ends on the required accuracy and the message delay through a network. For example, setting the clock of a PC using NTP over a network that has an average latency of 50 ms is probably okay for the majority of applications, whereas trying to achieve submicrosecond level time alignment, as is needed for O-RAN, over a network even a hundred times better would be nonstarter. Two-way protocol (Fig. 8-13)-The inaccuracy caused by the finite delays in a network can be largely eliminated by using a two-way protocol, in which messages flow in both directions between the master and slave. These methods combine one-way time delivery with a ranging mechanism that is used the gauge the message delay between the master and the slave, which can then be used by the slave to correct the received time. By far the most common two-way time transport protocol, and the one of relevance to O-RAN, is PTP, as defined by the IEEE 1588-2008 standard-hence it is alternate name of just 1588. The following section gives an overview of PTP. An introduction to PTP PTP was originally designed for time synchronization across small, localized, networks for industrial automation applications such as the synchronization of robots on a production line. However, it was quickly appreciated that the underlying concept, though not the partic- ular initial implementation, could be used for a wide range of applications across both large and small networks. Therefore a second, incompatible, version of PTP was defined and rati- fied in the 2008 release of the IEEE 1588 standard (the original dating from 2002). A subse- quent third release of the IEEE 1588 standard in 2019 added some additional functionality Master sends time over network to slave Master Round trip delay measurement allows master to slave delay Slave to be estimated and corrected for FIGURE 8-13 Two-way time distribution. 280 Open Radio Access Network (O-RAN) Systems Architecture and Design and clarification but is fully backward compatible with the 2008 version, and none of the new changes a