Fiber nonlinearity and chromatic dispersion can severely degrade the signal quality in externally modulated long-haul AM-VSB CATV transport systems. Standard single-mode fiber chromatic dispersion will convert inherent laser phase noise fluctuations to intensity noise. This dispersion-induced intensity noise increases with frequency, and thereby, imposes a fundamental limit on CNR. Even in the absence of stimulated Brillouin scattering, self-phase-modulation and chromatic dispersion induces CSO distortion. For transmission over 70 km fiber, CNR and CSO degradation associated with fiber nonlinearity and chromatic dispersion limits laser linewidth and fiber launch power to less than 1 MHz and less than 18 dBm, respectively.