, or long-term transient: Does not apply to the T-TC because it is not phase aligned to the PTP message stream. The testing specification for the T-TC mirrors, as much as possible, the same values as the T-BC. This is very handy for budgeting purposes because it allows the T-BC and T-TC to be treated much the same, certainly from the point of view of noise generation performance. Over the past few years, the authors have not seen many operators performing T-TC testing, but that is because the T- TC has not been a popular option for deployment. Most effort goes into testing the T-BC. Testing (Cascaded) Media Converters There is a slightly different consideration when testing T-BC clocks that consist of two network ele- ments connected to each other in a back to back configuration. In PTP terms, these devices are known as "media converters," and there are sections of the ITU-T recommendations (such as Annex A in G.8273) describing their performance. This state may arise in a lab where a T-BC does not have interfaces that can be connected directly into the timing tester. Perhaps the DUT needs testing with an Ethernet variant that is not supported on a tester (such as 400 GE at the time of writing) or some specialized optic that the tester does not support or recognize. A similar problem arises when the DUT is part of a transport link using technology such as mi- crowave, passive optical networking (PON), or cable that almost certainly comes as a pair. It might have an Ethernet port that can accept a timing signal from the tester but lack a port to return an out- put signal for measurement. The return signal comes out of a second device. In this situation, the way to test it is as a back to back pair, as shown in Figure 12-22. The tester introduces the generated test signal to one device that does the conversion to the special- ized media. Another device at the other end of that link recovers the clock and returns the timing signal to the tester with PTP and SyncE. The tester then measures the TE characteristics of t