s. So, there are two types of hold- over available in a T-BC/T-TSC: one where only the PTP disappears and one where both signals are lost. In the first case, a T-BC/T-TSC loses the PTP packet timing signal and yet the physical layer fre- quency reference remains traceable to a PRC/PRS. This is also referred to as frequency-assisted holdover, and it helps with holdover performance because the frequency reference is used to keep the time output "ticking" at close to the correct rate. And so, it follows that the holdover depends on the quality of the frequency reference. If the fre- quency remains traceable to a PRC, it is likely to maintain very accurate phase/time for an extended period. A PRC-traceable frequency reference extends the period of holdover (at microsecond accur- acy) for network elements from hours to more than a week. The requirements for holdover are divided based on what class of T-BC/T-TSC that the device per- formance needs to meet. Table 9-7 summarizes the holdover requirements for Classes A and B for a maximum observation interval, tau (T) of 1000 seconds with both constant and variable temperat- ure. Note also that the holdover performance is a function of observation interval itself. Table 9-7 Holdover Limits During Loss of PTP Input for T-BC/T-TSC Classes A and B at Constant Temperature Temperature Condition MTIE (ns) Observation Interval T (s) Approximate Range (ns) 22 + 40 .1 1<0< 100 62-85 Constant 22 + 25.257 0.2 100 <T< 1000 85-123 22 + 40T 0.1 + 0.57 1<T<100 62.5-135 Variable 72 + 25.257 0.2 100 <T< 1000 135-173 For further study 1000 10 000 Source: ITU-T G.8273.2, Tables 7-9 and 7-10 Figure 9-31 illustrates the permissible limits for phase error of T-BC/T-TSC clocks for Classes A and B holdover performance for 1000-s duration (starting immediately after the clock moves to hol- dover). Under constant temperature conditions (within +1 K) the maximum observation interval is 1000 S. 123 ns 100 ns 85 ns 62 ns 10 ns Figure 9-31 Phase Error Limits for T-BC/T-TSC Classes A and B During Holdo