Wavelength and frequency
λ [nm] = 299 792.458 / f [THz] — the speed of light in vacuum divided by the frequency. ITU-T grids are defined in frequency, and the wavelengths quoted for them are vacuum wavelengths.
DWDM: the ITU-T G.694.1 grid
The grid is anchored at 193.1 THz. On the 100 GHz grid neighbouring channels are 0.1 THz apart — about 0.8 nm in the C-band; the 50 GHz grid adds a channel halfway between each pair, about 0.4 nm.
The channel number used by most vendors is n = (f − 190 THz) / 0.1 THz: C21 is 192.1 THz, C31 is 193.1 THz and C34 is 193.4 THz. A 50 GHz channel between two of them is written C21.5 or H21, depending on the vendor — the frequency is the name that cannot be misread.
CWDM: the ITU-T G.694.2 grid
18 wavelengths 20 nm apart, from 1271 to 1611 nm. A CWDM laser may sit up to ±6.5 nm from its nominal wavelength, which the wide filters of a CWDM mux accept. Older documents and labels name the same channels 1270–1610 nm.
The water peak — absorption by OH⁻ ions around 1383 nm — decides three channels: 1371, 1391 and 1411 nm. Fibre made before low-water-peak production (G.652.A/B) loses up to 1–2 dB/km there instead of about 0.35 dB/km, so on such plant these channels are usually skipped; low-water-peak fibre (G.652.C/D) opens all 18. The eight channels from 1471 to 1611 nm are the most common: they work on any standard single-mode fibre. When the fibre type is unknown, measure its spectral attenuation before planning the E-band.
More in the documentation: DWDM and the ITU grid, CWDM channel plan, CWDM mux and budget, Wavelength bands.