 as shown in Fig. 8-19. As can be seen in Fig. 8-19, a high priority message arrives in a queue to leave a switch just as a lower priority message has started being sent. Even though the new message has a higher priority it has to wait for the earlier one to complete since Ethernet does not provide any mechanism for preempting transmissions once started. The effects of this type of block- ing can be large. As an example, Fig. 8-20 shows the measured variation for Sync messages (dark) and Delay Request messages (grey) going through a network of 10 switches that are also carrying other traffic at a constant rate. It can be seen that in this particular test, which 286 Open Radio Access Network (O-RAN) Systems Architecture and Design Packets [1], [2] and [3] all wants to egress the same port. [2] is given priority over [1] given it is tagged as higher priority, but it is still delayed by [3] since that packet is already in progress FIGURE 8-19 Head of line blocking. Average (Sync message) Average (Delay Request message) FIGURE 8-20 Example network PDV. PDV, Packet Delay Variation. comes from an older ITU-T standard (G.8261) and is probably rather more draconian than would ever be seen in a real network, the PDV very closely matches the traffic loading on the network. The effect of PDV on the syntonization and synchronization calculations is that each time the calculations are done there will be a different result. For example, during the syntoniza- tion phase, it is meaningless to compare the t2 and tl timestamp differences between two consecutive Sync messages if those messages actually took different times to get through the network. In order for a PTP slave to provide useful time recovery, each one of these factors needs to be considered and mitigated in some way. Looking at each one in turn: Timestamping accuracy-The key to reducing errors related to the timestamping of the event messages is to design the solution such that the messages are timestamping as Chapter 8 Synchronization in open radio access networks 2