a PTP message flow. The probe uses the timestamps obtained from this PTP session to monitor and measure the phase/time alignment error of the network T-BC node. With these input sources, the probes can measure and monitor the performance of a clock node or clock distribution network in real time. The exact type of measurement and monitoring is dependent on the type of deployment. For example, for PTS deployments, a probe can measure the PDV intro- duced by the unaware nodes. For FTS deployments, a probe might monitor the 2wayTE, and for frequency-only deployments, it might generate MTIE/TDEV graphs of frequency wander. There are generally two categories of probes, which are as follows: Active probes: These devices act and behave like just another slave clock node in the network. The network element in the timing network treats the active probe connected to it as a real slave clock. In an FTS network deployment, the engineer would connect these probes to a T-BC node, and the probe is treated just like any other T-TSC in network. The active probe can then either ex- port synchronization key performance indicators (KPIs, such as TE and/or TIE) periodically or trigger notifications based on certain criteria (such as cTE or dTE exceeding predefined thresholds) to a centralized NMS. This helps to pinpoint the type and location of a problem or allows early detection of developing issues in specific regions of the timing network. Passive probes: These probes do not become part of the synchronization network, but they are able to monitor and measure synchronization quality in real time if provided with timing in- formation from active network elements. Because these probes do not directly connect to timing network elements, they are mostly used for monitoring PTP sessions, based only on time data shared from active PTP clocks. For this to work, the passive probe needs to capture timestamp data (t1 and t4 timestamps) from a PTP master port to allow measurement and analysis. PTP messages containing timestamps are either mir