Two prominent applications of high-accuracy time and frequency transfer can be identified: the operation of a global navigation satellite system (GNSS) and of telecommunication networks. Each of them has ambitious requirements regarding accuracy, availability, and security. Mobile telecommunication networks are operated according to the Long Term Evolution-Advanced (LTE-A) standard and are going to be prepared for future fifth generation (5G) standards. Mobile time-division duplex (TDD) operation, new features for increased spectrum efficiency like enhanced inter-cell interference cancellation (eICIC), future new mobile location-based services, and single-frequency network-based multi- and broadcast applications (MBSFN) services need not only frequency syntonization, but also time synchronization. In order to reach the required network synchronization quality [1], dedicated synchronization chains are implemented, structured as a hierarchical and layered synchronization network. Each network equipment draws its synchronization signal from a superior hierarchy element located closer to the primary synchronization source(s). An example of such a network is shown in Fig. 1. It comprises the network production part, responsible for routine operation of the network, with the requirement of continuous 24/7 operation, and a primary clock supervision part. At the network core level, the highest accuracy synchronization equipment including a number of primary reference time clock (PRTC) functions [2] is used, which is responsible for passing down the network reference time along the hierarchy. In order to increase the network synchronization stability and minimize GNSS related risks, enhanced PRTCs (ePRTCs) [3] and coherent network PRTCs (cnPRTCs) [4] combine GNSS receivers with atomic (cesium) clocks. This approach has recently been proposed for standardization by the respective committees of the International Telecommunication Union — Telecommunication Sector (ITU-T). Over a few core locations, ePRTCs are going to be distributed geographically in the network. The ITU-T specified maximum absolute time error (max|TE|) allowed for ordinary PRTC function is 100 ns [2], whereas the related value for ePRTC is more tightly set at 30 ns [3]. In addition, stability specifications for dynamic time error, expressed as maximum time interval error (MTIE) and time deviation (TDEV), apply. For 24/7 synchronization of the network production part, technologies such as synchronous Ethernet (SyncE) and Precision Time Protocol (PTP) with full timing support from the network (PTP-FTS) according to the ITU-T Recommendation G.826x series for frequency and G.827x for time synchronization, are able to ensure the required level of accuracy at the end application.