ation emitted as a result of an electron from a cesium-133 atom transitioning between the two hyperfine levels of the ground state under certain specified conditions. What does this all mean? The details do not really matter at all (unless you are a physicists working for a standards institute), but the key point is that this definition allows the duration of a second to be determined by anyone, at any time, in any place, just as long as they have the right equipment. The significance of this, or rather the fact that the same premise does not apply to time, will become clear later. Of course, no one measuring frequency, or generating a signal of a certain frequency, would expect to use a cesium-based atomic clock. Instead, all practical applications use less- precise standards that can be calibrated, either during manufacture, or periodically for more accuracy, against a more precise standard, that itself can be calibrated against an even more precise one, etc., until ultimately it is calibrated against a cesium source. An example might be a quartz crystal that you buy with a datasheet accuracy of 20 ppm. This means that if Chapter 8 Synchronization in open radio access networks 267 used properly, it will oscillator to within 20 ppm or, 0.002%, of its nominal frequency, and this is something that the manufacturer will have ensured by testing it against a more accu- rate standard, maybe even a cesium standard itself, at manufacture. Time and phase Before moving on to look at time, it is important to distinguish the difference between two related terms-time and phase. These words are often used interchangeably and without a clear definition. However, the generally accepted definition is that "time" refers to a unique instance that can be found out from a clock and a calendar. For example, 12:31 pm on February 2, 2018 is a time. Of course, while we typically express time down to the resolution of minutes, or occasionally seconds, there is no reason that it cannot be stated considerably more precisely and, as you wil