Chain Ring of Class D Boundary Clocks

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In real-world telecom deployments, it is typical to have a chain ring of Class D Boundary Clocks, enabling precise and stable Time Synchronization over multiple hops, even in the events of a failover or holdover.

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For this test case, we had two Microchip TimeProvider® Telecom Grandmasters (T-GMs ), T-GM -A and T-GM -B, with a 10 nanosecond (ns) offset configured between them. We also had six Boundary Clocks (BCs) set up in a chain ring, three of which are measured by the Calnex Paragon-Neo PAM4 and the Keysight Time Sync Analyzer.

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All BCs had local priorities set to T-GM -A, and T-GM -B was therefore used as the backup T-GM.

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[[Figure 123: Chain Ring of Class D Boundary Clocks - Setup>>image:434865443417686017_5.18.png||alt="Figure 123" width="550"]]

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Initially, T-GM-A provided PTP and SyncE downstream. The measurement was started, and after a short period, the GNSS connection to T-GM -A was cut, causing a Failover of all BCs onto the PTP path of T-GM-B prompted by the change in clockClass from 6 to 7 from T-GM-A.

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After 200 seconds, the time measurement was restarted, and once again, after a short period, the GNSS connection to T-GM-B was cut, causing all T-BCs to enter PTP Holdover. The measurement was left running for 200 seconds.

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Following this, the time error measurement was restarted once more, and after 30 seconds, the GNSS connection to T-GM -B was restored, causing all T-BCs to lock back onto T-GM-B on PTP after some time. Once all BCs were locked back onto the PTPpath of T-GM-B, the measurement was left running for 300 more seconds.

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Finally, the time error measurement was restarted, and after 30 seconds, the GNSS connection to T-GM -A was restored. After the lock-acquisition period, all BCs locked back onto the PTP path of T-GM-A. The measurement was then left running for 300 seconds before being stopped.

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During all of these procedures, the network passed the requirement defined for clock accuracy class level 6B as per ITU-T G.8271, even during the failover and holdover periods.

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