 signal the clock traceability and quality. As seen previously, SSM is used in SDH to exchange the quality level of the frequency clock between the network elements-carried with several bits located in the frame header. SyncE, on the other hand, uses the Ethernet Synchronization Message Channel (ESMC), which is a packet-based mechanism, to exchange clock quality information. PTP also deploys messages, the Announce mes- sage, to exchange information about clock traceability and quality levels. See the preceding section "Clock Quality Traceability" and Table 3-1 for information on traceability through a network. Chapter 6 covers SSM and ESMC in much more detail, SO it is enough just to summarize them here with one example. Figure 3-21 indicates how ESMC clock quality is trans- ferred with SyncE (ESMC is specified in ITU-T G.8264). Primary Reference Ethernet Switch / Router Ethernet Switch / Router Cellular Base Station Clock layer layer layer layer layer layer stability reference status messages Recovered Ethernet Ethernet Ethernet Ethernet Ethernet Ethernet Figure 3-21 ESMC Distribution in a SyncE-Enabled Network 3.5 Consumer of Time and Sync (the End Application) Of course, the whole goal behind building a timing distribution network is to allow some applica- tion that needs the synchronization service to function correctly. It is vitally important, before de- ploying such a solution, that the requirements of the end application are clearly understood. This cannot be overstated, as it is not unusual to see engineers build a timing distribution network that is not what was needed. A key component, then, is to understand what the timing requirements are from the end application. The following list should be some of the first questions asked before starting a project: What forms of synchronization are required: frequency, phase, or time? What is the timing budget, meaning what level of performance and accuracy is needed at the end application? What is the requirement for holdover performance-how long must the end ap