One of the promises of coherent systems is that they can handle very high levels of polarization mode dispersion (PMD), but is that really the case? Each wavelength has a different instantaneous PMD (referred to as differential group delay or DGD) that is uncorrelated with the next wavelength and changes over time. The average of all these DGDs at individual wavelengths is what we call PMD. Coherent systems feature digital signal processing (DSP) technology that tracks and compensates in real-time for the variations in DGD on a per-wavelength basis. However, DSPs can compensate for a limited range of DGD, and they have a limited reaction time to changes in DGD. Since DGD changes over time, very high DGD values can occur, and may therefore exceed the compensation range of the DSP. Similarly, DGD sometimes changes very rapidly, and even faster than the DSP is able to track, leading to loss of PMD compensation. It can also be sudden changes in state of polarization that lead to a complete loss of signal. When that happens, the DSP tries to resynchronize and rediscover the level of PMD that needs to be compensated for. While it usually does this very quickly, many bits will have gone by uncompensated for, potentially generating a lot of errors in the process