her, is defined to have three basic modes of operation: locked, free-run, and holdover. As described in Chapter 3, in locked mode, the output of the clock is considered synchronized with the input clock, such that the output frequency is tracking (or aligned) to the input reference frequency. This state of operation is the locked condition for a slave clock. If the input reference is lost, the slave clock can enter a holdover state. The holdover state is achieved by using stored data, acquired during locked mode, to control the frequency and phase of the output clock. In this mode, the output clock is a "reproduction" of what was the input reference. A transition back to the locked mode would occur when the input reference becomes available again. The final mode of operation is called free running. The output clock in this mode of operation is based on the oscillator of the slave clock and is not tracking the input reference signal or using stored data. This mode is the typical mode of operation when the slave clock is first powered on. It is also achieved when there is no stored data available in which to transition to the holdover state. Refer to the section "Clock Modes" in Chapter 3 for a detailed description of the different modes of operation of a clock and the different possible transitions between the modes. 6.3 Clocking Hierarchy The basic task of network synchronization is to distribute the reference signal of the most accurate clock available to all network elements that require synchronization. Different methods (such as ex- ternal timing and line timing) that distribute the reference signal in the network mostly use a single strategy, known as master-slave synchronization. This method uses master clock and slave clock nodes whereby a slave clock always receives timing information from a master clock. And this strategy is then replicated throughout the network. Note that this means that a device slaved to an upstream master clock can also take the master clock role to other downstream equipment. The r