 To provide 16 channels on a single fiber, CWDM uses the entire frequency band spanning the second and third transmission windows (1310/1550 nm respectively) including the critical frequencies where OH scattering may occur.OH-free silica fibers are recommended if the wavelengths between second and third transmission windows is to be used.Avoiding this region, the channels 47, 49, 51, 53, 55, 57, 59, 61 remain and these are the most commonly used.