R Fiber in the WAN Chapter Six In this chapter we discuss the use of fiber in the wide area network (WAN). First, we will briefly discuss the evolution of the use of fiber as a mechanism to provide high-speed communications between commu- nication carrier central offices. We will also discuss the rationale behind the use of fiber in the WAN. Once this is accomplished, we will turn our attention to the development of standardized optical transmission systems in the form of SONET in the United States and the Synchro- nous Digital Hierarchy (SDH), the European equivalent of SONET. After examining the characteristics and operation of SONET and SDH, we will then focus on two techniques that were developed to signifi- cantly increase the transmission capacity of a fiber: wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM). Evolution and Rationale The use of optical fiber in the WAN dates to the 1980s, when Sprint and AT&T began installing fiber throughout their backbone networks. In 1983 AT&T installed its FT3C lightwave transmission system in what was then referred to as the Northeast Corridor Project, with fiber routed from Boston to Washington, DC. The AT&T FT3C lightwave transmission system is notable because it used an early version of wavelength division multiplexing (WDM) to transport three 90-Mbit/s signals on a common fiber. This transmis- sion method enabled over 240,000 simultaneous voice conversations to be carried on one cable, which was a record for its time. In fact, the AT&T WDM system can be considered to represent a very early ver- sion of WDM. That system was based on the use of LEDs and multi- mode fiber, with the output of three LEDs operating at different fre- quencies coupled onto a common fiber. The result was the transmission of three data streams, each operating at 90 Mbits/s on dis- tinct optical frequencies. Thus, the AT&T FT3C lightwave transmission system seems to be based on the concept of frequency division multi- plexing. By early 1991 Sprint became 