e issue with the underlying trans- port system or optical devices rather than the PTP clocks. There is traceability to different clocks for frequency and phase that causes issues because of a slight misalignment between the frequency signal and the movement of the phase signal. Troubleshooting is a learned skill. So, the more experience that you have performing the task, the better you become at doing it. Obviously, you do not want to have SO many issues that you become an expert. Thankfully, in just about all cases, this technology simply works when using equipment that is designed to properly support it. Most issues come from flaws in design, non-timing-aware nodes, or shortcomings from the transport and optical systems. The next task is to understand what the slave clock can tell the operator about the results of the clock recovery mechanism and its alignment with the master. For that, operations staff like to mon- itor the values calculated from the PTP timestamps, the offset from master (OFM) and the mean path delay (MPD). 12.5.1 Monitoring Offset from Master and Mean Path Delay Remember that four PTP timestamps are used by the slave to calculate two values, the OFM and MPD. Many operators know how to extract these values from the servo, and some monitor it quite closely. A common fallacy is that somehow the OFM value provides a reliable indication of how accurate the slave clock is, when in fact it simply tells you what values the calculations produced for the last set of timestamps. And obviously, because of factors covered in preceding sections, these calculated values can vary quite markedly for every discrete set of timestamps. This is what you can learn from monitoring these two values: If the value of OFM is varying widely, that would be caused by both PDV and varying asym- metry. The same is true for MPD, but a real example might make it easier to comprehend. As- sume a situation where the path delay between a master and a slave is 100 us in both directions, the slave has arrived at this value over m