re (the tunnel labels and tags might make it difficult for the TC to recognize PTP messages). Provision/sell some timing transparent circuit/servicesuch as dark fiber-that has no active components between the two ends of the fiber. There is a fourth option available, and that is where the CE SP delivers an accurate (but independ- ent) PTP message stream with a stable SyncE at any location where the MSP needs it. The major difference is that the timing signal comes from inside the CE network and is not part of the MSP timing domain. For this reason, it is frequently called time as a service. Figure 11-5 illustrates how this works. PTP & SyncE Mobile cell sites PTP & No sync needed SyncE Carrier Ethernet network Leased circuit Central mobile core Service demarcation Figure 11-5 Carrier Ethernet SP Offering Time as a Service This is similar in concept to having a standalone GNSS receiver at the cell site. If the frequency is within specification (traceable to PRC/PRS) and the phase is well within 1.1 of UTC, then it does not matter where the timing signal comes from. Of course, the MSP must ensure that its timing do- main is closely aligned to UTC for this scenario to work (the MSP could not run an autonomous timing source without it being calibrated or aligned to UTC). So, a CE SP can build its network into one that supports accurate timing and use it to offer MSP and other customers a traceable timing signal as a service. If that supplied (for example) 200-300 ns offset from UTC and an accurate SyncE, most operators would leap at it. Many would happily pass their timing distribution problems onto somebody else. Of course, the attractiveness of such a service comes down to cost, but the authors are of the opin- ion that it would gain a lot of interest and are aware of several large SPs that are currently trialing or planning such a network. 11.2 Frequency-Only Deployments The solution options for deploying frequency timing are covered in "Chapter 9. PTP Deployment Considerations", for packet and in "Chapter 6. Phys