 port requires the frequency to be 10 MHz. Thus, a SETS device can take multiple frequency and phase inputs, select frequency and phase ref- erence sources from these inputs, and then output multiple frequency and phase signals that are locked to a configured reference source. It also provides the capability to monitor clock quality and clock signal fail conditions, which allows the software to take appropriate actions for selecting the next best reference source. 12.1.1 Frequency Synchronization Clock As discussed at different points in earlier chapters, there can be many frequency sources for net- work equipment (for redundancy and resiliency purposes). These frequency sources can primarily be categorized into two different categories: External timing interfaces: Recall from Chapter 6 that a system can have dedicated timing in- terfaces. These interfaces are used only for frequency synchronization purposes and not for data transfer. The BITS and 10-MHz ports (from a 10-MHz interface on a primary reference clock/primary reference source or a GNSS receiver) are examples of dedicated timing interfaces. Line timing interfaces: In addition to carrying network data, these interfaces carry timing in- formation as well. In recent times, Ethernet interfaces supporting synchronous Ethernet (SyncE) are the most prevalent interfaces carrying frequency along with data. Engineers expect that network equipment being used for clock synchronization is able to support both interface types. They also expect that there should be minimum possible restrictions on the types of ports that can be used for clock synchronization. For example, one expects all the Ethernet ports on an NE are SyncE capable, and the equipment allows configuration to recover frequency from any of these ports. Figure 12-2 illustrates a high-level hardware design for the frequency synchronization for such equipment. It shows SyncE-capable Ethernet ports (on the front panel); external timing interfaces, such as the SSU/BITS and 10-MHz interfaces; and an embedded