequency and phase of the clock slowly starts to drift as the local oscillator wanders off its proper frequency. This is because the oscillator in the clock is affected by its physical environment, and SO the data the clock is using to steer the oscillator becomes stale and increasingly out of date. As time progresses, the accuracy progressively degrades to a point where any advantage to being in the holdover state is gone. To guard against this degradation in accuracy being passed to downstream devices, the clock estim- ates how much accuracy might have degraded since it lost connection to the reference. When it de- cides that it is no longer accurate enough to remain in holdover, it reverts to free running (and sig- nals that downstream). Another way that holdover mode terminates is that the equipment reacquires the signal from a trace- able reference clock and moves back to locked mode. 3.2.1 ANSI Stratum Levels of Frequency Clocks Clocks distributing frequency are defined based on their performance, and there are several cloc