The optional Passive Port Monitoring (PPM) feature, as defined in Annex G of ITU-T G.8275.1, has demonstrated its value in the operation of synchronization networks. PPM can be utilized to monitor the Precision Time Protocol (PTP) phase/time difference between the passive port and the slave port of the Telecom Boundary Clock (T-BC) or Telecom Time Synchronous Clock (T-TSC), and is applicable in the following scenarios:

1. Measuring and compensating for asymmetry within network nodes
2. Monitoring and comparing the PTP phase/time differences between various clock sources across the network
3. Continuously measuring the PTP phase/time difference between ports on a specific T-BC or T-TSC node from upstream time sources to help identify devices that affect clock quality.
   To verify the functionality of this feature, we deployed a scenario with one Telecom Grandmaster (T-GM), one impairment emulator, two T-BCs, and a frequency/phase analyzer. If a real T-GM, the Microchip TimeProvider 4500 was used, then the Calnex SNE Ignite was used as the impairment emulator to produce an artificial delay;
   If an emulated T-GM, like the Calnex Paragon neo, Paragon neo-A, or the Keysight Time Sync Analyzer, was used, then the impairment was directly applied by the emulated T-GM.

T-BC-1 and T-BC-2 were both locked to the T-GM and using it as their primary time reference, with the link between the T-GM and T-BC-2 being impaired. T-BC-2 was then acting as a Master towards T-BC-1’s passive port. Both T-BCs' output was then measured by a frequency/phase analyzer, either the Calnex Paragon neo, Paragon neo-A, or the Keysight Time Sync Analyzer.
The impairment emulator first introduced an asymmetric delay of +540 ns, followed by a -300 ns asymmetric delay. These delays induce a time error of +270 ns and -150 ns, respectively, on the downstream T-BC-2, since the time error on that node will be half of the introduced one-way delay. That offset should be measured on the passive port of T-BC-1.
Nine test combinations were performed, and all T-BC-1s properly displayed their PPM capabilities by measuring the +270 ns/-150 ns time offset relative to their primary time reference, the T-GM.

Table 72: Passive Port Monitoring - Test combinations