racteristics of the interfaces used to carry physical timing signals, such as 1PPS or a 2-MHz signal Appendix I, "Measuring the performance of a PRTC or a PRTC combined with T-GM" (a very useful source of information when you are considering testing a PRTC in a lab) Note that that there is currently no requirement for holdover from a PRTC. If you are interested in the PRTC, you may also find very helpful the technical report on GNSS sys- items, "GSTR-GNSS Considerations on the use of GNSS as a primary time reference in telecommu- nications." It is covered later in the chapter in the "Other Documents" section, which includes a link for its location. Link to recommendation: https://handle.itu.int/11.1002/1000/14211 G.8272.1: Timing Characteristics of Enhanced Primary Reference Time Clocks ITU-T G.8272.1 provides the requirements for higher accuracy ePRTC clocks suitable as a source of time, phase, and frequency synchronization in packet networks. From the scope of the recommendation: The ePRTC provides a reference time signal traceable to a recognized time standard (e.g., coordinated universal time (UTC)) and also a frequency reference. Compared to the primary reference time clock (PRTC) as defined in [ITU-T G.8272], the ePRTC is subject to more stringent output performance requirements and includes a frequency input directly from an autonomous primary reference clock. This Recommendation defines the ePRTC output requirements, including the ePRTC integ- rated with a T-GM clock. A difference between the PRTC and ePRTC cases is that the ePRTC has a (stringent) holdover re- quirement that requires the stable source of frequency. This means that the ePRTC basically re- quires an atomic clock (PRC) as an oscillator to meet its performance requirements. The case for a combined ePRTC + T-GM is also covered-it allows no additional allowance for time error or wander when the ePRTC has a T-GM function embedded within it. There are also two classes of device covered in this recommendation: ePRTC-A and ePRTC-B, with the ePRTC-