12-1: Frequency recovery PLL: This PLL, sometimes also referred as the TO PLL (as labeled in Figure 12-1), is used to recover the frequency reference signal for the whole system. This PLL recovers frequency from the selected frequency reference and outputs the recovered frequency to be used as the system frequency. The output frequency is fed to different ports (such as SyncE-enabled ports) and potentially to other components of the system requiring the recovered frequency. Phase recovery PLL: The SETS device also hosts a separate PLL to recover phase inde- pendently alongside the frequency recovery PLL. The reference source for phase recovery could be a phase reference signal input to the equipment, such as a 1 pulse per second (1PPS) signal from an embedded GNSS receiver port or a dedicated 1PPS port on the equipment. Refer to the section "1PPS" in "Chapter 3. Synchronization and Timing Con- cepts", for details on 1PPS. In the absence of these reference sources, this PLL is used by the PTP slave software to re- cover phase from PTP packets. Having a separate PLL for phase, different from frequency, keeps the phase and frequency domains separate and hence helps implement a good hybrid mode function on a network element (NE). With separate PLLs, the NE is also able to sim- ultaneously output separate frequency and phase to downstream clocks. Recall that the term hybrid mode is used to refer to phase recovery from PTP, whereas frequency synchroniz- ation comes from a physical frequency source. Figure 12-1 shows a separate frequency reference input to the frequency PLL and the phase reference to the phase PLL. However, some SETS devices also have a connection between the frequency PLL and the phase PLL to feed (or receive) a recovered frequency or phase signal from one PLL to another. For example, although the phase PLL might facilitate phase recovery from PTP packets, the phase PLL will also require a frequency reference (for ex- ample, a SyncE input) to drive both phase and frequency for the whole system. T4 PLL: