equency has no concept of the time. There is little point in having the most accurate and stable clock when it is not set to the correct time. Again, this calibra- tion can come from a GNSS receiver or some reference signal provided as a service (in many cases) from a national physics laboratory. In addition to frequency, a PRTC needs to be able to output a phase and time signal. More details on that topic are provided later in the chapter in the section "Source of Frequency, Phase, and Time: PRTC" that discusses the 1PPS and time of day (ToD) signals. The ITU-T has recommendations that define the performance and characteristics of various levels of performance in the PRTC. In line with the introduction of the enhanced version of the PRC, there is now also an enhanced primary reference time clock (ePRTC) offering a more accurate source of phase/time. These clocks are specified in ITU-T G.8272 (PRTC) and G.8272.1 (ePRTC) and can be thought of as the phase/time equivalents of G.811 and G.811.1 for PRC and ePRC clocks. See Chapter 8 for more details on these standards. Packet Clocks In the packet-based network, a master clock is the source of time for the connected network ele- ments lower in the synchronization hierarchy, and the packet signal generated by the master clock is provided to downstream slave clocks. These clocks are referred to in some ITU-T documents as packet-based equipment clocks (PEC)-one being the PEC-master (PEC-M) and the other being the PEC-slave (PEC-S). Refer to ITU-T G.8265 for more information. Earlier chapters mentioned PTP and NTP as methods to distribute timing across the packet network, but there are other options available, as covered in Chapter 9. Mostly, this chapter will focus on PTP as the method to distribute time in a packet network, but be aware that there are other options. Chapter 7 covers PTP in much more detail, and SO here, the basic concepts of packet timing will be outlined using PTP as an example. Within any given part of the PTP network, there is always a single master