ion within that equipment, that is involved in the distribution and use of time or frequency-it does not imply a clock in the traditional sense of something that keeps time. Chapter 8 Synchronization in open radio access networks 277 For the purpose of this discussion, the details of what the SyncE standards define are not really necessary beyond knowing that the "clean up" function shown in Fig. -10 is a low- pass filter with a bandwidth normally between 1 and 10 Hz. This allows each switch in the network to filter out noise on the SyncE signal, rather than allowing it to accumulate unchecked node-to-node, allowing for more switches between the master and slave. Another aspect of SyncE, defined in the ITU-T G.8264 recommendation, is a messaging protocol that operates between nodes to indicate the quality of the clock carried on the cor- responding link. After all, the receiver has no simple way of knowing if the frequency it is recovering has come from a GNSS receiver or from a cheap crystal in the upstream node. Therefore this message protocol, which is referred to as the Ethernet Synchronization Messaging Channel is used to send Synchronization Status Messages (SSMs) between nodes. A multiport switch can look at the SSMs received on each port to decide which ports carry a suitable SyncE clock and then select one of these to use as the transmit clock. This is shown in Fig. 8-11. Rx Port Tx Port Rx Port Tx Port Filter Filter Clock is recovered from Rx side, filtered and used as Tx clock FIGURE 8-10 SyncE propagation. Ethernet Receiver QL-PRC Device QL-DNU QL-PRC QL-DNU Switch selects and propagates Ethernet QL-DNU clock from GNSS marked as QL-PRC Device (primary reference quality) Free-running clock is marked DNU - do not use - and is ignored FIGURE 8-11 SyncE ESMC Messaging. ESMC, Ethernet synchronization messaging channel. 278 Open Radio Access Network (O-RAN) Systems Architecture and Design SyncE provides a good way of distributing frequency and can achieve high accuracies since it uses the physical layer dir