ascinating story in Wired magazine, chronicling a project called FLAG that involved a 28,000-km fiber loop around the globe. I encourage you to read it. Its title is "Mother Earth Mother Board: The Hacker Tourist Ventures Forth Across the Wide and Wondrous Meatspace of Three Continents, Chronicling the Laying of the Longest Wire on Earth," and it is available at http://wired-vig.wired.com/wired/archive/ 4.12/ffglass_pr.html. An optical multiplexing hierarchy was the predecessor to WDM: Synchronous Digital Hierarchy (SDH) and Synchronous Optical Network (SONET). SDH/ SONET is a time division multiplexed system, and SDH/SONET fiber cables make use of just one wavelength. (SDH/SONET is discussed in detail in Chapter 4, "The PSTN.") DWDM can operate over 16 or more wavelengths. Products that are cur- rently shipping support upward of 192 wavelengths and operate at data rates of Chapter 2 Traditional Transmission Media 2.5Gbps to 10Gbps. New systems have emerged that operate at 40Gbps. Mean- while, research is also under way with dense wavelength division multiplexers that will be capable of supporting as many as 15,000 wavelengths. These developments are just the tip of the iceberg of what we can expect in coming years. (WDM and DWDM, as well as many other aspects of optical networking, are discussed in more detail in Chapter 11.) Forecasting Optical Developments The basic equation in assessing the development of optics is that every year, the data rate that can be supported on a wavelength doubles and the number of wavelengths that can be supported on a fiber doubles as well. Applications of Fiber Optics Fiber has a number of key applications. It is used in both public and private net- work backbones, SO the vast majority of the backbones of the PSTNs worldwide have been upgraded to fiber. The backbones of Internet providers are fiber. Cable TV systems and power utilities have reengineered and upgraded their backbones to fiber as well. Surprisingly, electric power utilities are the second largest network operator af