which the clock advances, is a characteristic based on the physical design and natural characteristics of the component. Because it is impossible to design os- cillators that all have the exact same characteristics, every clock will oscillate at a different natural frequency-the difference might be small, but it will be there (and will very likely change over time as well). You can buy desktop equipment to measure frequency, what is known as a frequency counter. Fre- quency counters measure the number of cycles of oscillation, or pulses per second in a periodic electronic signal (in Hertz). Frequency counters with today's technology can count frequencies above 1 X 10 11 Hz, which allows accurate measure of very small time intervals. These devices are commonly found in lab environments for testing time accuracy. Oscillators based on cesium-133 atoms are widely used as primary reference clocks (PRC) provide a very stable and accurate source of frequency. To make a real "clock" out of it, you would simply add a counter that increments by 1 after it detects 9,192,631,770 cycles of cesium radiation. This counter would then increment almost exactly once per second, an almost perfect ticking clock (there are no perfect ones). Oscillators based on quartz crystals (see "Chapter 5. Clocks, Time Error, and Noise"), being more affordable, are widely used to provide a frequency signal within telecommunications networks. Al- though quartz is an adequate oscillator capable of maintaining a somewhat stable frequency, it is not accurate enough for digital communications and nowhere near as stable as a cesium clock. It is the distribution of the frequency reference signal from the cesium PRC to all the devices in the network that allows the embedded quartz oscillator in each network node to accurately deliver the frequency for error-free communications. 3.1.1 Defining Phase Synchronization As introduced in "Chapter 1. Introduction to Synchronization and Timing" phase synchronization is when separate, but regularly occurring, event