 on the previous discussion, you might be forgiven for thinking that the obvious way to synchronize the nodes in an O-RAN network is to just build a GNSS receiver into each O-RU and use these to achieve time alignment between all the radios, as shown in Fig. 8-6. Indeed, prior to the growth of heterogenous networks, back when most of the radios in a cellular network were outdoor macrocells, this was the standard method of synchronization, and as we shall see later, this is actually allowed as a method by the O-RAN specification. However, there are three good reasons not to follow this approach, all stemming from the limitations of GNSS outlined previously: Cost-The inclusion of a GNSS receiver and associated antenna will always have some cost impact. Practicality-Many O-RUs, particularly for millimeter-wave applications, are likely to be in locations where the need for a clear view of the sky for GNSS to work makes the use of GNSS simply impractical. Reliability-The vulnerability of GNSS to jamming and spoofing makes it a risky proposition to use directly at the radio nodes. Therefore it is expected that the majority of O-RAN networks will instead use an alternate method of synchronization for the radios, and this takes the form of network-based synchronization. 274 Open Radio Access Network (O-RAN) Systems Architecture and Design Local GNSS in all radios Receiver Receiver Receiver No Sync Across Link FIGURE 8-6 GNSS synchronization of O-RAN. GNSS, Global Navigation by Satellite Systems; O-RAN, open radio access networks. Network-based synchronization The basis of any network-based synchronization strategy is to use the network itself to dis- tribute time and/or frequency between nodes in the network, as shown in Fig. 8 - -7. Specifically, one node, commonly called the master, receives time and/or frequency from outside the network, normally through a GNSS receiver, and then transports this across the network to the other nodes-the slaves. The result is that the slaves align their time and fre- quency to that of 