Architictures for Digital Multimedia On-Demand Services in Integrated Networks

网络的进一步发展加上存储技术的迅速发展将使建立多媒体按需服务器成为可能，这种服务器能够提供类似于大都市地区网络上的邻里录像带租赁店的服务。在这项研究中，我们研究了多媒体服务器的体系结构，它可以允许多个订阅者在集成的计算机网络上交互式地记录和播放多媒体对象。在开发这种多用户多媒体服务器时，我们提出了两个主要要求：(1)构成多媒体对象的每种媒体的播放的连续性；(2)构成多媒体对象的不同媒体的播放的相互同步。为了满足连续性要求，我们开发了允许控制算法，使用该算法，多媒体服务器可以同时满足最大数量的订阅者，而不会破坏其中任何一个的播放连续性。其中一种算法是质量比例方案，它以与其各自的播放速率平均成比例的速率检索媒体单元，但使用交错切换技术，通过该技术，检索到的连续数量的媒体块被单独微调，以同时接收和服务最优数量的订阅者。初始性能估计表明，与直接的准入控制策略（如每个磁盘头服务一个订阅者和订阅者的轮询服务）相比，质量比例策略具有更高的可伸缩性。在检索多媒体对象期间，播放该对象的不同媒体组件的媒体电话可能具有记录和播放速率不匹配。此外，新兴的集成网络，如ATM，预计会引入抖动，然而，在媒体单元从服务器到媒体电话的传输中，抖动可以被限制。为了确保多媒体对象的同步播放，我们研究了一种反馈技术，其中多媒体服务器使用由媒体电话生成并传输回服务器的称为反馈单元的轻量级消息来检测播放期间的异步。我们研究了各种重同步策略，如主动、保守和概率，并比较了它们在视频/音频播放中的性能。初始性能估计表明，激进策略只有在较低的播放速率下才能表现得最好，而保守策略只有在较高的播放速率下才能表现得最好。相比之下，概率策略有可能在所有回放速率下都表现得很好。在这个项目中开发的策略和算法构成了UCSD多媒体实验室正在开发的原型多媒体服务器的基础。

Further advances in networking coupled with rapid advances in storage technologies will make it feasible to build multimedia on-demand servers capable of providing services similar to those of a neighborhood videotape rental store on a metropolitan-area network. In this research we investigate architectures for multimedia servers that can permit multiple subscribers to interactively record and playback multimedia objects over integrated computer networks. We address two main requirements in the development of such a multi-subscriber multimedia server: (1) Continuity of playback of each media constituting a multimedia object, and (2) mutual synchrony of playback of the different media constituting a multimedia object. In order to address the continuity requirement, we develop admission control algorithms with which a multimedia server can satisfy the maximum number of subscribers simultaneously without violating continuity of playback for any of them. One such algorithm is a quality proportional scheme that retrieves media units at a rate proportional on an average to their respective playback rates, but uses a staggered toggling technique by which successive numbers of media blocks retrieved are fine tuned individually to admit and service an optimal number of subscribers simultaneously. Initial performance estimation shows that the quality proportional policy is an order of magnitude more scalable compared to straightforward admission control policies such as servicing one subscriber per disk head and round robin servicing of subscribers. During retrieval of a multimedia object, mediaphones that playback the different media components of the object may have recording and playback rate mismatches. Furthermore, emerging integrated networks such as ATM are expected to introduce jitter that can nevertheless be bounded in the transport of media units from servers to mediaphones. In order to ensure synchronous playback of multimedia objects, we study a feedback technique in which a multimedia server uses lightweight messages called feedback units generated by mediaphones and transmitted back to the server to detect asynchronies during playback. We study various resynchronization policies such as aggressive, conservative, and probabilistic and compare their performance for video/audio playback. Initial performance estimation shows that whereas aggressive policies are expected to perform best only at lower playback rates, conservative policies are expected to perform best only at higher playback rates. In contrast, probabilistic policies have the potential to perform uniformly well at all playback rates. The policies and algorithms developed in this project form the basis for a prototype multimedia server being developed at the UCSD Multimedia Laboratory.