sted case). In the case that a frequency source such as SyncE is still available, the limits are the same as the T-BC/T-TSC case. The test calculates the dTE with an LPF applied and there is an MTIE mask for both constant and variable temperature indicating pass/fail. The values are 62-123 ns MTIE after 1000 S with the constant temperature and 63-173 ns MTIE after 1000 S with the variable temperature. Once again, holdover tests should only be initiated following a period that allows the clock to settle down following setup. For accurate long-term tests, that should be up to 24 hours. In the authors' experience, much like the test cases for the T-BC-A and T-TSC-A assisted clocks, these tests are not widely executed. Generally, engineers are more interested in the T-BC cases from G.8273.2. Testing T-TC to G.8273.3 One slightly different form of testing involves that of T-TCs. The testing layout is very much the same as the case for the T-BC with PTP and SyncE delivered to one port on the T-TC and returned out another port to be measured by the tester. The major difference is that the tester directly con- firms the accuracy of the values written into the correctionField of the Sync and Delay_Req mes- sages. Figure 12-21 shows the topology needed to support testing. T-TC Testing Reference Tester input Traffic injected to stress the T-TC. T-TC output T-TC input PTP & SyncE PTP & SyncE PTP + SyncE 10 MHz Figure 12-21 Testing T-TC Transparent Clocks The tester can determine the actual latency by timestamping the packets on egress and again on in- gress. It will then be able to compare the difference in value between the latency and the values written into the CF by the T-TC (the propagation time for the cable is entered into the tester and cal- ibrated out). The engineer can configure the tester to generate both one-step and two-step PTP packet streams to ensure proper behavior for both schemes. It is also possible to run these tests un- der varying traffic patterns and priorities to confirm whether the T-TC keeps its a