流媒体分发与传输关键技术研究

随着Internet技术的飞速发展，流媒体分发技术取得了长足的进步，同时，日益增加的用户、视频数据和流媒体交互式需求，也给大规模流媒体分发服务带来了新的挑战。另外一方面，随着宽带无线接入技术发展的日新月异，如何有效利用多种接入方式为用户提供更高质量的流媒体服务是当前通信技术领域迫切需要解决的问题。本文对基于应用层支持交互式的P2P流媒体分发技术及基于传输层支持Multihoming的SCTP流媒体传输技术进行了相关研究，主要贡献和创新点如下： 第一、提出了一种支持随机访问的协作式P2P流媒体分发方案。该方案采用平衡二叉树将媒体对象进行分布式分段预取缓存，用于媒体流快速定位，利用缓存重叠机制构建非结构化网络，用于节点间常规媒体流分发。分别设计了媒体预取算法及邻居节点搜索算法，给出了节点失效恢复策略及支持随机访问的协作过程。仿真试验结果表明，在随机访问及网络波动过程中，该方案能够提供高平滑质量的P2P媒体流服务。 第二、提出了一种基于数据驱动及分布式存储的P2P VoD解决方案。该方案融合了平衡多叉树与基于数据驱动的随机图网络两种拓扑。利用gossip算法构建和维护随机图网络中的邻居节点关系，采用数据驱动机制进行邻居节点间媒体流的分发，进一步降低了源服务器的负载。设计了一种基于平衡多叉树的分布式预取缓存算法，提高了随机访问的搜索效率。仿真试验结果和分析表明，两种网络拓扑的协作机制有效地解决了P2P VoD中视频传输及交互式操作问题。 第三、设计和实现了一种基于Trace驱动及SCTP的MPEG-4视频传输评估平台Evalvid-SCTP。Evalvid-SCTP提供了在仿真环境下SCTP流媒体实时传输及视频质量评估的解决方案。在Multihoming环境下，Evalvid-SCTP可以有效地分析SCTP在不同网络条件和负载下的流媒体传输行为特征和传输质量。 第四、提出了SCTP流媒体单路径传输最佳协议参数配置策略及多路径并行传输策略。评估了在单路径传输机制下快速重传策略、超时重传策略、路径故障检测门限值设置，三者配合时在多种网络条件下的流媒体传输性能，并综合以上各种发现提出了在不同的路径条件下应该采取的重传路径选择策略以及路径故障检测门限值设置方案。评估了在多路径并行传输下融合SCTP部分可靠特性的流媒体传输性能，提出了针对流媒体传输，多路径并行传输应采取的策略。

Sustainable development of coastal and marine areas in small island developing states: A basis for integrated coastal management

Agenda 21, the 40-chapter action plan, agreed to by all nations participating in the 1992 Earth Summit represents an ambitious effort to provide policy guidance across the entire spectrum of environment, development, and social issues confronting mankind. In the area of oceans and coasts (Chapter 17 of Agenda 21), the Earth Summit underscored that the management of oceans and coasts should be ‘integrated in content and anticipatory in ambit.’ To assist those responsible for implementing the Earth Summit guidelines on ocean and coastal management, this article first reviews the fundamental shift in paradigm reflected in the Earth Summit agreements as well as the specific recommendations contained in Chapter 17. Next, the article examines the central concept of ‘integrated management,’ noting both its importance and its limits. A general or ‘synthesis’ model of ‘integrated coastal management’ is then presented, addressing such questions as management goals, what is being managed, where, how, and by whom. In a concluding section, methods are proposed whereby the general or ‘synthesis model’ can be tailored to diverse national contexts, involving varying physical, socio-economic, and political conditions.

The effect of land management on carbon and nitrogen status in plants and soils of alpine meadows on the Tibetan plateau

Large-scale grassland rehabilitation has been carried out on the severely degraded lands of the Tibetan plateau. The grasslands created provide a useful model for evaluating the recovery of ecosystem properties. The purposes of this research were: (1) to examine the relative influence of various rehabilitation practices on carbon and nitrogen in plants and soils in early secondary succession; and (2) to evaluate the degree to which severely degraded grassland altered plant and soil properties relative to the non-disturbed native community. The results showed: (1) The aboveground tissue C and N content in the control were 105-97 g m(-2) and 3.356gm(-2), respectively. The aboveground tissue C content in the mixed seed treatment, the single seed treatment, the natural recovery treatment and the severely degraded treatment was 137 per cent, 98 per cent, 49 per cent and 38 per cent, respectively, of that in the control. The corresponding aboveground tissue N content was 109 per cent, 84 per cent, 60 per cent and 47 per cent, respectively, of that in the control. (2) Root C and N content in 0-20 cm depths of the control had an 2 2 average 1606 gm(-2) and 30-36 gm(-2) respectively. Root C and N content in the rehabilitation treatments were in the range of 26-36 per cent and 35-53 per cent, while those in the severely degraded treatment were only 17 per cent and 26 per cent of that in the control. (3) In the control the average soil C and N content at 0-20 cm was 11307 gm(-2) and 846 gm(-2), respectively. Soil C content in the uppermost 20 cm in the seeded treatments, the natural recovery treatment and the severely degraded treatment was 67 per cent, 73 per cent and 57 per cent, respectively, while soil N content in the uppermost 20cm was 72 per cent, 82 per cent and 79 per cent, respectively, of that in the control. The severely degraded land was a major C source. Restoring the severely degraded lands to perennial vegetation was an alternative approach to sequestering C in former degraded systems. N was a limiting factor in seeding grassland. It is necessary for sustainable utilization of seeding grassland to supply extra N fertilizer to the soil or to add legume species into the seed mix. Copyright (c) 2005 John Wiley & Sons, Ltd.