The Split and Merge Protocol for Interactive Video-on-Demand

V ideo-on-demand (VoD) combines the service quality of cable TV with the interaction capabilities of a VCR (see Figure 1). Its birth followed technical advances in information retrieval, consumer electronics, networking, and digital signal processing, and the convergence of communications, computers, and entertainment. With VoD services, customers may select programs from massive, remote video archives, view them when they wish, and interact with the programs using VCR-like functions, such as fastforward and rewind. A true VoD system satisfies three important requirements: viewing any video, at any time, using any VCR-like user interactions. A system that does not satisfy all these requirements is called a near-VoD system. To compete with existing video rental services requires true VoD, which most existing VoD field trials1 do indeed provide. One solution for true VoD assigns a dedicated video stream to each customer. (A video stream typically consists of video segments retrieved by a read-write head from the video server, transported over a high-speed network, and delivered to set-top boxes at the customer’s premises.) This gets expensive, since each stream requires high-speed data transport. For example, existing VoD field trials typically deliver MPEG-1 or MPEG-2 compressed video, requiring 1.5 Mbps and 3 Mbps, respectively. To be commercially viable, VoD service pricing must compete with existing video rental pricing. Batching2 can reduce the per-user video delivery cost. (Batching here does not necessarily mean waiting a certain amount of time before serving user requests—it simply means resource sharing.) In a batching approach the same video stream is multicast to, and shared by, multiple users. This sharing, however, means that one user’s interaction affects all other users on the same stream. For example, to satisfy one user’s desire to rewind the video, all other users sharing the same stream would have to rewind. This won’t work. We want to make the sharing transparent to users while allowing true user interactivity. Existing batching solutions fail to achieve this goal. For example, staggered VoD3 broadcasts multiple copies (streams) of the same video program at staggered times, with one stream serving multiple users. Jumping to a different stream is used to perform a user interaction. However, not all user The Split and Merge Protocol for Interactive Video-onDemand