arge value at the start of the measurement interval and starts converging slowly toward zero, it might suggest that the clock or PLL is not yet locked to the reference clock or is slow in locking or responding to the reference input. Note that the measurement of the change in TE at each interval really becomes measurement of a short-term variation, or jitter, in the clock signals, or what is sometimes called timing jitter. And as TIE is a measurement of the change in phase, it becomes a perfect measurement for capturing jitter. Now it is time to revisit the concepts of accuracy and stability from "Chapter 1. Introduction to Synchronization and Timing". These are two terms that have a very precise meaning in the context of timing. For synchronization, accuracy measures how close one clock is to the reference clock. And this measurement is related to the maximum absolute TE (or max|TE|). So, a clock that closely follows the reference clock with a very small offset is an accurate clock. On the other hand, stability refers to the change and the speed of change in the clock during a given observation interval, while saying nothing about how close it is to a reference clock. As shown in Figure 5-17, an accurate clock might not be a very stable clock, or a very stable clock might not be accurate, and SO on. As the end goal is to have the most stable as well as the most ac- curate clock possible, a clock is measured on both aspects. The metrics that quantify both these as- pects become the timing characteristics of a clock. Error Accurate and stable clock Error Inaccurate and stable clock Error Accurate and unstable clock Error Inaccurate and unstable clock Figure 5-17 Considering Clock Accuracy and Stability As you should know by now, the timing characteristics of a clock depend on several factors, includ- ing the performance of the in-built oscillator. So, it is possible to use that to differentiate between different classes of performance that a clock exhibits. Consequently, the ITU-T has classified clocks by the diff