Attenuation in optical fiber refers to the reduction in power of the light signal as it is transmitted through the fiber. Attenuation is caused by passive media components such as cables, cable splices, and connectors. The total attenuation is the sum of all losses, and it is usually expressed in decibels per kilometer (dB/km) . The expression used to calculate the attenuation coefficient α is where P(z) is the optical power at a position z from the origin, and P(0) is the power at the origin. The value of the attenuation factor depends greatly on the fiber material and the manufacturing tolerances. The optical losses of a fiber are wavelength-dependent, and Rayleigh scattering accounts for about 96% of attenuation in optical fiber. Any wavelength that is below 800nm is unusable for optical communication because attenuation due to Rayleigh scattering is too high. An efficient optical data link must have enough light available to overcome attenuation.