ed" or syntonized (the correct tech- nical term). If you are synchronizing the frequency of machines such as computers, radios, or even clocks, you are basically doing the same thing as a piano tuner. Each device, such as the quartz wristwatch, your home computer, or your mobile phone, contains an oscillator (possibly more than one). The silicon engineer designs this oscillator to run at a very precise frequency, known as the nominal frequency. This nominal frequency will be designed for a target application while meeting other requirements like power consumption, SO the nominal frequency might vary-for example, 20.00 MHz in a network router or 32768 Hz in a quartz watch. The first thing to realize is that physics comes into play and therefore a device will never naturally oscillate at exactly its correct nominal frequency. Out of the box, it will be either fast or slow, even if only by a miniscule amount. The amount the oscillator can be expected to deviate is part of the specification of the oscillator (frequency accuracy) and can be expressed in terms of parts per mil- lion (ppm) or parts per billion (ppb). The better the oscillator, the lower that number will be-a typ- ical value for a 20-MHz oscillator in a Cisco router might be +4.6 ppm, which means the measured frequency could be anywhere between 19,999,908 and 20,000,092 Hz. Furthermore, the actual output frequency can dynamically change because of many factors, SO an engineer cannot simply measure the actual output frequency of an oscillator and somehow correct for it. As you would expect, it will change based on temperature, but it might surprise you to know that as the device gets older, the frequency will change slowly over time (a process called aging) and might change by several ppb per day. Oscillators will be discussed in more depth in "Chapter 5. Clocks, Time Error, and Noise". Frequency synchronization is the process whereby one uses another, more accurate, and stable source of frequency to make the oscillator tick over much more closely to its 