Nortel White Paper, "Comparing Optical Ethernet Resilient Packet Ring to Packet over SONET," 56029.25/01-01, January 2001. 15. D. Minoli and O. Eldib, Telecommuting (Boston, Mass.: Artech House, 1995). 16. Terri Gimpelson, "Nortel's Alternative for Metro Ethernet Recovery," Network World (October 1, 2001). 17. Terri Gimpelson, "MSPPs, Next-Gen SONET Mixing Optical Signals," The Edge, October 9, 2001. CHAPTER Dense Wavelength Division Multiplexing (DWDM) Systems Chapter 5 Wavelength division multiplexing (WDM) and high-capacity WDM systems called dense WDM (DWDM) have seen major deployment in the United States since the late 1990s. By using frequency-division multiplexing (FDM) techniques on a fiber transmission system, you can achieve fiber- optic pair gain where multiple signals (called lambdas [X]) operating at dif- ferent frequencies can each carry OC-48 (2.5-Gbps) or OC-192 (10-Gbps) channels between endpoints.1 WDM is viewed as operating at the optical layer; Synchronous Optical Network (SONET) is viewed as an overlay onto this newly established layer. WDM multiplies the traffic-carrying capacity of existing fiber spans by combining two or more optical signals of different wavelengths on a single fiber pair.2 DWDM coupler devices perform the actual multiplexing/demultiplexing of the different optical wavelengths. The multiplexing/demultiplexing of 2 to 1,000 channels has been demon- strated, with typical commercial systems in the field carrying 16 or 32 chan- nels operating at OC-48 (2.5-Gbps) or OC-192 (10-Gbps) data rates. The International Telecommunications Union (ITU) has defined a set of stan- dard wavelengths for manufacturers to use in DWDM in order to facilitate interworking between equipment. So far, this technology has been opti- mized for and has seen major deployment in long-haul networks based on very traditional cost-of-transmission-versus-cost-of-multiplexing economic trade-off considerations. Proponents of DWDM make the claim that acceptance of the technology will drive the expansion of the 