ism to signal clock quality. Table 3-1 Clock Quality Traceability Methods Method Explanation SSM bits indicating the quality of the clock. The synchronous digital hierarchy SDH/SONET (SDH) and synchronous optical network (SONET) use different values for these bits and different levels of clock quality. Ethernet SyncE The Ethernet synchronization message channel (ESMC) sends packets to indicate the quality of source for SyncE. eESMC The enhanced Ethernet synchronization messaging channel (eESMC) includes Enhanced SyncE additional information in ESMC to support enhanced SyncE (eSyncE). clock- PTP sends Announce messages with clock quality information (including clock- Class Class) that indicates the quality of the time source at the grandmaster clock (see "Chapter 7. Precision Time Protocol"). Health GPS signals GPS satellites transmit whether their timing data is valid or not. Global naviga- Most GNSS receivers have some method to signal an estimate of the clock qual- tion satellite sys- Time of ity. This can be included in the time of day messages between the receiver and tem (GNSS) the packet grandmaster. See Annex A of ITU-T G.8271 for one approach. receiver One thing to recognize is the problem of suffering from timing loops in the synchronization net- work. It is possible to build and configure a network where the timing information generates a loop (for example, where A gives time to B that gives time to C that gives time back to A). Some clock traceability methods, such as ESMC, have mechanisms to help mitigate such problems, although it is still possible to construct such a topology. Chapter 6 covers the details of the specific traceability methods used by SDH/SONET, while Chapter 7 covers the methods used for packets. Chapter 6 also discusses frequency timing loops in detail, while Chapter 9 examines ways to mitigate them in an operational network. 3.2 Clocks As mentioned previously, a clock is an oscillator plus a counter. Now it is time to revisit many as- pects of the clock in further detail. A free-ru