al offices the bottom of the hierarchy. Each clock level depended on the one above it for synchronization. If it lost the reference signal from above, the local clock was permitted to operate within a range wider than the higher reference until the higher-level reference was once again available. 4 NETWORK TECHNOLOGY AND METHODOLOGY As you can imagine, there's not much of a disturbance when the reference clock disappears. But what about when the reference clock returns or if it is intermittent? Dealing with these situations led to the development of a hold-over specification, which basically requires the clock to remain within, or hold its frequency of operation for a given time and, in the interest of overall stability, not switch back to the higher hierarchy reference clock until after some period of stable operation, and if possible, switch gracefully, sometimes called hitless switching. Primary reference clock (PRC) products compliant with ANSI and ITU standards are available from many sources. These clock sources can run independently, or they can be referenced and locked to other sources traceable to the World Timing Standard for time-of-day, called Coordinated Universal Time (UTC).2 UTC is the result of combining TAI (International Atomic Time) and Universal Time 1 (UT1). TAI is a timing reference derived by averaging outputs from the clocks of approximately 100 countries. These clocks keep the timing relationship to each other within 2 to 3 millionths of a second over a year. UT1 provides a correction to compensate for the difference in solar time and TAI caused by slightly elliptical orbit and polar inclination, both of which affect solar time. Distributing the PRC to all the elements in early digital network infrastructure was expensive, cumbersome, and complex. However, advances in lowering the cost of clock reference products and the availability of the global positioning system (GPS) and Internet-based references running under network time protocol (NTP) have dramat- ically reduced cost on all fronts