l metrics are not fond of retiring working equipment. Early deployments of WDM (in the early 1990s) were based on wideband technology. Figure 5-20 illustrates how wideband WDM (WWDM) doubles the capacity of fiber plant by optically coupling the outputs of two terminals in a fiber-optic transmission system (FOTS); one terminal operates in the 1,310-nm range and the other operates in the 1,550-nm range. Although this is a cost-effective solution for applications with restricted reach, wide- band WDM systems, which tend to consist of a little more than an optical Dense Wavelength Division Multiplexing (DWDM) Systems Electronic Figure 5-19 Regenerators Economic SONET SONET advantages of WDM. SONET SONET SONET SONET SONET SONET SONET Electronic Regenerators SONET SONET SONET SONET SONET SONET SONET 1310-nm 1310-nm Figure 5-20 Original WDM Terminal Terminal system. 1550-nm 1550-nm Optical Coupler Optical Splitter/Filter Terminal Terminal coupler and splitter, suffer from the absence of maintenance capabilities and scalability for long-haul applications. 13 However, if they are priced cor- rectly, these WWDM or CWDM solutions, perhaps with some added fea- tures, may be applicable to metro applications. Chapter 5 More recently, narrowband or DWDM systems have been deployed. With DWDM, the outputs of 2, 4, 8, 16, or more SONET terminals are optically multiplexed into one fiber pair, as shown in Figure 5-21. As noted earlier, the wavelengths used are all within the 1,550-nm range to enable optical amplification using EDFA technology to maximize reach (stated differently, the EDFA operates in this optical region). The wavelength of the optical output from a SONET network element (NE) is roughly centered at 1,310 or 1,550 nm and has an approximate tolerance of +20 nm. Such wave- lengths would have to be SO widely spaced that after optical multiplexing only a few would be within the EDFA passband. Therefore, DWDM systems must translate the output wavelength from the SONET NE to a specific, stable, and narrow-width wavelength 