here the local clock is a PTP clock, but only when it uses 1PPS. Figure 12-20 shows that the DUT is also equipped with a PTP port (marked S')-the slave port aligned with a remote T-GM over a partially aware network. This port is to be used as a backup source during outage of the local clock. For the purposes of this testing, this input port is like the T- BC case, although there are no test cases that include frequency input on that port. In the center of Figure 12-20, a PRTC+T-GM is shown as the reference source for the PTP slave port, but for some test cases, the tester would have to be configured as the master input to that PTP slave port on the DUT, SO that it can inject noise into the signal. A problem arises where the T-BC-A/T-TSC-A has an embedded GNSS receiver as the local time source. In this case, the tester does not have a port to inject the generated test because that connector is internal to the DUT. The T-TSC-A may be even more difficult to test because it may be equipped only with a single 1PPS port, and if that 1PPS is needed to inject the generated signal from the tester, then there is no 1PPS available to use as a measurement port. Of course, the T-TSC-A cannot be measured using a PTP master port, whereas that would be an option if it was a T-BC-A. The G.8273.4 recommendation has not yet specified holdover limits for the clock relying on the re- mote PTP GM; it specifies only the case where both the local time source is lost and the backup PTP source is unavailable (the oscillator-assisted case). For that reason, these components do not need to be there unless the engineer wants a network phase alignment test during failure of the local source but with the PTP slave clock providing a secondary signal (see the section "Testing Net- works for Phase Alignment (Packet)" earlier in the chapter). Regarding the equipment limits, a good amount of reference information is available in the recom- mendations, specifically the clock limits in G.8273.4 (in clause 7) and the network limits from G.8271.2. The 