cancels out if it is equal in the forward and reverse direction through the T- TC. This means the Sync (forward) and Delay_Req (reverse) messages have equal offsets (in opposite directions). However, any differences between the offset in the forward and reverse direction will produce asymmetry and result in cTE on the input to the slave clock. Variable error (dTE): This is caused by packet-to-packet variation in CF accuracy. The classic example of dTE results from the resolution or granularity of the timestamping unit updating the CF. Any dTE reflected in the CF results in the slave clock not being able to accurately com- pensate for the PDV. ITU-T G.8273.3 defines these limits for a T-TC, and Table 9-8 summarizes the three main paramet- ers for noise generation for a T-TC: max|TE|, cTE, and dTE. Table 9-8 Maximum Noise Generation of a T-TC Maximum Limits (as defined by ITU-T G.8273.3) Noise Type Class A Class B Class C 100 ns 70 ns For further study +50 ns +20 ns +10 ns dTEL $ 40 ns (MTIE) 40 ns (MTIE) 10 ns (MTIE) 70 ns 70 ns For further study For constant as well as variable temperature except for Class C, which is constant temperature only For the noise transfer behavior, the T-TC is not expected to amplify any input time error on its out- put. Both noise tolerance and transient response limits are for further study, and there is no holdover capability supported in a transparent clock. 9.4.1 T-BC-A, T-TSC-A As explained in Chapter 7, a T-BC with assisted partial support (T-BC-A) is a boundary clock with only partial support from the network but assisted by having a local time reference (such as a GNSS receiver) as a primary source of time. The PTP slave port on this BC recovers the PTP clock over the PTS network and uses it as a reference only when the local time source fails. In the same fash- ion, a T-TSC with assisted partial support (T-TSC-A) is the slave-only equivalent of the T-BC-A boundary clock. ITU-T G.8273.4 defines the performance limits for the T-BC-A and T-TSC-A. Refer to the section on G.8273.4