king between the PNT and CES domains, including adaptive and differential clocks). Appendix IV, "Applications and use cases" (provides examples of applications and use cases for frequency synchronization for various mobile technologies). Appendix VI, "Measurement guidelines for packet-based methods" (gives test cases and scen- arios to confirm the required performance metrics). Appendix XI, "Relationship between requirements contained in this Recommendation and other key synchronization related recommendations" (this is a very good reference appendix). Appendix XII, "Basic principles of timing over packet networks." Appendix VI contains a set of 17 test cases with different topologies, traffic conditions, and fault scenarios, including models for background traffic. Many operators use these test cases to confirm performance of packet clocks in the lab. It is very important to be aware of the existence of Ap- pendix VI in G.8261, especially when you start to explore testing of frequency synchronization. Link to recommendation: https://handle.itu.int/11.1002/1000/14526 G.8261.1: Packet Delay Variation Network Limits Applicable to Packet-Based Methods (Frequency Synchronization) ITU-T G.8261.1 covers the packet delay variation (PDV) network limits applicable when frequency synchronization is carried via packets. It also specifies several hypothetical reference models (HRM) that correspond to the worst-case models for most of the mobile back-haul networks. From the scope of the recommendation: Two main applications are addressed in this Recommendation: the distribution of a syn- chronization network clock signal via a packet-based method (e.g., using PTP or NTP pack- ets and using an adaptive approach), and the distribution of a service clock signal over a packet network according to an adaptive clock recovery method This recommendation also covers two main use cases: When carrying a reference timing signal for frequency synchronization, there must be limits on the PDV generated in the network in order to meet accept