r, switching the selected input to the input selector based on the software configuration. The SETS device has an input selector to choose the selected reference source for the system. It also has the capability to continuously monitor the input references, shown as Input monit- oring in Figure 12-2. The input monitoring module in the SETS device updates the software on the state of the clock quality. The frequency transfer using physical methods is usually accompanied by synchronization status messages (SSM) bits or Ethernet synchronization messaging channel (ESMC) protocol data units (PDU). These messages carry quality level (QL) information describing the quality of the frequency source being carried over the physical layer. The selection of the best frequency source for the equipment is done according to the QL val- ues, the physical link status, and the configuration on the equipment. This selection is also re- ferred to as the synchronization selection process and is defined in ITU-T G.781 (Annex A). This synchronization selection process is usually implemented in a software module running on the control plane CPU, as shown in Figure 12-2. Changes in QL values (or link status) triggers the synchronization selection process. This pro- cess involves selecting the best frequency source available on the equipment following the re- arrangement. To achieve this, the SETS device monitors all the available frequency sources and, when necessary, triggers the software module to execute the synchronization selection process. If the clock quality of the selected frequency source degrades (or the physical link goes down), the SETS device passes that information to the software module. The software then starts a new synchronization selection process triggered by this event. Figure 12-2 shows the SETS device connected to the local oscillator, which is used by the PLLs in the SETS device for locking to any inputs and generating any output synchronized frequen- cies. Refer to Chapter 6 for a survey of the different types of