The recommendations from the ITU-T that define the performance and characteristics of different types of clocks are based on the role of the clock and the timing network architecture. Table 9-4 lists the ITU-T recommendations for different clocks applied to each timing network architecture. Table 9-4 ITU-T Recommendations for Phase and Time Delivery Clocks ITU-T Recommendation Clock Type Network Type G.8272 G.8272.1 ePRTC G.8273.2 T-BC / T-TSC Full timing support G.8273.3 G.8273.4 (Clause 7) T-BC-A / T-TSC-A Assisted partial support G.8273.4 (Clause 8) T-BC-P / T-TSC-P Partial support There are at least three aspects to consider when trying to understand the performance characterist- ics recommended by ITU-T standards: Performance metrics: Performance metrics are specific to the clock type and are very pre- cisely defined. If there is an MTIE or TDEV mask specified, usually it is different for each type of clock. For example, it would not serve any purpose if a T-BC were to fail the strict MTIE mask defined for a PRTC clock. Measurement approach: As the performance metrics are developed, there is a set of interfaces used to measure and qualify against the recommended metrics. It is necessary to understand at which position the data for measurement can be collected. For the case of phase/time delivery via packets, the data can be collected via PTP packets, at the output of the 1PPS interface, from frequency outputs of the clock, or at all of them. Key elements of clock specifications: As listed in the earlier section "Parameters for Timing Performance", the five key elements of ITU-T clock performance are noise generation, noise tolerance, noise transfer, holdover, and transient response. Limits and values for these met- rics appear in almost all the clock specifications covering performance. In Chapter 5, the sections "Noise," "Holdover Performance", and "Transient Response" explain these terms in detail. Although Chapter 5 deals with the metrics for frequency delivery, the general approach for clock performance 