he OTN. Two frequency combs have been standardized by ITU-T for the OMS: the DWDM comb and the coarse wavelength division multiplexing (CWDM) comb: the standard wave- lengths are shown in Figures 3.34 and 3.35, respectively. In these figures also, the standard names the ITU-T has assigned to the different bands on the near infrared where fiber transmission can be realized are reported. O-Band E-Band S-Band L-Band 1260-1360 1360-1460 1460-1560 1565- 1530- Water 1271 12911311 1331 1351 1371 1391 14111431 1451 14911511 1531 155115711591 1471 1611 LcWDM Wavelength (nm) wavelengths ITU DWDM wavelengths (8 wavelengths) (channels 20 through 59) FIGURE 3.34 ITU-T standard wavelengths for DWDM transmission systems. In the figure, the standardized names of the optical bandwidths and the reference attenuation curve of a standard transmission fiber are also reported. Networks Fundamentals and Present Architectures O-Band E-Band S-Band L-Band 1260-1360 1360-1460 1460- 1565- 1530- Water 15711591 FIGURE 3.35 ITU-T standard wavelengths for CWDM transmission systems. DWDM systems have wavelength in the C and L bands, often in extended versions of these bands depending on the needs of the transmission system. The DWDM standard comb is constituted by 132 wavelengths with a spacing of 50 GHz. This comb can be used in different ways: Selecting a subset of wavelengths 50 GHz spaced Selecting a subset constituted only by even or odd wavelengths, thus obtaining a 100 GHz spaced comb Selecting a subset of 200 GHz spaced wavelengths In all these cases, the available bandwidth around the central wavelength is also standard- ized, being smaller in the former case and larger in the last. The CWDM comb is defined with the scope to create a standard suitable for the use of uncooled lasers. The channel spacing of 21 nm is derived from the analysis of frequency fluctuations of an uncooled distributed feedback (DFB) laser. It is evident that the fiber attenuation changes greatly from channel to channel (the attenuation curve is reported in the fi