【作者】 张扬；

【导师】 孙棣华；

【作者基本信息】 重庆大学 ， 控制理论与控制工程， 2002， 硕士

【摘要】 网络及计算机技术的进一步普及，促使对网络上提供高质量的视频/音频等多媒体服务的需求越来越大。尽管目前网络带宽呈现逐步增加的趋势，但由于多媒体特别是视频数据在传输时，所需带宽往往大于网络所能提供的容量，因而网络上常常出现拥塞现象，导致视频数据在传输过程中大量的丢失，引起接收到的视频图像质量下降。如何在带宽有限的条件下尽可能提高多媒体数据特别是视频图像的传输质量，是当前网络多媒体应用领域的研究热点。同时也是多媒体网络应用软件实用化所必需解决的问题。目前，对基于TCP协议点对点传输模式中的拥塞控制研究较多，而组播中的拥塞控制的研究还处于实验研究阶段，大多数基于组播的应用还没有一种合适的拥塞控制机制或完全没有采用拥塞控制。本文以多点对多点传输方式的会议系统为应用背景，重点研究了组播环境中的自适应网络传输控制技术。借鉴传统点对点的网络拥塞控制原理，运用系统控制的思想对网络传输控制进行研究，给出了用于组播传输的网络反馈控制信息模型。在这个模型的基础上提出了一种组播环境下基于分层的自适应网络拥塞控制策略及算法。利用视频编码的特点，将视频数据编码帧按不同的压缩比划分为多个层次，并制定出相应的占用不同带宽的多层视频编码传输方案。接收方则根据反馈的网络环境状态值，自动地对网络上出现的拥塞情况作出适当的反应，缓解网络拥塞，改善网络环境，提高传输质量。模拟实验结果表明，该算法对网络带宽波动具有良好的适应能力。为确保规范化应用所提出的自适应拥塞控制策略机制，针对该策略算法，给出了用于网络自适应传输的应用层协议。将所提出的自适应拥塞控制策略算法应用于视频会议系统，设计了相应的系统体系结构和软件框架模型。提出了该策略算法的详细实施方案，并完成了算法在发送端和接收端的具体实现。最后，对全文的工作进行了总结，并指出今后有待进一步完成的工作。更多还原

【Abstract】 With the widespread use of the computer network, the demands for high quality of online multimedia services are ever increasing. Although the bandwidth is trending to gradual increase for the improvement of technology, it is not enough to provide satisfied quality of services at the present time yet.Usually, bandwidth needed by transmission of multimedia data, especially video, is exceeding the throughput of network providing. Video frame will be dropped when the network experiences congestion. The performance of data transmission may therefore be affected seriously. How to improve the quality of service for multimedia data on condition of bandwidth limit is the hotspot of research in multimedia application field recently.There were many researches focusing on point-to-point congestion control based on TCP. Studies of congestion control in multicast are still in experiment phase. Hardly is there any congestion avoidance control used in most multicast applications.In this paper, firstly, existing network congestion control mechanisms are studied and some shortages of current control mechanism in multicast environment are pointed out. Then a network transfer feedback-control model is established. Based on this model, an application level layer-based congestion avoiding mechanism using by multicast is given. It takes advantage of layered video coding, sets down a set of video transfer scheme with different bandwidth. Close-loop feedback control allows receivers to select one of scheme automatically according to the network status carried back by the feedback information. Also, a protocol is presented in order to make the mechanism standardized.In this research, simulation software is taken to evaluate the algorithm’s performance. Results demonstrate that the control mechanism can tolerate occasional bandwidth changes in the presence of transient network congestion.Then, this adaptive control mechanism is implemented in videoconference system. Corresponding system architecture and software framework are designed at the same time. The process of implement the given mechanism is described and algorithm both in sender and receiver are carried outFinally, the conclusions of the mechanism and the future trends of study are given.更多还原