external reference clock signal. However, even when a local clock is determined to be synchronized in both phase and frequency to a reference clock, there are always errors that can be seen in the final synchronized clock signal. These errors can be attributed to various factors such as the quality and configuration of input cir- cuit being used as a reference; any environmental factors that influence the synchronization process; and the quality of oscillator providing the initial clock. To quantify the error in any synchronized clock, various parameters have been defined by the stand- ard development organizations (SDO). To understand the errors and the various metrics to quantify these errors, refer to Figure 5-13, which illustrates the output of a clock synchronized to an external reference clock. Because the measurement is taken from the synchronized clock that is receiving a reference signal, this synchronized clock will be referred to as the measured clock. (n 1) Reference clock Measured clock X(n- 1) X(n+ 1) Figure 5-13 Comparing Clock Signals It is also important to note that the error being referred to here is an error in significant instants or phase of a clock. This is called phase error, which is a measure of the difference between the signi- ficant instants of a measured signal and a reference signal. Phase error is usually expressed in de- grees, radians, or units of time; however, in telecommunication networks, phase error is typically represented as a unit of time (such as ns). The two main aspects that you need to keep in mind for understanding clock measurements are as follows: Just as the measured clock is synchronized to a reference clock, the measured clock is always compared to a reference clock (ideally the same one). Therefore, the measurement represents the relative difference between the two signals. Much like checking the accuracy of a wrist- watch, measurement of a clock has no meaning unless it is compared to a reference. The error in a clock varies over time, SO it is important that 