Optical transmissions are not transmitted continuously over the whole electromagnetic spectrum. The spectrum for transmission in single-mode optical fibers has been broken into the following wavelength ranges, or bands:

• O Band (Original) from 1260 to 1360 nm
• E Band (Extended) from 1360 to 1460 nm
• S Band (Short wavelength) from 1460 to 1530 nm
• C Band (Conventional) from 1530 to 1565 nm
• L Band (Long wavelength) from 1565 to 1625 nm
• U Band (Ultra-long wavelength) from 1625 to 1675 nm

The wavelengths transmitted in multimode fibers are typically around 850 and 1310 nm, known originally as first window and second window.

O-band covers the second transmission window, whereas S, C, and L-bands cover the third transmission window. Presently, the vast majority of DWDM transmissions are done in the C-band.