Passive port monitoring

The optional feature of Passive Port Monitoring (PPM), as defined in Annex G of ITU-T G.8275.1, has proven to be a valuable asset in the operation of synchronization networks.
PPM can be used to monitor the Precision Time Protocol (PTP) phase/time difference between the passive port and the slave port of T-T-BC /T-TSC nodes and can furthermore be used for the following use cases:

• Measuring and compensating for asymmetry in network nodes
• Monitoring and comparing the PTP phase/time difference between different clock sources throughout the network
• Continuously measuring the PTP phase/time difference between ports on a given T-BC /T-TSC node from upstream time sources to help identify devices that might impact the clock quality.
  In this test, we verified two scenarios involving PPM, where an Impairment Device introduced asymmetry, enabling us to observe an offset with PPM.
      
  In the first scenario, Telecom Boundary Clock-1 (T-BC-1) was directly connected and locked to the Telecom Grandmaster (T-GM). Telecom Boundary Clock-2 was also locked to the Telecom Grandmaster; however, with an impairment device, the Calnex SNE Ignite, in between. If the Keysight Time Sync Analyzer was used as an emulated T-GM, then the output of the emulated T-GM was impaired directly without an external impairment device.
  540 nanoseconds (ns) of delay asymmetry was introduced initially, then changed to -300ns for the second stage.
  T-BC-1  and T-BC-2  were also physically connected, with the port on T-BC-2 acting as a master to T-BC-1  and the port on T-BC-1  configured for PPM.
  This test was conducted with five different combinations, all of which passed. All T-BC-1s were able to measure the +270ns and -150ns offsets due to the delay asymmetry.

For the second scenario, T-BC-1  was directly locked to the GNSS and used it as its primary reference instead of the T-GM. T-BC-2  was still locked to the T-GM with an impairment device, the Calnex SNE Ignite, in between, once again introducing 540ns/-300ns of delay asymmetry. If the T-GM was emulated by the Keysight Time Sync Analyzer, then once again no external impairment device was used, as the impairment was introduced by the emulated T-GM.
The connection and relation between T-BC-1  and T-BC-2  did not change when compared to the first scenario; T-BC-2  still had a master port towards T-BC-1 's PPM port.
This test was conducted in three different combinations, which all once again passed, as all T-BC -1s were still able to measure the +270ns and -150ns offset due to the delay asymmetry.