ITR: Support for Video Traffic in Ad Hoc Networks

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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.