 metry in the CE network, but that has its own trade-offs (see the section "Assisted Partial Timing Support with G.8275.2" later in the chapter). For both local phase/time assurance monitoring and APTS deployments, a local GNSS receiver would be needed at the cell site and the CE SP would be merely delivering a backup source of time via the PTP. On the other hand, the CE SP must guarantee that accurate phase/time can be recovered by its cus- tomers when the PTP the CE SP is carrying for them is receiving no on-path assistance. To be suc- cessful, the CE SP must design and deliver a service that has only a few hundred nanoseconds of asymmetry and extremely low PDV. The CE SP must also be able to monitor the timing perform- ance of this network and demonstrate compliance to its service level agreements (SLA) for timing performance. This requires careful network engineering. MEF has defined some of these service characteristics in MEF 22.3 section 12.3.1, "Performance for Synchronization Traffic Class," and section 13, "Synchronization." See the MEF website (https://www.mef.net/) for its documentation on providing time-aware back- haul services. Basically, the best option from a timing point of view is being able to lease dark fiber, but this can be difficult to procure in every location where it is needed by the MSP. The other alternative is for the CE SP to offer to deliver time as a service (TAAS). 11.1.1 Time as a Service The earlier section "Third-Party Circuits" shows that there are basically three options available to carry timing where the MSP does not own or control their own back-haul network. The options for the CE SP are Carry the MSP (packet) timing signal transparently using some tunneling technology without any attempt to provide on-path hardware assistance at the intervening nodes in the CE network. Carry the MSP timing signal using a TC at every step in the CE network to update the correc- tionField of the transit packets. For this to work, the PTP messages must be visible to the under- lying hardwa