on of optical networking in the WAN, let's turn our attention to specifics, namely, SONET and SDH. SONET and SDH Although the use of optical fiber significantly increased during the 1980s, one key difficulty faced by equipment purchasers was a lack of standards. Most optical equipment was proprietary, which precluded the ability of communications carriers to mix equipment from different vendors. In addition to creating interoperability problems, the lack of standards also resulted in the inability of carriers to obtain multiple bids for additional equipment after a primary vendor was selected. This had the effect of locking a communications carrier to a specific optical component vendor. Recognizing the problems mentioned above, the Exchange Carriers Standards Association (ECSA) initiated action during 1984 to develop a standard to govern the connection of one optical fiber system to anoth- er. While this effort governed the interconnection of optical fiber sys- items, it wasn't until 1989 that a comprehensive standard was developed by the predecessor of the ITU, the Consultative Committee for Interna- tional Telecommunications and Telegraphy (CCITT). The effort of the CCITT resulted in the Synchronous Digital Hierarchy (SDH) standard, which begins at a data transmission rate of 155.52 Mbits/s. SDH was oriented as a transport mechanism for the European E car- rier that was based on a format of thirty-two 64 Kbit/s DSO channels. In North America the T carrier uses a DS1 signal consisting of 24 DSO channels. Thus, the effort of the American National Standards Institute (ANSI) focused on the DS1 signal. Fortunately, there are many common features between the DS1 and E1 signals; thus, in an ANSI standard pro- mulgated during 1997, SONET and SDH were defined as being compati- Team-Fly® Fiber in the WAN ble at rates of 155.2 Mbits/s and above. In this section we first discuss SONET, examining its line structure, frame format, and transmission hierarchy. Once this is accomplished, we will then examine SDH and conclude t