he pair, including the media. Note that we are not testing the transport of PTP over these transports. In fact, for some of these transport schemes, it is known that they produce unacceptable amounts of PDV and asymmetry. That is what would happen if these devices were completely PTP-unaware, which is not the case here. In this scenario, these pairs of devices act as a two-device distributed T-BC; they recover the clock from the PTP on the slave port, carry the frequency and phase using some native mechanism, and regenerate the clock signals at the master port of the second device. Media Converter Testing Reference Tester input PTP and SyncE 400 GE T-BC output T-BC input PTP & SyncE PTP & SyncE 10 MHz PON, DSL Microwave DOCSIS Figure 12-22 Testing T-BC Performance in Media Converters The problem arises because the tester is now testing two nodes as a T-BC and not one, SO a new method is needed to compare the equipment limits in the recommendations. Appendix V of G.8273.2 gives details on addressing this scenario when testing T-BC performance. The following shows how the various test cases are treated in the case of cascaded media: Noise generation: In summary, the cTE adds linearly, the dTEL accumulates as a root mean square (RMS), while the dTEH stays at the same value as for one clock. When testing for cTE, the requirement is +50 ns to meet Class A in the case of a single T-BC clock. If the clock is made up of two nodes, then they would still meet Class A if the cTE was less than +100 ns (adding +50 ns from each). The dTEL accumulation is treated differently because of the results of filtering. Normally, the requirement for Class A is 40 ns, but when testing two devices back to back the RMS of the ac- cumulated errors is used. When combining two Class A devices, the RMS of 40 ns + 40 ns is used, which comes to almost 57 ns. This can be rounded up to 60 ns, SO testing a pair of clocks in this method would meet Class A if the measured dTEL is under 60 ns. The dTEH does not accumulate because the noise from the fi