Delay Asymmetry DetectionMeasurement

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Controlling and handling asymmetric delay on links that carry PTP messages is of the utmost importance and is one of the big challenges for network time synchronization.

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This test was performed in four different combinations. The setup consisted of one Grandmaster (either real or emulated), whose link towards Boundary Clock-1 is impaired; if an emulated T-GM was used, like the Calnex Paragon-Neo PAM4 and Keysight Time Sync Analyzer, then the impairment was provided directly by the emulated T-GM; if a real T-GM, the Microchip TimeProvider® 4500 was used, then an external impairment device, the Calnex SNE Ignite, was used.

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T-BC-1 is initially directly connected to a GNSS reference and Boundary Clock-2 (T-BC-2), which is connected to a time error measurement device (Calnex Paragon-Neo PAM4 or Keysight Time Sync Analyzer).

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In three of the four combinations, PTP profile ITU-T G.8275.2 was used throughout the entire chain; in the fourth combination, the interworking function (IWF) from PTP profile ITU-T G.8275.2 to ITU-T G.8275.1 was used between T-BC-1 (Ciena 5134) and T-BC-2 (Juniper PTX10002-36QDD) and the connection between T-BC-2 and the time error measurement device was done over an 800G link.

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[[Figure 128: Delay Asymmetry Detection/Measurement - Setup>>image:434430579187449857_5.20.png||alt="Figure 128" width="550"]]

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At first, the GM and T-BC-1 were connected to the GNSS, and T-BC-2 used T-BC-1 as its primary time source. The GNSS was then disconnected from T-BC-1; T-BC-1 then locked onto the GM as its new time source, with the link between the GM and T-BC-1 being impaired by introducing an asymmetric delay of 500ns. During this process, the time error was measured on T-BC-2, and none of the measurements reflected the 500ns asymmetry, proving that all T-BC-1s could detect and eliminate the asymmetry.

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The network passed the class accuracy level of 6 for all test runs, as per ITU-T G.8271, throughout the entire test run, including the switchover; the first three test runs, which used PTP profile ITU-T G.8275.2 throughout the entire chain all passed class accuracy level 6C. The IWF test run passed the class accuracy level of 6A.

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These results are remarkable and a big victory for Time Synchronization. They prove that asymmetric delays can very well be handled, beating one of the big challenges of Time Synchronization.

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|=GM-A|=T-BC-1|=T-BC-2|=Impairment device|=Timing Test Equipment

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|Calnex Paragon-neo PAM4|Microchip TimeProvider® 4500|Huawei ATN 910D-A||Calnex Paragon-neo PAM4

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|Keysight Time Sync Analyzer|Ciena 5134|H3C S12500R-48Y8C||Keysight Time Sync Analyzer

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