By John Trail and Dawn Emms, Harmonic Inc.
In the future an increasing amount of video content will be provided to the cable subscriber as an on-demand service rather than a broadcast service. This advanced service can be expected to become a very significant portion of the revenue stream for cable operators. Historically the high costs of video-on-demand (VOD) have discouraged significant deployment. The falling costs of computing power and digital hardware, combined with innovative hybrid fiber-coax (HFC) transport techniques such as dense wavelength division multiplexing (DWDM), are now making the business case much more appealing. In this paper we will review how one can centralize the video server hardware in the headend and use DWDM to transport the digital video streams in on channel QAM256 format out to the hubs. Once at the hubs, the VOD channels can be easily combined with the broadcast content and then routed out to the subscriber. The paper will review the technical issues, performance, and approximate costs of a 16 and 32 wavelength DWDM system for VOD transport. We will also review the latest technical developments in the area of digital stream manipulation required for successful, low-cost VOD deployment: PID filtering and program assignment, DVB/Simulcrypt scrambling, conditional access provisioning and QAM modulation. Particular emphasis will be placed on how this is implemented in an open-standard configuration, taking into account OpenCAS and other standards-based initiatives. Several such systems are in deployment in trials worldwide. In the last section of the paper we will review the deployment currently underway by Telewest in Britain.