physical and packet, have defined method of clock traceability to distribute information about the quality of the signal they are offer- ing. Clock traceability allows the network elements to always select the best clock available to them and know when a valid source of time disappears. This mechanism allows the clock distribution network to optimally configure itself by continuously discovering and connecting to its best sources of synchronization. No matter what the method of time distribution, there will always be some mechanism used to al- low any clock receiving a timing signal to determine the quality and accuracy of that signal. For fre- quency, SDH/SONET signaled this information in some overhead bits in the data frame, what are called the synchronization status message (SSM) bits. Figure 3-9 illustrates the use of this mechanism in frequency distribution using an SDH network as an example. The network element receives information from the connected devices about the source of timing they are traceable to. In the figure, the quality level (QL) is shown being distributed along with the timing signal. One of the best sources of frequency in an SDH network is the PRC, while the SSU-A is the next level down. What clock signal shall I use? SSU-A Figure 3-9 Selecting the More Accurate Source of Frequency The SSU-A QL indicates traceability to a primary level synchronization supply unit (SSU), which is a level of accuracy below that of a PRC (see the section "Clock Traceability" in Chapter 6 for more details). Even in the small number of timing or frequency signals that contain no quality information (for ex- ample the 2-MHz or 10-MHz sine waves), there will be some mechanism to signal to the receiver of the signal when the source of time is invalid. In these cases, the upstream reference would simply squelch the signal (as in, turn it off), which indicates to the downstream slave that it should try to find a new source of frequency. Table 3-1 gives an overview of the common distribution methods and their mechan