Grooming Mechanisms in SONET/SDH and Next-Generation SONET/SDH

Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH) are very closely related standards, which came into being primarily as a means of transporting telephone traffic in large volumes utilizing optical fiber transmission systems. SONET was brought forth by Bellcore (Telcordia), with coordination from International Telecommunication Union (ITU) as well as other standards organizations, and is primarily in use in North America, whereas SDH was developed by ITU, came into use slightly later than SONET, and is in use in the rest of the world. The two standards so closely resemble each other in most significant concepts as well as details that they are often spoken of as a single entity, and denoted as SONET/SDH. In what follows in the rest of this chapter as well as elsewhere in the book, the distinctions between the two are not important, and we continue to mean " SONET/SDH " , even when we mention just one of them, for ease of reference. From the grooming point of view, SONET/SDH are very important entities. Transmission systems for telephony, originally analog, adopted digital systems starting with the introduction of the T-carrier system in 1961. From the very beginning, such systems adopted a " base rate " which was defined by the digitization of a single voice line, and successively higher rates formed by multiplexing larger number of lower rate channels, thanks to the hierarchical nature of the telephony architecture; this is the origin of the term digital hierarchy. The term Plesiochronous Digital Hierarchy (PDH) is used to describe all such digital standards before the advent of SONET/SDH. This indicates a system in which all parts operate on clock signals which have exactly the same rate (within a bounded error), but may have different phases. Such a characteristic is arguably not a design decision as much as it is an adjustment to the realistically inevitable. The move to optical transmission systems was accompanied by a move to a synchronous architecture, which means that SONET/SDH performs multiplexing in a strictly time division multiplexed