ONET and SyncE, as well as digital subscriber line (DSL), passive optical network (PON), DOCSIS cable, and SO on Packet-based (Layer 2 and Layer 3) distribution: NTP, PTP, White Rabbit, adaptive clock recovery (ACR), and SO on Radio or satellite navigation systems: GNSS systems, but also other radio systems such as LORAN-C, IRIDIUM, WWVB, DCF77 and many others The GNSS and radio systems have been treated extensively above as a source of time, but this sec- tion outlines the different approaches and the tradeoff with each of them. Physical Layer In this solution approach, nodes in the network hierarchy carries the synchronization signals over in layer 1, as part of the communications method itself. It has been shown that legacy TDM and SDH/SONET networks carry their frequency reference at the physical layer. It uses a hierarchy of clocks to distribute network synchronization from the PRC/PRS down to the network element clocks. As shown previously in Figure 3-13, the highest accuracy clock, the PRC/PRS, is deployed at the top of the hierarchy. The next level of hierarchy is the SSU/BITS layer that supports better holdover and stability. The third level is SEC or SONET minimum clock (SMC). And this three-layer hier- archy will deliver a frequency that is traceable to the PRC/PRS, SO long as the path is not broken. In this hierarchy, the PRC and SSU/BITS are standalone timing elements, while SEC/SMC devices are implemented as a component inside the network elements. SyncE has been designed along the same principles as SDH, where the Ethernet physical layer provides traceability to a PRC/PRS frequency reference. Because the first version of Ethernet was originally designed for asynchronous data traffic; it was initially not suitable to carry synchronous signals. In fact, the original 10 Mbps Ethernet cannot provide a synchronization signal over physical layer because it periodically stops transmission. These mechanisms are only used to carry frequency. There are systems that use physical signals to carry phase (you co