propagation for different As in a fiber, such as when using bidirectional fiber. The second method, measuring static asymmetry, is the only method to discover the asymmetry when it cannot be discovered ahead of time. For example, the cable patching problem is a problem that may only be reasonably detected by measurement. The main disadvantage with measurement is that it can be operationally difficult and expensive. And then, when some change is made to the net- work, the measurement process may need to be repeated, which is an unwanted operational expense. The upcoming section "Correction and Calibration" shows the details of the correction and calibra- tion methods that PTP uses to treat and counteract static asymmetry. Dynamic Asymmetry Unlike the static kind, dynamic asymmetry does not appear to be predictable; it seems to vary with the phases of the moon, the temperature of the ice sheets, or just because it is Tuesday. Although dynamic asymmetry appears somewhat random, there are logical reasons for it (such as the laws of physics), but those reasons are too complex and chaotic to understand, model, and compensate for. One of the most common sources of dynamic asymmetry comes from external traffic crossing the path between the master and the slave. This traffic is competing with the PTP message stream for access to the transport link, SO the changing patterns of this traffic causes the PTP to suffer differing amounts of PDV (in each direction). Figure 9-19 illustrates how an unbalanced traffic pattern between the forward path and reverse path can affect PTP packets. Queue Traffic Traffic Traffic Traffic Traffic Traffic Traffic Traffic Queue Traffic Traffic Traffic Traffic Traffic Figure 9-19 Differing Traffic Patterns Causing PDV and Asymmetry Even with perfect quality of service (QoS) and priority queues, in a PTP-unaware node, it is still highly likely that a time-sensitive PTP message can be held up. This is because a large jumbo frame may have begun transmitting or passing through a congested pathway, an