nerated synthetically on the tester and given to the head of the timing chain and the phase from the last node is returned to the tester and compared to the input. The tester in this case is measuring the relative phase offset between the incoming and outgoing sig- nals; there is no connection to UTC. The absolute case is different because the phase that is input at the head of the timing chain is sourced from an independent source, such as a PRTC and T-GM combination with traceability to UTC. In this case, the tester must measure the output of the last node in the chain against a trusted source of UTC time. Therefore, the tester needs a 1PPS reference from a UTC time source that it can compare against the phase received from the network. It could be the same PRTC as the one giving input at the head of the chain, but if the tester is remote from that device, then it must use a different PRTC as its source. That is why this case is called absolute: it is determining real-world phase/time. (The pedantic might say it is still a relative test against UTC, but many regard UTC as "absolute" time.) As always, the tester also needs a frequency standard to condition its oscillator. From the PTP test signal, the tester measures the 2wayTE (see Chapter 9 for details), and for a 1PPS input test signal, the tester measures TE. The engineer can then determine a pass/fail against any desired limit for max|TE|, such as the +1.1/1.5-us phase alignment limit for the 5G mobile case. Figure 12-16 shows the MTIE graph of the phase TE after crossing a (small) network of routers connected to an ePRTC with 10-GE links. Because this is a network test, the appropriate threshold for pass/fail is the 1.1-us 2wayTE network limit from G.8271.1 (the mobile requirement). As in the preceding case with SyncE, this capture comes from the CAT reporting tool and the Sentinel tester. Phase Test-Network the 1 2Way Time Error Date 2020-10-20 File channel1 FWD POV.dset include Correction Field True 1 200 Threshold 1 000 2Way Time Error =1000 -1 200 -1 4