itch has identical, high-stability clocks operating at the same nominal rate. When we say high stability, we mean a clock stability range from 1 X 1011 to 5 X 10-13 per month. Such stabilities can only be achieved with atomic clocks, rubidium, or cesium. The accuracy and stability of each clock are such that there is almost complete coincidence in time-keeping. And the phase drift among many clocks is, in theory, avoided or the slip rate is acceptably low. This requires that all switching nodes, no matter how small, have such high-precision clocks. For commercial telecommunication networks, this is somewhat of a high cost burden. Synchronous operation (a) Plesiochronous operation Despotic synchronization Mutual synchronization (b) Master-slave (c) Hierarchical (d) External (e) Single-ended (f) Double-ended master-slave reference control control Figure 6.24 Digital network synchronization methods. DIGITAL NETWORKS Another synchronization scheme is mutual synchronization, which is illustrated in Figure 6.24e and 6.24 f. Here all nodes in the network exchange frequency references, thereby establishing a common network clock frequency. Each node averages the incom- ing references and uses the result to correct its local transmitted clock. After an initial- ization period, the network aggregate clock converges to a single stable frequency. It is important here to understand how we can "exchange frequency references." One method would be to have a separate synchronization channel connected to all nodes in the network. This is wasteful of facility assets. We can do just as well by synchronizing the switch clock from incoming bit streams carrying traffic, such as a DS1 or E1 bit stream. However, this (these) incoming bit stream(s) must derive from a source (a switch) that has an equal or higher-level clock. One method of assigning clock levels based on clock stability is described later in this section. The synchronization information is carried in the transitions of the bit stream of interest. A phase-lock loop slaves t