mpares the reference input signal to the noisy signal from the clock under test. Chapter 6. Physical Frequency Synchronization In this chapter, you will learn the following: Evolution of frequency synchronization: Covers how frequency synchronization strategies evolved over time and what are the different approaches to achieve frequency synchronization. BITS and SSU: Describes these pieces of timing equipment and their role in achieving net- work-wide frequency synchronization. Clocking hierarchy: Provides an overview of a clocking hierarchy within a frequency syn- chronization network. Synchronous Ethernet: Covers how Ethernet evolved the ability to distribute frequency over Ethernet links. Enhanced synchronous Ethernet: Describes the need for higher-performance synchronization and how synchronous Ethernet is being enhanced to achieve it. Clock traceability: Covers techniques to transport the quality of a clock to all network nodes and how nodes use this information to select the best timing source available. Synchronization network chain: Describes the principles to aid in deploying and managing network nodes to build a frequency distribution network. Clock selection process: Provides an overview of the methods a clock uses to select the best source of reference timing signal. Timing loops: Covers what timing loops are and how timing loops can be avoided in networks. Standardization: Provides an overview of the standards development organizations that are ap- plicable to frequency synchronization. In the previous chapters, you have read that synchronization mechanisms are essential in both mo- bile and fixed communications. Synchronization is one of the fundamental technologies that tele- communication operators use to build and deliver network services, and they have introduced it into many of their networks. "Chapter 3. Synchronization and Timing Concepts", introduced the differ- ent types of synchronization, classified as frequency, time, and phase synchronization. You also learned that the state in which th