e clock frequencies of different systems match is called frequency synchronization, and the state in which timings between clocks agree is called phase synchronization. This chapter focuses primarily on how frequency synchronization is achieved across clocks that are connected to each other in a network. The central focus will be on those as- pects specific to frequency synchronization. 6.1 Evolution of Frequency Synchronization Frequency synchronization had an immense impact on the evolution of traditional telecommunica- tion, especially in enabling the transition of transmission and switching from analog to digital. Tra- ditional public switched telephone networks (PSTN) started the digitization process with isolated digital transmission links between analog switching machines or radio transmission systems. The fact that digital technology was being applied was transparent to the interfaces, SO the internal clock rate in one system was not required to be synchronized to the internal clock rate of another system. Even networking systems that combined higher bit-rate multiplexing with lower-speed tributaries did not require synchronization across systems. It was the introduction of circuit-switched data net- works and the Integrated Services Digital Network (ISDN) that first needed more stringent syn- chronization. With this development, the evolution of network synchronization began. A few different network synchronization strategies evolved, but for frequency synchronization, the one that became most prevalent was for the clock distribution of frequency signals via a separate external network (refer to the section "Legacy Synchronization Networks" in "Chapter 2. Usage of Synchronization and Timing"). This led all the major network service providers to build national frequency synchronization networks to distribute a common timing reference to each node of their telecommunication networks. At the same time, various standard bodies, such as the International Telecommunication Union Telecommunication Standardizati