Video-on-demand: The Holy Grail of the cable industry, VOD is getting a boost from underutilized ("dark") fiber optic networks. Early attempts at VOD were glitchy at best, but computer technology is increasingly making the service a reality via large-capacity servers that can offer thousands of hours of programming to thousands of digital cable subscribers. Many of the fiber networks are owned by telecommunications companies that lease use to cable providers. Cox Communications Inc., Time Warner Inc., and Comcast Corporation have all bet heavily on the potential of fiber optics to deliver more to their subscribers. "80% to 90% of the fiber installed during the telecom boom is still sitting unused," reports Peter Grant in a recent analysis in <I>The Wall Street Journal</I>.