 operated in the 800-nm window with 4 channels, each operating at 140 Mbps. These systems were primarily used in cable TV links. However, CWDM systems did not generate significant interest among service providers-that is, until now. As metro carriers seek cost-effective solutions to their transport needs, CWDM is becoming more widely accepted as an important transport technology. As noted, unlike DWDM, systems based on CWDM technology utilize uncooled DFB lasers and wideband optical filters. These optical elements provide several advan- tages to CWDM systems such as lower power dissipation, smaller size, and lower cost. The commercial availability of CWDM systems offering these benefits makes the technology a viable alternative to DWDM systems for many metro and access applications. The bandwidth of a fiber-optic link can be increased by transmitting data faster or transmitting multiple wavelengths in a single fiber. As dis- cussed at length in Chapter 5, "DWDM Systems," WDM is accomplished by using a multiplexer to combine wavelengths traveling on different SONET-managed fibers into a single fiber. At the receiver end of the link, a demultiplexer separates the wavelengths and routes them into different fibers, which all terminate at separate receivers (see Figure 6-3). The spacing between the individual wavelengths transmitted through the Chapter 6 Figure 6-3 WDM revisited. Wavelength Division Multiplexing Multiplexer Demultiplexer 11 Signal 11 Signal Single fiber 12 Signal 12 Signal 13 Signal 13 Signal 14 Signal 14 Signal WDM is accomplished using a multiplexer to combine wavelengths traveling on separate fibers into a single fiber. At the receiver end of the link, a demultiplexer separates the wavelengths and routes them into different fibers. same fiber serves as the basis for differentiating DWDM from CWDM (see Figure 6-4). DWDM systems typically use wavelength separations of 200 GHz (1.6 nm), 100 GHz (0.8 nm), or 50 GHz (0.4 nm), with future systems projected to have even narrower spacing at 25 GHz or less.