l Hierarchy Optical communications can transmit data for many end applications. However, it is advan- tageous to limit the number of datarates and data protocols in order to ensure compatibility of the equipment used and to constrain costs. Nearly all data conveyed by transcontinental, national, and regional networks, as well as a large percentage of the data on metropolitan networks, conforms to a set of standards known as Synchronous Digital Hierarchy (SDH) (Sexton & Reid, 1997). The details of SDH, which is used in most countries, are available from the wide-ranging recommendations of the Telecommunications Sector of the Interna- tional Telecommunications Union (ITU-T) (ITU-T G.707, 2007; ITU-T G.780, 2004; ITU-T G.691). A slightly different standard, Synchronous Optical Network (SONET), has been adopted in North America. However, because many of the standardized datarates and other operat- ing characteristics in SDH and SONET are identical, throughout this chapter we refer only to SDH. Interested readers should refer to Shepard (2001) for a more SONET-oriented treatment. In addition to the benefits of cost and interoperability, optical communications systems based on SDH offer several advantages. These include The lease of routing, dropping, and inserting in the time domain at digital cross- connects, thanks to the synchronous nature of the data Advanced network management functions, which are enabled via a sophisticated data frame structure The ability to interoperate with other data protocols, such as Internet Protocol (IP) and Asynchronous Transfer Mode (ATM) The availability of protection switching in the event of fiber or equipment failure Most of the details of SDH are outside the scope of this chapter, but detailed explanations are provided in Chapter 10 and in Sexton and Reid (1997). One aspect of SDH that does directly relate to optical fiber transmission is the channel datarates used. As the acronym SDH specifies, there is a hierarchy of datarates. They are all designated "STM-m," which means "Synch