The PMD coefficient, often called the PMD delay coefficient expresses a delay that accumulates as the square root of distance. But these calculated values may not represent accurately the DGD of the installed fiber. Due to the nature of random mode coupling, PMD depends on wavelength and environmental effects. Although calculations can give you an idea of DGD, actual measurements on a system provide the most reliable values, but even those measured values may change slightly over time and over a range of wavelengths. In effect, DGD represents a statistical value calculated from measurements.
Ambient conditions change the stress on a cable, and the stress alters the cable's birefringence.
For FO cable mounted on telephone poles, stresses from wind and temperature alter the optical path, resulting in a DGD value that changes with time. Plotted data lets you determine the maximum PMD a fiber link can tolerate at a given distance for a given data rate. As data rates increase above 10 Gbps, DGD can significantly alter digital signals. At 10 Gbps, a signal has a 25-ps bit time, so for effective communications at this rate, DGD cannot exceed 2.5 ps. The graph in Figure 2 shows the relationship between maximum transmission distance, PMD coefficient, and transmission rate.