of agreed wavelengths allows operators to provide "midspan meet," which is the point at the physical or logical boundary Table 6.2 Optical Fiber Telecommunications Transmission Band Names and Wavelength Ranges Wavelength Range (nm) O-band ("original") 1260-1360 E-band ("extended") 1360-1460 S-band ("short") 1460-1530 C-band ("conventional") 1530-1565 L-band ("long") 1565-1625 U-band ("ultra-long") 1625-1675 Chapter 6 Optical Fiber Transmission where two networks must exchange signals. Expensive interfacing equipment would be required in the absence of standardized channel plans. Agreeing on the working wave- lengths also helps to reduce equipment costs. Production volumes increase and unit costs decrease if all component manufacturers supply their products to the same set of wave- lengths. Overall, the greatest benefit of WDM standardization is to the end customer of the telecommunications services. Coarse Wavelength Division Multiplexing The ITU-T standard G.694.2 (ITU-T G.694.2) specifies a grid of wavelengths for CWDM, as illustrated in Figure 6.2. The objective is to use a large portion of the ~400-nm fiber operating range at a low cost by spacing the channels far apart (Thiele & Nebeling, 2007). Up to eigh- teen channels are available but the ones at 1271 and 1291 nm are less commonly used than the others. The 20-nm channel separation depicted in Figure 6.2 is designed to allow rela- tively cheap filters and lasers. The emission wavelength of a semiconductor laser varies with temperature and SO a Thermo-Electric Cooler (TEC) and its control circuits are normally required to achieve a constant output. However, if the channels are positioned at 20-nm intervals, as shown, it is possible to tolerate temperature-induced drifting, which can be up to +6.5 nm. Therefore, we can omit the TEC and SO reduce costs. Furthermore, with such coarse spacing we can employ wider passband filters, usually based on relatively cheap optical thin-film structures. An important application for unidirectional CWDM is in metropolitan 