In optical transmission, the transmission ranges are divided intoITU grids by the International Telecommunication Union( ITU-T) according to ITU-T G.694.2.
These wavelength ranges are also referred to as transmission windows or telecommunication windows and are characterized by low attenuation and low dispersion.
There are six wavelength ranges between 1,260 nm and 1,675 nm. The O-band (original) is in the second optical window with wavelengths between 1,260 nm and 1,360 nm. It consists of five wavelengths spaced 20 nm apart, which are used for Coarse Wavelength Division Multiplexing( CWDM). Above the O-band is the E-band, which stands for Extended.