ITR: Support for Video Traffic in Ad Hoc Networks

自组织网络是一种新的网络模式，在这种模式下，网络节点按需创建网络。自组织网络方法具有许多特征：高度动态的网络体系结构(节点经常在没有警告的情况下加入和离开网络)、完全分布式的体系结构以及多跳路由。由于自组织网络体系结构的高重新配置率，许多(如果不是大多数)传统路由协议在这种类型的环境中不能很好地执行。因此，人们提出了许多专门适用于自组织网络的新型路由协议。一个新的互联网工程任务组工作组--MANET工作组已经成立，以解决自组织网络中的路由问题。在自组织网络中的路由的上下文中没有充分覆盖的一个主题是用于多媒体应用的一般的Qos路由的问题，特别是用于实时业务的路由的问题。更具体地说，自组织网络可能在网络拓扑中经历频繁且不可预测的变化，这导致网络路径的生命周期相对较短。因此，路径经常变得无效，而且，更严重的问题是，可能几乎没有路径出现故障的警告。尽管对于非实时应用来说，这可能不是一个实质性的问题，但这种频繁的路径无效通常会导致实时通信的严重降级。因此，需要一种机制来补偿自组织网络的这种行为。以前在技术文献中已经提出了许多方法。例如，维护次要路由，以便当主路由发生故障时，系统可以在检测到故障时立即回退到次要路由，这已被证明是一种很好的策略。然而，由于网络中的路径状态通常高度相关，所以在许多情况下，次要路由机制是不够的。因此，主路径的故障通常意味着辅助路径可能也不可用。此外，转换时间可能持续太长时间，从而导致在此期间信号质量的可察觉降级。自组织网络的一个特征是在源和目的地之间有许多路径。因此，利用这些大量路径的机制必然比上述两路径方法执行得更好(即，在支持实时业务的QOS方面)。此外，与在任何时间选择用于特定连接的单个路径不同，更好的方案将总是在多个路径之间分发信息，可能在各个路径上的信息之间具有某种相关性，以便防止路径的某个子集发生故障。因此，所提出的机制包括四个步骤：i)发现源节点和目的节点之间的多条路径，并评估路径可用性中的相关性；ii)基于一些好的度量(例如路径的预期可用性、路径的容量、路径的延迟和抖动等)和允许评估网络路径的这些度量的方案，将路径的子集选择到活动路径集合(APS)中；iii)在路径之间编码和传播信息的方法(包括将路径与业务类型的特定要求相匹配)；IV)一种方案，用于监视AP路径、估计其Qos参数、并基于路径的状态及其相关性来更新AP。上述方法是通用的，可以应用于各种实时业务类型。然而，为了使研究更具现实性，研究人员选择了视频通信作为实时测试应用。因此，研究人员打算在视频通信的上下文中提出一组特定的算法/协议来解决上述四个步骤。例如，他们将确定哪些参数与压缩视频流量在不稳定路径上的传输相关，并提出将视频源编码为可在多条路径上传播的多个相关描述的方案。研究人员打算将上述用于视频业务的多路径传输方案集成到对自组织网络环境的全面模拟中，其中将包括无线电传播模型、节点移动性模型、MAC协议和路由算法(以发现网络路径)。研究人员将从模拟中收集性能指标，使他们能够根据所用模型的参数确定应用程序级别的视频质量。研究人员希望能够回答各种问题，从在自组织网络环境中支持实时流量的可行性这一非常基本的问题，到哪种类型的路由协议最适合实时流量。这项研究将通过分析和模拟工具进行。特别是，对于模拟，研究人员将使用高级模型来预测无线电传播环境、用户移动性模型和流量生成模型。

Ad-hoc networks are a new networking paradigm, in which the network nodes create a network "on demand." There is a number of characteristic attributes associated with the ad-hoc networking approach: highly dynamic network architecture (nodes join and leave the network often and without warning), totally distributed architecture, and multi-hop routing. Due to the high reconfiguration rate of the ad-hoc network architectures, many, if not most of the conventional routing protocols do not perform well in this type of environment. Consequently, a number of novel routing protocols, specifically suited for the ad-hoc networks, were proposed. A new Internet Engineering Task Force working group, the MANET Working Group, has been established to address the issues of routing in ad-hoc networks. One topic that has not been adequately covered in the context of routing in ad-hoc networks is the issue of QoS-routing for multimedia applications, in general, and the issue of routing for real-time traffic, in particular. More specifically, an ad-hoc network may undergo frequent and unpredictable changes in the network topology, which results in relatively short lifetime of the network paths. Thus, paths become frequently invalid, and, what is more of a problem, there may be little warning of a path going down. Although this might not be a substantial problem for non real-time applications, such frequent path invalidation will most often lead to severe degradation of real-time communication. Thus, a mechanism is needed that will compensate for this behavior of ad-hoc networks. A number of approaches have been previously proposed in the technical literature. For instance, maintaining a secondary route, so that when the primary route fails, the system can fall back onto the secondary route as soon as the failure is detected, has proven a good strategy. However, the secondary route mechanism is insufficient in many cases, as the state of paths in the network is usually highly correlated. Thus, failure of the primary path usually means that the secondary path may not be available as well. Also, the change-over time may last too long , so as to cause a perceptible degradation of the signal quality during this period. One characteristic of the ad-hoc networks is that there are many paths between a source and a destination. Thus, a mechanism that takes advantage of these multitude of paths is bound to perform better (i.e., in supporting QoS for real-time traffic) than the above two-path approach. Moreover, rather than selecting a single path at any time to use for a specific connection, a better scheme would be to always distribute the information among multiple paths, possibly with some correlation between the information on the various paths, so as to protect against failure of some subset of the paths. The proposed mechanism thus consists of four steps: i) discovery of multiple paths between the source and the destination nodes and evaluation of the correlation in the paths' availability; ii) selection of a subset of the paths into an Active Path Set (APS), based on some "goodness'' measures (such as the expected availability of the path, the capacity of the path, the delay and jitter of the path, etc), and a scheme that allows to evaluate these measures for the network paths; iii) a method of coding and spreading the information among the paths(including matching the paths with the specific requirements of a traffic type); iv) a scheme to monitor the APS paths, estimate their QoS parameters, and update the APS based on the state of the paths and their correlation. The above approach is general and can be applied to a variety of real-time traffic types. However, to make the study more realistic, the researchers chose video communication as the real-time test application. Thus, the researchers intend to propose a specific set of algorithms/protocols that addresses the four steps as outlined above, in the context of video communication. For instance, they will determine what are the parameters relevant to transmission of compressed video traffic over unstable paths and propose schemes to code video source into multiple correlated descriptions that can be spread over multiple paths. The researchers intend to integrate the above multi-path transport scheme for video traffic into a comprehensive simulation of the ad-hoc networking environment, that will include a radio propagation model, nodal mobility model, MAC protocol, and a routing algorithm (to discover the network paths). The researchers will gather performance measures from the simulation that will allow them to determine the quality of video at the application level depending on the parameters of the models used. The researchers expect to be able to answer questions, ranging from the very basic issue of viability of supporting real-time traffic in an ad-hoc networking environment to what type of routing protocol is most suitable for real-time traffic. The research will be performed through both analytical and simulation tools. In particular, for the simulation, the researchers will use advanced models for prediction of the radio propagation environment, the user mobility model, and the traffic generation model.