MPEG, in addition to being an international standard, is currently the most popular coding scheme for stored video. For several applications that require stored video, such as video-on-demand, it is advantageous to group MPEG frames into segments. Retrieving segments instead of individual frames can result in a more efficient use of disk retrieval resources and consequently, in the support of a larger number of users and in a more cost-effective system. Depending on the value of parameters such as desired user quality of service, size and cost of required buffers, number of disks to stripe a segment across, etc., a certain segment length (defined as the number of frames per segment) may be preferred over others. In this paper, we propose a generalized grouping scheme that may be used to achieve any desired segment length. Properties of such a grouping scheme and formulae that may be used to achieve such a grouping have been provided. Since a segment is an atomic unit of video retrieval, retrieval of some segments is skipped during fast playback. Some frames belonging to retrieved segments may be discarded due to the unavailability of other frames necessary for their decoding. Tradeoffs between different kinds of fast playback (i.e., the choice of skipped segments) and the number of discarded frames is discussed.

Video-on-demand (VOD) servers need to be efficiently designed in order to support a large number of users viewing the same or different videos at different rates. While considering a disk-array based VOD server, use of a shared buffer at the server end may be more economical than the sole use of dedicated buffers at each user's end. In this paper we propose a simple buffer sharing architecture that may be used when disk-array based video servers are used. Our aim is to support the maximum number of users for a given number of video server disks while employing a simple scheme requiring less buffer space. The number of video segment retrievals that can occur within a certain time (the service round) is maximum when the scan disk scheduling algorithm is used. Consequently, we shall assume use of the scan algorithm for disk retrieval. The VOD server has a buffer manager that directs retrieved segments to appropriate buffer locations depending on their release and deadlines. The release and deadlines of segments are such that buffer requirement at the user's set-top box is minimized to two video segments while avoiding video starvation and buffer overflow at the user's end. We propose a novel scheme for the operation of the shared buffer that aims at increasing buffer utilization and decreasing cell loss due to buffer overflow. An ATM based broadband network is assumed and all segments are stored in buffers as fixed length ATM cells.