 use cases: Physical methods Legacy TDM and Circuit Emulation Legacy synchronization (not covered here) Frequency-only 2G/3G/4G Frequency in packet networks: Packet-based time/phase: Packet methods SyncE and ESMC PTP phase/time Frequency-only 2G/3G/4G Time/phase 4G/5G mobile Frequency with packet methods PTP telecom profiles PTP G.8265.1 telecom profile G.8275.1 and G.8275.2 profiles 8.1.1 Types of Recommendations There are different types of recommendations written for different purposes, SO it is important to know what sort of recommendation you need to consult and for what purpose. They are layered in approach, starting with the top-level recommendations that define an end-to-end problem to be ad- dressed and the assumptions and requirements to use when solving it. Subsequent recommendations address different levels and facets of the synchronization solution. Figure 8-1 illustrates how these recommendations work to support each other. The architecture documents set up the assumptions and define the problem. The network limits doc- uments then define the end-to-end performance budget. The equipment limits documents define the performance of each component in the chain. Again, there is some overlap, and the boundaries are sometimes fuzzy, but this is the general approach. Architecture and definitions Reference clocks Interfaces Clocks (PRC, PRTC) Interfaces Equipment Network limit Clocks limits profiles Interfaces Interfaces Clocks Figure 8-1 Categories of ITU-T Recommendations on Synchronization and Timing The following list details the types of recommendations within each category of synchronization method: Functional architectures can contain definitions, terminology, and abbreviations as well as the architecture, framework, and requirements of the timing scenario. Synchronization layer functions contains atomic functions or function blocks for the scenario. These recommendations may also include fundamental logic flow and state diagrams for the control-plane functions. Many engineers regard them as describin