ectly, but it does have a couple of drawbacks: SyncE is only useable for frequency distribution. There is no way that it can be used directly to transport time. For SyncE to work, every switch in the network between the master and slave must support it. If any switch in the link doesn't then synchronization is lost-just like breaking a single link in a chain renders the chain useless. Despite these limitations, we shall see that SyncE has a role to play even in time distribu- tion and since the incremental cost of including SyncE capability in new networking equip- ment is relatively low most new-build networks now being deployed include it. However, the majority of legacy networks do not support SyncE because of the fact that doing so would require new equipment to be deployed throughout the network. Packet time transport and precision time protocol To be able to transport time, and not just frequency, across a network requires a different tactic-specifically, a packet-based time transport that uses messages on the network to transfer time from the master to the slaves. There are two fundamental ways of achieving this: One-way protocol (Fig. 8-12)-The master simply sends time to the slave devices, either through a broadcast or one-by-one or in response to a request from the slave, and the slave uses this to update its clock. This is the method used by the original forms of Network Time Protocol (NTP). The advantage of this method is that it is relatively simple. However, it had a significant drawback in that it cannot compensate for the facts that the messages carrying the time information experience a finite delay between the master and Master sends time over network to slave Master No compensation for Slave network delay FIGURE 8-12 One-way time distribution. Chapter 8 Synchronization in open radio access networks 279 slave. In other words, when the slave receives the time message it only knows what the time was at some point before now, rather than what it actually is now. Whether this is a problem or not dep