ng chapters: Chapter 4, "Standards Development Organizations," takes a slightly different tack and intro- duces the amazing standards-setting community that underpins much of what makes the world's information and communications systems work. Of course, there is the 3GPP, the IEEE, and the ITU-T, but also later entrants such as the CPRI and the O-RAN Alliance, defining the modern radio access network (RAN). Chapter 5, "Clocks, Time Error, and Noise," is where the material starts to get more intense, covering the details of clocks, clock signals, and components of clocks. This is where many of the metrics for timing are first explored in topics such as TE, TIE, MTIE, and TDEV along with an explanation of jitter and wander. This sets up the foundation knowledge for a detailed exam- ination of distributing frequency using physical methods. Chapter 6, "Physical Frequency Synchronization," provides comprehensive coverage of the topic of frequency synchronization, mainly using older TDM techniques such as E1/T1/SDH/SONET and the newer SyncE and eSyncE. It includes discussion of building a clock hierarchy using clocks of different quality or stratum levels-such as BITS/SSU devices. For this to work, there needs to be a traceability mechanism to support the transport of clock quality information-a mechanism that also helps to avoid timing loops, which this chapter then covers in a separate section. Chapter 7, "Precision Time Protocol," is all about PTP and the details of many of its character- istics and features contained first in IEEE 1588-2008 and then 1588-2019. The chapter provides details on the PTP datasets, the messages and how they are encapsulated, the BMCA, the clock types, and the mathematics used to recover frequency and time/phase. It ends with coverage of the different PTP profiles, including the ITU-T telecom profiles, and PTP security. Part III, "Standards, Recommendations, and Deployment Considerations," provides an overview of the various ITU-T recommendations and discusses the factors involved in dist