ical Frequency Synchronization", for non- packet (such as SyncE). This section focuses more on the deployments typically selected by most mobile operators. See the relevant chapters for other use cases such as circuit emulation. A source of accurate frequency has always been a required element of radio installations, going back almost a full century. The thing that has changed is the method of achieving it. For MSPs, the options were typically provided by the PDH/SDH/SONET back-haul network; however, for 3G and later network generations, those options began to disappear. The following sections examine what sort of solutions the mobile operators have deployed for frequency synchronization. 11.2.1 Solution Options In summary, here is a list of the possible options to deploying a timing solution to support accurate and stable frequency synchronization: GNSS receiver inserted everywhere that a frequency signal is needed and feeding a physical TDM circuit (such as E1/T1 SSU/BITS), or a 2-MHz/10-MHz signal over a short-run cable. A standalone atomic clock acting as a PRC/PRS wherever frequency is needed. TDM E1/T1 or SSU/BITS network link to bring frequency from a remote PRC/PRS source over a frequency distribution network. Synchronous Ethernet (SyncE) frequency from a remote PRC/PRS source. Packet-based methods and protocols such as PTP or NTP, or some other packet technique such as adaptive clock recovery (ACR) or differential clock recovery (DCR). See Chapter 9 for more details of these solutions. Some radio technique; this is no longer common, although ground-based radio technology (for example, eLORAN) might make a comeback as alternatives to GNSS receivers. First, deploying atomic clocks (as a PRC/PRS) wherever frequency is needed is possible, but it is prohibitively expensive unless it is needed at only a few locations. The normal way would be to use these clocks as sources of frequency placed at strategic sites and then transport frequency via the network. Traditionally, that was how SSU/BITS networks were desi