gned. The vast majority of MSPs used atomic clocks to provide a frequency signal and then built a stan- dalone SSU/BITS network to carry that signal to their main facilities, central offices, and points of presence (POP). To get that frequency from the local POP out to the mobile cell site, the TDM- based back-haul link from the nearest POP carried it via line-clocking along with the data and voice circuits. As TDM was replaced (eventually) by Ethernet, SyncE became widely adopted, but only where it was possible, given that technical constraints exist. For instance, for sites that are difficult to reach, the only transport available might have been microwave, which originally offered only limited sup- port for SyncE. For these and similar reasons, most MSPs settled on one of the following options: Install a GNSS receiver at the cell site giving frequency with a short-run cable (such as 10-MHz signal over coax cable) Maintain a legacy TDM network link (at least one) to the cell site to carry only frequency (it was no longer required for voice and data, as that was using Ethernet) Bring a SyncE timing signal over the back-haul network where technically possible Transport frequency with a packet-based method, almost exclusively using the G.8265.1 PT- PoIP telecom profile for frequency (at least one major mobile equipment vendor used NTP instead) To understand the trade-offs when using deployments based on GNSS, refer to the upcoming sec- tion "GNSS Everywhere" because the trade-offs are identical to using them for phase/time use cases. Using the frequency recovered from a TDM back-haul circuit is what many SPs already have a long history with. But many are now getting rid of it, as it means maintaining another separate network (such as SSU/BITS) solely for the purpose of carrying frequency. Not only that, but the equipment is very expensive and, in many cases, becoming impossible to maintain as vendors drop support for TDM and therefore the cost to maintain these links soars. Another factor was that cell site router