"One-Way Versus Two- Way PTP" and "G.8265.1-Telecom Profile for Frequency Synchronization" in "Chapter 7. Preci- sion Time Protocol". For this reason, asymmetry is not an issue with G.8265.1 deployments, but ex- cessive PDV is a serious problem that must be controlled (more on PDV in a following section). Probably the two most deployed use cases for G.8265.1 are circuit emulation and mobile networks where phase is not yet necessary (for example, 3G/4G frequency division duplex [FDD] radios). As the mobile use case begins to require phase/time, the chief remaining use for this solution is CEM. And again, that applies only where other alternatives are not availablebecause SyncE would be the preferred option. Figure 9-9 shows how the flexibility of G.8265.1 can be used to provide a form of "glue" different frequency domains together, especially in those areas where no physical methods to carry frequency are available. In this case, the right-hand Ethernet network between router C and D does not support SyncE, which would normally be a problem. clockClass IP/TDM IP/TDM Ethernet/IP E1/T1/TDM Ethernet/IP SyncE output Frequency Recovered Frequency SyncE outp traceable to recovered Recovered frequency recovered Frequency traceable t input PRC/PRS from SyncE drives PTP clock from PTP frequency recovered from PRC/PRS SyncE drives TDM clock TDM line clock No SyncE Figure 9-9 Frequency Synchronization Between E1/T1, SyncE, and G.8265.1 PTP Profile The frequency is sourced from a PRC/PRS on the far left and used as a frequency source in router A. The QL information indicating the source of frequency would be carried using the SSM bits (if supported by the BITS signal). Router A uses that frequency as a source for the frequency on the outgoing SyncE link to router B. The QL information is carried over that same link in ESMC packets. Router B uses the ESMC QL information to help select the SyncE input from A as a network clock. That frequency is then used as the clock source for a TDM link to router C. As there is no CEM in thi