h boundary clocks everywhere. Putting PTP and SyncE together Having just learned about PTP and how it can be used to achieve both syntonization and synchronization-in other words, provide time and frequency to the slave-you might be for- given for thinking that SyncE as a frequency-only transport does not have any role to play. After all, PTP can do it all. However, in practice, it turns out that there are significant benefits of using both SyncE and PTP in conjunction to achieve the best overall solution. First, as we will see in the next section, O-RAN defines both frequency and time align- ment requirements. As already said, being a physical-layer technique, SyncE can provide very stable frequency to the slave. Therefore although PTP could be used to provide both fre- quency and time to an O-RU, it makes perfect sense to use SyncE for frequency aspect if it is available in the network itself. 290 Open Radio Access Network (O-RAN) Systems Architecture and Design The second benefit of SyncE comes into play in determining the performance of PTP itself. The key to being able to achieve accurate time synchronization with PTP is to first achieve fre- quency syntonization. If the frequency that forms the tick rate of the local clock in the slave device is itself unstable, it makes being able to synchronize time that much harder. Syntonizing the slave tick rate is effectively a necessary steppingstone to synchronizing the actual time. However, if you already have a stable frequency from SyncE then why not bypass the PTP synto- nization step and go straight to synchronization? The actual details are somewhat more involved, since although the frequency from SyncE is likely to be more stable than the frequency recov- ered from PTP, accumulated noise in the network can mean that the frequencies are not per- fectly aligned, which would, in turn, cause more problems with synchronization and, therefore, has to be allowed for. However, for the purposes of this discussion, the message to take away is that PTP + SyncE will alwa