A laser light shone down the fibre carries data along the fibre's length. The goal is for the data, on reaching the far end, to emerge identical to how it began. The signal is absorbed by and scattered by the fibre due to the low-level impurities and imperfections remaining from the fibre manufacturing process. Therefore, in a long-distance cable, regenerators or optical amplifiers are required at periodic intervals to restore the signal. The speed that the light travels down the fibre depends on the wavelength.
Dispersion is a physical property of the fibre cable.
The fibre itself can be designed to exhibit zero dispersion at almost any wavelength, for example. Much of the fibre already installed in Europe and North America has zero dispersion at a wavelength of 1.3 microns, but the amplifiers used tend only to work for signal wavelengths of 1.5 microns and above, so the fibre exhibits an unavoidably large dispersion effect. After launch into and propagation along the fibre, the signal disperses then the DCMs force it to return to its original state, by compensating for the changes that have occurred. The amount and incidence of PMD depends on many external influences, such as the quality of the fibre and the way the fibre is installed. PMD is unpredictable and its effects are not uniform, so compensating for it is very difficult.