 radios SO that they transmitted on the exact frequency that the mobile operator intended. So, hardware engineers designed the radios to use the oscillators that were being assisted by the frequency recovered from the external source. Radios for 3G and later generations evolved over time from carrying mobile traffic using a TDM transport to carrying mobile traffic using packet-based methods (for example, Ethernet, IP, and IP/MPLS). But because the transmission of data in packet networks is not synchronous, the Ethernet circuits do not require frequency synchronization. This created a problem because these links to the cell site no longer (by default) provided a source of frequency synchronization for the radio equip- ment. Some operators solved this problem by retaining a single E1/T1 circuit connected to the radio purely for the purposes of providing it a source of frequency. It was this deployment of packet-based methods that made SyncE popular because it added the cap- ability for the packet network to carry frequency synchronization in the Ethernet's electrical and op- tical layers. Figure 2-4 shows how the frequency synchronization of those networks was achieved using either a single TDM circuit or enabling SyncE (only one of the two options was required). SDH or SONET Ethernet SyncE with IP/MPLS No sync needed Central Ethernet mobile core Figure 2-4 Frequency Synchronization Using Packet back-haul Enabling Ethernet to propagate frequency hop-by-hop through a network required hardware sup- port, SO it was not something that could be enabled only by software changes. But as equipment manufacturers increasingly incorporated SyncE support into mobile back-haul routers, operators moved to decommission any costly TDM links and provided mobile synchronization via the Ether- net connections (this process is largely complete for many operators). The section "Circuit Emulation" later in this chapter as well as Chapter 7 and "Chapter 9. PTP De- ployment Considerations", describe how a packet-based solution based on th