By understanding frequency and wavelength, we can understand where on the electromagnetic spectrum optical communications occur. The spectrum represents the range of electromagnetic phenomena. At one end sit gamma rays (around 1 ZHz), the kind of radiation released in a nuclear blast, and at the other end are radio transmissions (1 KHz to 1 GHz). Optical transmission happens just above visible light and below the ultraviolet band. There's some overlap in these regions, as the divisions in the spectrum are entirely manmade (see Figure 3.8).
Figure 3.8 The electromagnetic spectrum.
To promote interoperability between optical equipment, the International Telecommunications Union (ITU), the standards body responsible for global telecommunications standards, defined six bands for long-distance optical communications. A seventh band is used for short-haul transmissions (Figure 3.9).
Figure 3.9 The transmission bands.