plex and exists only to align with the Earth's motion-something that O-RAN networks really does not care about. Even if 1972 Jan 1 TAI-UTC = 10.0s Current TAI-UTC = 37sec 19751980198519901995 2000 2005 FIGURE 8-2 The relationship of TAI to UTC. TAI, International Atomic Time; UTC, Coordinated Universal Time. Chapter 8 Synchronization in open radio access networks 269 we collapse all these to just counting seconds, the number of seconds since a starting point of over 2000 years ago is a large number that will result in an unnecessary amount of calculation. Therefore to make things more manageable, it is common to define a different epoch-one that is closer to the present day. There are two such epochs that are relevant to O-RAN: The Precision Time Protocol (PTP) epoch, as used by IEEE 1588. This counts seconds from January 1, 1970 00:00:00 TAI (which at that point was already 8 seconds ahead of UTC). The GPS epoch, which counts from January 6, 1980 00:00:00 TAI. O-RAN defines frame numbering in terms of GPS time, SO seconds from the GPS epoch. However, the PTP epoch is more relevant for synchronization on the network, as we will see when we look at IEEE 1588, and PTP time can easily be converted to GPS time by subtracting a constant value. Therefore when we talk about time in the following sections, we will mean PTP time. One thing that should be clear from the preceding discussion is that compared to fre- quency, time is very arbitrary. Indeed, it is impossible for two people to agree on the current time without them having synchronized their clocks at some point in the past. Compare this to a frequency where, as stated earlier, any two people can agree on frequency, by virtue of agreeing on the length of a second, provided they have the right equipment-no prior syn- chronization is needed. This is a key consideration when it comes to time synchronization in networks, since it means that time must be transported from one place to another. Syntonized versus synchronized As a final piece of terminology, it is neces