As discussed previously in the section "Clocking Hierarchy", there exists a hierarchical relationship between stratum levels and a master-slave relationship is used to distribute frequency. Remember that the basic rule is that a slave should lock to a clock of higher or equal stability. This does require a method to communicate the stability level of a master clock to a slave clock along with the fre- quency signal itself. Because the ability to receive a stable timing signal has become a critical need, a good timing distri- bution network is designed such that there are multiple frequency sources (or master clocks) avail- able to the slave clock. This design is used to provide a backup timing signal during clock outages as well during physical link failures. In such cases, a decision is required at the slave clock to pick the best clock signal from those available to it. This decision to pick the best available clock at the slave node is also sometimes referred to as the clock selection algorithm. Figure 6-8 shows a simple case of a network with SEC nodes, and Figure 6-9 shows a possible con- dition where during a fault, the clock available at different SECs will be traceable to different stratum levels. PRC/PRS PRC traceable frequency throughout the network SSU/BITS SSU/BITS SSU/BITS PRC traceable clock Figure 6-8 SEC Nodes Timing Distribution with SSU/BITS Different traceable frequency in case of fault in network SSU/BITS SSU/BITS SSU/BITS Fault PRC traceable clock SSU traceable clock SEC/EEC traceable clock Figure 6-9 SEC Nodes Timing Distribution with Network Link Failure To know the clock stability level at any piece of equipment, it is necessary that every clock be able to trace its received reference signal back to its source. For example, as shown in Figure 6-8, reference clock that all the NEs are receiving during the no-fault condition is traceable to the PRC/PRS. However, as shown in Figure 6-9, after a physical interface fault, a few NEs in the net- work will receive a SEC traceable clock signal whil