eory, this is a nice solution; however, you need to understand a couple of things when consider- ing this design. Remember from Chapter 9 that there are two basic forms of asymmetry, static and dynamic. If the asymmetry being measured by the T-TSC-A (or Telecom Boundary Clock with as- sisted partial support [T-BC-A] is mostly static in nature (for example, differing fiber lengths), then measuring the value and using it to compensate the recovered PTP time will deliver quite accurate results. On the other hand, if the asymmetry is largely dynamic, and prone to mostly random and dramatic movements in value, then this method has limited use. This is because the last recorded value (even with some filtering to reduce the jitter) will not remain valid for the next measurement period. Fairly reasonable and common behavior such as rapidly changing traffic patterns would lead to this form of dynamic noise. The second factor to consider is that a complex back-haul network is hardly a static setting because it is constantly changing and adapting to changes in the environment. Equipment heats up, links go down, routing protocols remap preferred paths, people dig up cables, and SO on. The point to make is that the last-known-good value is only good for as long as the current situation remains stable. Once there is a significant change in the path between the T-TSC-A and the T-GM, the measured asymmetry becomes stale and could quickly become highly erroneous. And with IP routing looking after all those details at a lower layer, the T-TSC-A slave clock will not know anything happened (unless the transient resulted in a very sudden and dramatic phase shift). And even if the slave does detect a very sudden shift in phase, what is it to do about it? Should it ignore that shift as erroneous or follow the phase data it is getting from the T-GM? Trying to second-guess the data received from an authoritative timing signal is a dangerous assumption. From the discussion of these points, it is clear that the APTS approach is a valid one