John shares his knowledge of fiber optics by answering your specific questions
Does CWDM technology support 6 or 18 channels? I have seen both referenced and I was wondering which one was correct.
In 2002 the ITU standardized a channel spacing grid for use with CWDM (Coarse Wavelength Division Multiplexing, ITU-T G.694.2), using the wavelengths from 1271 nm through 1611 nm with a channel spacing of 20 nm (1271, 1291, 1311, etc.)
Originally 16 channels were adopted for CWDM, and 2 channels were skipped because of high water peak attenuation in G.652 compliant singlemode fibers at those specific wavelengths.
Fibers conforming to the newer G.652.C and G.652.D standards, such as Corning SMF-28e+ and Ultra singlemode nearly eliminate water peak attenuation. These new fibers allowed CWDM systems to add the 2 wavelengths that were originally unusable, so now CWDM can operate with up to 18 channels.