ction between clocks and a reference frequency signal. This means testing for aspects of frequency performance such as wander. Much of this form of testing has been inherited from the TDM technologies and standards. SyncE adds an extra di- mension, but they have a lot in common. Because this testing involves frequency, the test equipment is connected via an interface carry- ing frequency. If the DUT has a BITS port, then it can be measured using it or via some legacy interface such as E1/T1. Testers that can perform these tests for TDM interfaces have been available for decades and are still operated by people using circuit emulation. Nowadays, Ether- net interfaces and SyncE inputs/outputs are more commonly used for testing. Phase/time testing: Measures what happens to the phase in interactions with the clock. This often involves measuring TE and calculating various statistics derived from those measure- ments. Therefore, the test equipment should include software to do those calculations, either as an embedded function or via some external application. Because this testing involves phase, the test equipment is connected via an interface that can signal phase. This is either a 1PPS signal (to measure output from the DUT) or PTP over an Ethernet interface. By using an Ethernet interface, it might be possible to capture phase/time (PTP) and frequency information (SyncE) at the same time. Testing the output of the ToD signal is possible on some test equipment, but almost no operators test it, unless they are interested in interoperability of the ToD feed. The only other type of testing that might arise in some cases is the testing for PDV and packet se- lection. Not many operators perform this class of tests, although it can become relevant when some clock will not align over an unaware network that displays a high level of PDV. When mobile oper- ators were first deploying G.8265.1 (frequency PTP), this type of testing was sometimes used be- cause that profile does not allow on-path support-every node between maste