the master, which in turn is aligned to actual time from the GNSS. This technique of network-based synchronization directly addresses the limitations of relying on GNSS everywhere: Cost-Although providing network synchronization will increase device complexity somewhat, with a corresponding cost penalty, since the existing data transport is being used which will almost certainly be significantly less than providing GNSS receivers at each node. Practicality-By decoupling where time is received, in other words where the master node with the GNSS receiver is, from where time is used (the radios) it becomes possible to locate the master in a more preferable location for GNSS reception independent of the radiolocation. Reliability-While 100% reliability can never be achieved, placing multiple masters in the network, each with their own GNSS receiver, goes a long way to mitigating potential concerns by providing multiple, geographically distinct, sources of time. In particular, this can help against jamming and spoofing attacks since these will in general be isolated to a relatively small area. Chapter 8 Synchronization in open radio access networks 275 Remote GNSS Time Source Receiver Time Provided Through Network FIGURE 8-7 Network time distribution. So, what tools do we have to transport time and frequency across a network? There are two distinct mechanisms that come into play-physical-layer transport and packet transport, and both are relevant to O-RAN. Physical-layer transport and Synchronous Ethernet Every network itself already has a mechanism to transport data over a wire or optical fiber and this can generally be used directly to transport a frequency from one node to another. This can be done because the transmitting side has to use a clock signal to clock the data onto the wire, whereas the receiving end typically needs to recreate a clock aligned to the bit rate of the incoming datastream to allow it to be decoded. This is shown in Fig. 8-8. Ethernet, which is the transport most likely to be used for any O-