of comparison, it is a good idea to compare this holdover to that of T-BC/T-TSC assisted by physical layer frequency reference (shown in Figure 9-31 in the preceding "Holdover" section). 100 ns 10 ns Figure 9-32 Permissible Phase Error for TBC-A/T-TSC-A During Holdover-Based on Figure 7-1 of ITU-T Recommendation G.8273.4 G.8273.4 defines one more holdover scenario for T-BC-A/T-TSC-A, one where the local time refer- ence input is lost but the PTP input is valid and ideal. This is a case where the holdover is assisted by PTP and SO is referred to as holdover based on PTP. Under constant temperature conditions and for a maximum observation interval of 1000 S, the MTIE limit recommended is 222 ns. 9.4.1 T-BC-P, T-TSC-P The T-BC with partial support (T-BC-P) is a BC with only partial timing support from the network and without any assistance from a local time reference. A physical frequency input signal is op- tional for this class of clock. The PTP slave port recovers clock over the PTS network, but this is its primary time reference signal, not a secondary/backup. In the same fashion, a T-TSC with partial support (T-TSC-P) is the slave-only equivalent of the T-BC-P boundary clock. Not only for the assisted clocks, G.8273.4 (in clause 8) also defines the performance requirements for the T-BC-P and T-TSC-P partial clocks. Refer to the section on G.8273.4 in Chapter 8 for an overview of this ITU-T recommendation. Noise Generation The noise generation of a T-BC-P/T-TSC-P represents the amount of noise produced at the output of the clock when locked to an ideal (wander-free) PTP signal as input. As with the T-BC-A case, the output of the T-BC-P is measured at either the 1PPS output or the PTP master port, whereas the out- put of the T-TSC-P is measured at the 1PPS output. Table 9-10 summarizes the cTE and dTE that a T-BC-P/T-TSC-P is permitted to generate from within the clock. The max|TE| for noise generation is for further study. Table 9-10 Maximum Noise Generation of T-BC-P/T-TSC-P Maximum Limits (as defined by ITU-T 