chronization The frequency timing recommendations using physical and TDM methods can be broadly grouped into these following categories: Definitions, architecture, and requirements: G.781, G.810 End-to-end solution and network performance: G.823, G.824, G.825 Node and clock performance: G.811, G.811.1, G.812, G.813 Other recommendation: G.703 Many readers might decide that this section on frequency transfer via physical means is not of great interest to them. Others, such as those interested in circuit emulation (of TDM circuits over packet networks) will recognize that they are important and relevant. But this section is also relevant for those interested in mobile synchronization because even the packet-based methods for frequency transfer (and SyncE) are underpinned by these physical recommendations. Another reason for coverage of these recommendations in the book becomes apparent in the section "Testing Timing" in "Chapter 12. Operating and Verifying Timing Solutions". These recommenda- tions define the frequency limits for both performance of standalone node clocks (of various quality levels) and end-to-end performance in a frequency distribution chain. When testing either stan- dalone network elements or network performance with lab equipment, the engineer will need to know what performance mask to apply to the test results to determine a pass or fail. Those masks are defined by these recommendations. 8.2.1 Definitions, Architecture, and Requirements These high-level recommendations are the basis for all the following physical frequency recom- mendations. They contain the definitions and terminologies as well as the architecture and require- ment for distributing frequency synchronization using physical methods. This is not the entire set but includes those directly applicable to physical frequency timing. G.781: Synchronization Layer Functions ITU-T G.781 defines sets of atomic functions that are part of what it defines in three layers: syn- chronization, network, and transport. These functions describe th