the synchronization quality depends on many factors including network PDV, asymmetry, and en- vironmental conditions. Besides monitoring the behavior and metrics of the transport nodes, one of the methods that can be employed is to integrate measurement and monitoring devices into timing distribution network itself. These devices act as probes (they are also referred to as sensors) in the live network, providing con- tinuous real-time monitoring information on the state of the time distribution. Operators deploy these probes at strategic points in their operational network; therefore, they differ from lab test sys- items, certainly in terms of cost and form factor. Compared with lab timing testers, probes are usu- ally much cheaper and are capable of being managed remotely. Because these probes are used for measurement purposes, they support some or all the following interfaces: GNSS receiver: The probes have a GNSS receiver that is connected to a GNSS antenna, and that becomes their source to use as a measurement reference. Of course, the probes can carry out meaningful measurements only while a signal is available at the GNSS receiver. Frequency test input signal (such as SyncE): Using a frequency test input signal, the probes can measure against the primary reference (such as that from the GNSS receiver). Generally, this is an Ethernet interface connected to a SyncE output on a T-BC/EEC router that is part of the timing distribution network. 1PPS phase/time test signal from a node in the timing network: The 1PPS signal can be used by probes to measure the phase of that node. Note that this might require some additional configuration on the T-BC/T-TSC timing transport node (allowing the 1PPS port to output a phase/time signal for measurement). The 1PPS signal might be used in the case where a net- work element does not feature a PTP master port (such as a T-TSC clock). PTP-enabled Ethernet interface: A PTP-enabled Ethernet interface allows the probe to estab- lish a PTP session with the test router and receive 