xing and Demultiplexing DWDM multiplexing/ demultiplexing. Multiplexer 1,2,3,X Demultiplexer 1,2,3,Y. also be packaged together as a transceiver. A transceiver transports input and output light signals through the system, whereas TxM and RxM can only transport light in one direction. The most common form of DWDM uses a fiber pair-one for transmission and one for reception. Systems exist in which a single fiber is used for bidirectional traffic, but these configurations must sacrifice some fiber capacity by setting aside a guard band to prevent channel mixing. They also degrade amplifier performance and reliability. DWDM multiplexing and demultiplexing (see Figure 1-22) requires sep- arating and combing wavelengths. There are three primary technologies used to perform the multiplexing and demultiplexing functions: thin-film filters, fiber Bragg gratings (FBGs), and arrayed waveguides. Although WDM has been a widely known technology for several years (a little more than FDM by another name), its early application was restricted to providing two or four widely separated wideband wavelengths. During Advances and Opportunities in Next-Generation SONET the mid- to late-1990s, the technology evolved to the point where parallel wavelengths could be densely packed and integrated into a transmission system with multiple signals in the 192- to 200-THz range. There are economic advantages to using DWDM when the transmission costs are high, such as in long-haul applications. Such use is justified by standard transmission-versus-multiplexing cost calculations. For example, transmit- ting 40 Gbps over 600 km using a traditional system would require 16 sep- arate fiber pairs with regenerators placed every 35 km for a total of 272 regenerators. A 16-channel DWDM system, on the other hand, uses a sin- gle-fiber pair and 4 amplifiers positioned every 120 km for a total of 600 km. This topic will be treated at length in Chapter 5. 1.5 Key Metro Access and Metro Core Players The following lists show some of the key players in the ser