By some accounts, only two-thirds of fiber currently in the ground will support 40-Gbit/sec transmission and even less will accommodate higher data rates, thanks to optical impairments caused by dispersion. Chromatic dispersion can be compensated using electrical or traditional optical techniques, including dispersion-compensating fiber (DCF) and tunable optical dispersion compensators (TODCs). But the effects of polarization-mode dispersion (PMD) are not nearly so easy to mitigate—either optically or electrically.