 operation before starting the test). The requirements for ePRTC-B are for further study but will obviously be tighter than that. Owing to the equipment needed and the time needed to test to these levels, it is only of interest to timing experts, and further discussion is outside the scope of this book. Frequency Testing Frequency testing is very closely related to the original TDM testing from decades past. This book will not cover much in the way of frequency testing for TDM interfaces, as the market is well served for literature on this topic. Just be aware that these tests are aligned with standards that have been in force for many years. Frequency measurement can be performed on any frequency signal transmitted by a piece of equip- ment. It could be a 10-MHz or 2048-kHz signal (or some other frequency), a BITS output such as E1/T1, or a TDM signal on a traffic interface. There are many (legacy) test devices that can be used to test these forms of signals, do the statistical analysis, and then check the results against the appro- priate ITU-T mask-such as G.811, G.811.1, G.812, and G.813. For frequency testing of a PRTC or PRC/PRS, the relevant recommendation is G.811 (see Chapter 8 for details on the ITU-T recommendations), and the enhanced PRC is in G.811.1. The testing of fre- quency for reference clocks is relatively simple because they are output-only devices, SO conducting tests like those for noise tolerance and transfer makes no sense. Frequency testing includes the following classes of tests: Frequency accuracy. For PRC/PRS/PRTC clocks this is 1 part in 10 11 over one week from G.811; for ePRC/ePRTC clocks this is 1 part in 10 12 over one week from G.811.1; and for other clocks this is 4.6 ppm in free-running state. Noise generation (jitter and wander) with MTIE and TDEV masks. Pull-in, hold-in, and pull-out (see Chapter 5; not used for PRC/PRS/PRTC clocks). Noise tolerance (jitter and wander; not used for PRC/PRS/PRTC clocks). Noise transfer (not for PRC/PRS/PRTC clocks). Transient response and hold