y of the oscillator and how it reacts to changing conditions such as temperature, power supply fluctuations, and aging. A clock with an oscillator in this state will signal a quality level based on the fundamental capabilities of its oscillator. For instance, a free-running clock with a cesium oscillator should signal that it is a better clock than one with a TCXO. In fact, after the warm-up, a standalone cesium clock is only ever in free-running mode (refer to the follow- ing section "Clock Types"). Acquiring The acquiring state is when the slave clock is attempting to align itself with the traceable reference clock. While the clock is acquiring, it may squelch the distribution of its clock signal to downstream slave clocks or signal a clock quality value indicating that it is not yet locked or aligned. Some other clocks, particularly frequency systems, might signal (following some settling time) the quality of the input clock rather than the quality of its own oscillator (like the warm-up case). Normally, acquiring is the step between free-running state and locked state, but a clock can also drop back to acquiring from out of the locked state. Locked Mode After some time acquiring, the clock determines that it is well aligned to the reference and enters locked mode. In this mode, the equipment has traceability to a reference clock signal, and its oscil- lator is synchronized with that signal. Being locked is the normal state of a slave because aligning to the input from the reference improves the accuracy of the clock. To stay locked and closely aligned, the control circuitry (see the section "PLLs" in Chapter 5) is constantly adjusting the clock to correct any drift from the reference. When the changes required to stay aligned are too large or too abrupt, the local clock may decide to go back and restart the acquir- ing process. Holdover Mode Consider a scenario where the equipment has been in a locked mode for an extended period. During this time, it has gathered data comparing the behavior of its local oscil