iod before it is restored 2. Switchover of the physical layer input while still maintaining the PTP input 3. Simultaneously switching both the physical layer and the PTP input This section discusses only case 1, because at the time of writing cases 2 and 3 are for further study. For the T-BC-P, the transient response is measured at the PTP and 1PPS output, while for T-TSC-P it is measured at the 1PPS output. Like the T-BC-A/T-TSC-A case, the clock undergoes a sequence of events during the short-term loss of the PTP timing signal in case 1. G.8273.4 specifies the transient response for each of the fol- lowing main steps: 1. The input PTP timing signal reference is lost. The transient response due to a loss of the PTP reference shall be less than 22-ns MTIE. 2. The clock enters holdover and maintains a holdover state for the short term. The performance requirement during this holdover is the same as the performance requirement during the regular holdover of the clock. This is discussed in the next section on holdover. 3. The PTP time signal is restored, and the clock selects the PTP as a reference input. The transi- ent response due to the selection (after restoration) shall be less than 22-ns MTIE. The clock locks to the PTP reference and the normal operation of the clock continues. Holdover The T-BC-P/T-TSC-P moves to holdover when the PTP timing signal from the network is lost and it is not assisted by a physical layer frequency reference. In this case, without any reference signal present, the holdover is driven by the performance of the oscillator. The holdover based on oscil- lator requirements for the T-BC-P/T-TSC-P are exactly the same as those for the T-BC-A/T-TSC-A case. Refer to the preceding section on T-BC-A/T-TSC-A holdover. The other type of holdover is when T-BC-P/T-TSC-P loses its PTP timing signal, but the physical layer frequency is valid and ideal. The performance limits for the holdover based on physical layer frequency assistance case is the same as that of T-BC/T-TSC. Refer to the preceding s