h as ITU-T, IEEE, and ANSI. ("Chapter 4. Standards Development Organizations", outlines the major SDOs and their relevance to both the fields of mobile and synchronization.) Chapter 8 covers the functioning of one of the major SDOs in this space, the ITU-T, in much more detail. It also outlines the processes behind the development of standards, and the concise summary of specific standards that are applicable to synchronization and timing. Chapter 8 also provides a detailed outline of all the applicable standards for frequency synchronization, covered in separate sections of that chapter. 6.11 Summary At a broad level, the mechanism to achieve frequency synchronization can be categorized as fre- quency distribution using the physical layer and packets. The primary focus of this chapter was fre- quency distribution using the physical layer. The chapter explained the SSU/BITS devices and their role in achieving and distributing frequency synchronization. Based on the accuracy and stability that a clock can provide, different clock levels are defined within a timing network hierarchy. These levels are segregated primarily based on different stratum levels, such as PRC/PRS belongs to stratum 1, SSU/BITS usually falls into stratum 2, and SECs/NEs to stratum 3 level. This chapter explained the hierarchical relationship between stratum levels, referred to as clocking or synchronization hierarchy. For proper frequency synchronization, a clock should always lock to another clock with a higher or at least equal stratum level. This requires a method to communicate the stability level of a master clock to a slave clock along with the frequency signal itself and is achieved via a clock quality (QL) indicator. The chapter covered the QL mechanism in detail, and how it is carried as SSM, ESMC, or enhanced ESMC from the master to the slave nodes. This forms the basis of clock traceability, with which every clock can trace its received reference signal from the source. A timing network chain is a connected network of nodes transp