uced by environmental changes over the course of the day (diurnal rhythms), such as the afternoon sun strik- ing the exterior metal panel of the clock. So, to gain confidence in the TDEV values (primarily to discard the rare noise occurrence in clock), the tests are usually quite long, at least several times longer than the usual MTIE observation inter- vals. For example, ITU-T G.811 calls for running TDEV tests for 12 times the maximum observa- tion interval. Compared to TDEV, you might notice that MTIE is perfectly suited to catch outliers in samples of TIE measurements. MTIE is a detector of peaks, but TDEV is a measure of wander over varying observation time intervals and is well suited to characterization of wander noise. It can do that be- cause it is very good at averaging out short-term and rare variations. So, it makes sense that TDEV is used alongside MTIE as metrics to characterize the quality of clock. As is the case for MTIE, ITU-T recommendations also specify TDEV masks for the same purpose. These masks show the maximum permissible values for both TDEV and MTIE for differ- ent clock types and in different conditions. 5.2.1 Noise In the timing world, noise is an unwanted disturbance in a clock signal-the signal is still there, and it is good, but it is not as perfect as one would like. Noise arises over both the short term, say for a single sampling period, as well as over much longer timeframes-up to the lifetime of the clock. Because there are several factors that can introduce noise, the amount and type of noise can vary greatly between different systems. The metrics that are discussed in this chapter try to quantify the noise that a clock can generate at its output. For example, max TE represents the maximum amount of noise that can be generated by a clock. Similarly, cTE and dTE also characterize different aspects of the noise of a clock. Noise is classified as either jitter or wander. As seen in the preceding section "Jitter and Wander," by convention, jitter is the short-term variation, and wa