Optical signal fading due to fiber polarization mode dispersion (PMD) has become a primary limiting factor in long haul optical communication links of high data rate of 40 Gb/s and above. Due to the random nature of the fiber PMD, this degradation can not be fully compensated. In this paper, we develop an analytic and intuitive pulse degradation formulism taking into account all major fiber transmission impairments including chromatic dispersion, nonlinearity, and PMD and the interaction among them. Benefited from its rigorous but intuitive nature, this formulism provides great insights in understanding the system performance impairments, for example, due to fiber PMD effects. In particular, we discuss the impact of fiber PMD for low and high data rates, where the mechanisms of performance degradation can be seen as characteristically different. The effectiveness of various PMD mitigation schemes in these situations is also discussed.