he local clock to these transitions. Remember that a transition is a change of state in the bit stream, a change from a binary 1 to a binary 0, and vice versa. A number of military systems as well as a growing number of civilian systems (e.g., Bell South in the United States; TelCel in Venezuela) use external synchronization, as illustrated in Figure 6.24d. Switch clocks use disciplined oscillators slaved to an exter- nal radio source. One of the most popular today is GPS (global positioning system), which disseminates universal coordinated time called UTC, an acronym deriving from the French. GPS is a multiple-satellite system such that there are always three or four satellites in view at once anywhere on the earth's surface. Its time-transfer capability is in the 10- to 100-nsec range from UTC. In North American synchronization parlance, it pro- vides timing at the stratum-1 level. The stratum levels are described in Section 6.12.2.2.1. We expect more and more digital networks to adopt the GPS external synchronization scheme. It adds notably to a network's survivability. Other time-dissemination methods by radio are also available, such as satellite-based Transit and GOES, or terrestrially based Omega and Loran C, which has spotty worldwide coverage. HF radio time transfer is deprecated. 6.12.2.2.1 North American Synchronization Plan Stratum Levels. The North Amer- ican network uses a hierarchical timing distribution system, as shown in Figure 6.24c. It is based on a four-level hierarchy and these levels are called strata (stratum in the singular). This hierarchical timing distribution system is illustrated in Figure 6.25. The timing requirements of each strata level are shown in Table 6.4. The parameters given in the table are defined as follows: 1. Free-Run Accuracy. This is the maximum fractional frequency offset that a clock may have when it has never had a reference or has been in holdover for an extended period, greater than several days or weeks. Table 6.4 Stratum-Level Specifications Stratum Level Free-