Network synchronization, at first unknown and considered irrelevant to the operation and performance of telecommunications networks, has played an increasingly important role throughout the evolution of telecommunications, especially since transmission and switching became digital. This survey deals with telecommunications network synchronization from a historical point of view, aiming to show how network synchronization issues have evolved with telephone networks over the last 30 years, beginning with old FDM networks up to the latest technologies through PDH, SDH/SONET, and ATM. For each case, the different synchronization needs and the peculiar techniques of timing transfer and distribution are pointed out, thus providing a comprehensive overview of the evolution steps of telecommunications network synchronization.
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