G radio network, the problem is much harder to solve be- cause the operator must phase align radio equipment, distributed around the whole country, to within a few microseconds (us) of any neighboring equipment. Time of Day Synchronization You can easily find examples of time synchronization in daily life. A parent might let their teen- agers go off alone in the shopping mall, if they promise to meet up again at a specific time. Before letting them wander off, the parent typically asks, "What time do you have now?" which is a crude method to ensure their "watches are synchronized." Of course, they all carry mobile phones these days, SO setting meeting times is almost a thing of the past, and additionally, all our phones are auto- matically time synchronized for us. However, once you set your watch to the correct time, the watch must rely on its oscillator to keep the time as best it can. For the 32768-Hz oscillator in a quartz watch, this means that every time the crystal undergoes 32768 oscillations, a counter will tick over one second and move the second hand by one graduation on the watch face. Therefore, the better the frequency accuracy of the oscillator (the closer it is to exactly 32768 Hz), the more accurate the watch will keep time. At the very high end, oscillators with exceptional frequency accuracy are called atomic clocks. The concept of time drift typically refers to the accuracy of the time reported by a device and how much it has moved over a fixed time period-for example, the accuracy of a quartz wristwatch might be expressed as 1.5 seconds per day. This means that one month after the hypothetical parent and teens in a shopping mall coordinate the time on their watches, they could expect the watches to disagree with each other by up to 45 seconds or SO. If a set of devices need to keep more accurate time over an extended period, they need a mechanism to synchronize time automatically to compensate for the inaccuracy in the frequency of the oscil- lator. Various mechanisms exist that do this, and 