, and other possibilities by assigning one to each wavelength according to cus- tomer needs. From the point of view of datarate and protocol, WDM is said to be "trans- parent," which means that logically the M channels act entirely independently of each other. The Operating Spectrum The most common type of transmission fiber in use in Europe and North America is often called "Standard Single-Mode Fiber" (SMF), and its specifications have been agreed on by the ITU-T, as detailed in ITU-T recommendation G.652 (ITU-T G.652). The fiber operates as a single-mode waveguide at wavelengths beyond about 1250 nm, and it has acceptable micro- and macro-bending losses out to nearly 1650 nm. In recent years it has become pos- sible to fabricate G.652 fibers with low absorptions due to OH ions and heavy metal impurities. Consequently, their losses are often less than 0.5 dB/km over the whole of the 1250- to 1650-nm range and their best performance is ~0.2 dB/km at 1550 nm. With the aid of WDM, such fibers therefore offer great operating flexibility and potentially vast total bandwidth. For convenience the available spectrum has been divided into bands, as stated in Table 6.2. WDM can be categorized in various ways, but in terms of carrier wavelengths we can speak of Coarse (CWDM) and Dense (DWDM). CWDM is used for low-cost but relatively low-capacity operation, while DWDM is a higher-cost technology that can provide very high capacities. Normally, both CWDM transmission and DWDM transmission are unidirec- tional, SO that two-way propagation between a pair of nodes requires two fibers. However, some very cost-sensitive applications use a variant of CWDM, which could be termed "wavelength diplexing," where two (or a small number of) channels are propagated in opposite directions in the same fiber. Wavelength and frequency plans for both CWDM and DWDM have been standardized by the ITU-T (ITU-T G.694.2; ITU-T G.694.1) because it is recognized that there are significant benefits to all affected parties in common operation. The use 