E val- ues for 2-s intervals). 6. Record the highest value against the 2-s horizontal axis position in the MTIE graph. 7. Repeat for other time intervals (4 S, 8 S, 16 S, etc.). Figure 5-19 shows the MTIE limit for a primary reference time clock (PRTC), as specified in ITU-T G.8272. PRTC-A 100ns PRTC-B Figure 5-19 MTIE Limits for Two Classes of PRTC Clock (Figure 1 from ITU-T 8272) The flat graph required by the recommendation for the MTIE values suggests that after a certain time period, there should not be any new maximum delay variation recorded for a PRTC clock. This maximum MTIE value, as recommended in ITU-T G.8272, is 100 ns for a PRTC class A (PRTC-A) clock and 40 ns for a PRTC class B (PRTC-B) clock. This line on the graph represents the limits be- low which the calculated values must remain and is referred to as a mask. The standards development organizations have specified many MTIE masks-for example, the ITU-T has written them into recommendations G.812, G.813, G.823, G.824, and G.8261 for fre- quency performance of different clock and network types. To summarize, MTIE values record the peak-to-peak phase variations or fluctuations, and these peak-to-peak phase variations can point to frequency shift in the clock signal. Thus, MTIE is very useful in identifying a shift in frequency. 5.2.1 Time Deviation Whereas MTIE shows the largest phase swings for various observation intervals, time deviation (TDEV) provides information about phase stability of a clock signal. TDEV is a metric to measure and characterize the degree of phase variations present in the clock signal, primarily calculated from TIE measurements. Unlike MTIE, which records the difference between the high and low peaks of phase variations, TDEV primarily focuses on how frequent and how stable (or unstable) such phase variations are OC- curring over a given time-note the importance of "over a given time." This is important because both "how frequent" and "how stable" parameters would change by changing the time duration of the measurements.