, now that this test confirms that the frequency signal is accurate and stable, the next task is to test the phase alignment. Testing Networks for Phase Alignment (Packet) Like the case with frequency, the goal of building a phase/time distribution network is to ensure that the timing signal arrives with the quality required. For applications that require phase synchroniza- tion, maintaining that phase alignment is especially critical because there are numerous factors that work against it. So, thorough testing before deployment is crucial, both on the individual clocks themselves (see following sections) and on the end-to-end network. End-to-end network phase/time testing is especially important to validate the lessons learned and assumptions made about the underlying packet transport. Additional tests that many operators like to perform include holdover testing when the PTP and SyncE reference signals to the last node are lost. Another is what happens to the phase alignment when there is a rearrangement and reconver- gence somewhere upstream in the network and a clock must select a new grandmaster (GM). Because this is a phase alignment test, it needs a phase signal, either PTP or 1PPS, from the last net- work node SO that the tester can measure the phase as recovered by that device. The left-hand side of Figure 12-15 shows the case of a relative test, as opposed to the absolute test case on the right- hand side. Just as in the frequency case, the test engineer must consider the cable lengths because the reference signals can only use short-run cables, and that determines the relative location of the test device and the reference sources. Relative Network Testing Absolute Network Testing Reference Reference Tester input Tester inputs T-BC input output PTP SyncE PTP & SyncE PTP & SyncE T-BC output PRTC T-GM SyncE 1 PPS T-TSC PTP SyncE T-TSC output output 10 MHz SyncE T-TSC SyncE output T-TSC output Timing network Timing network Figure 12-15 Testing Phase Alignment in a Network Relative means that the phase is ge