and the dTE noise generation. 9.3.1 Constant Time Error cTE is the most watched characteristic of a PTP clock, mainly because it is the principal metric used to select what class of boundary or slave clock it attains. The values for cTE and other metrics are outlined in the ITU-T Recommendation G.8273.2, which defines the performance characteristics for T-BC and T-TSC clocks. Chapter 12 contains a section on how G.8273.2 testing and measurement is performed. Without spoiling Chapter 12 too much, cTE for a boundary clock is determined by giving an ideal PTP and SyncE input signal to the device slave port and measuring any variance in the timestamps returned from the master port. By doing this, the engineer measures how much noise has been gen- erated as time transits through the node. These measurements are averaged over 1000 S to determine the cTE. The cTE is very much a characteristic of a clock; this does not mean the cTE is a fixed value, but that it will not vary significantly, because a well-designed BC has only a small number of moving parts that might generate a different value for TE depending on the day (dynamic TE describes that). So, a PTP-aware network element generates a similar cTE value to that of other devices made the same way with the same components. When building a network with several T-BC clocks, the cTE measures the TE that you could expect the T-BC to generate on an almost permanent basis. And that TE output from the first T-BC would be fed as an input to the next T-BC in the chain. Therefore, the effects of cTE are additive-over the length of the timing chain. The worst-case result could be when connecting ten Class B T-BC clocks together where every clock generated the maximum of +20-ns cTE. A chain of these clocks would generate up to 200 ns of cTE (10 X 20ns) at the output of the final clock. The same would be true if they were all -20 ns of cTE, giving -200 ns total. Of course, you would also need to add the cTE from the network links, which would arise from asymmetry in the transport