sary to understand the difference between the terms "synchronized" and "syntonized" when applied to network applications. In common use, the term synchronized is used to refer to the general process of two events, or systems, being aligned in some way. However, in network applications, the term "synchronized" has more specific meaning. If two nodes in the network are said to be synchronized, it means that those nodes share both a common frequency and a common time. On the other hand, if the nodes only share a common frequency, but not time, then they are said to be syntonized. The distinction is shown in Fig. 8-3. Unfortunately, to further complicate things, the term "synchronization" is typically used to refer to the process of achieving alignment between nodes, whether the aim is for those nodes to be actually synchronized or just syntonized. How do we get time? As stated earlier, unlike frequency, time cannot be created locally. Therefore where do we get time from? Are there some magical things that we can "plug into" and receive time? Luckily there are-GNSS (Global Navigation by Satellite Systems) services. 270 Open Radio Access Network (O-RAN) Systems Architecture and Design Neither Syntonised Or Synchronized Syntonized Synchronized FIGURE 8-3 Syntonization versus synchronization. Global navigation by satellite systems Since the 1980s, governments around the World have provided "free" time as part of so- called GNSS services. The best known of these is GPS, run by the United States military, but still there are other GNSS services operating-Glonass (Russian), Beidou (Chinese), and Galileo (European). These systems are all global, meaning that they are available worldwide, even though they are owned by individual countries or consortiums (Fig. 8-4). As the "N" in GNSS suggests, the primary purpose of these services, at least initially, was for navigation-at first for military applications but now equally, if not more, important for civilian and commercial uses-however, they very fortuitously provide precise t