With the latest developments in video coding technology and fast deployment of end-user broadband internet connections, real-time media applications become increasingly interesting for both private users and businesses. However, the internet remains a best-effort service network unable to guarantee the stringent requirements of the media application, in terms of high, constant bandwidth, low packet loss rate and transmission delay. Therefore, efficient adaptation mechanisms must be derived in order to bridge the application requirements with the transport medium characteristics. Lately, different network architectures, e.g., peer-to-peer networks, content distribution networks, parallel wireless services, emerge as potential solutions for reducing the cost of communication or infrastructure, and possibly improve the application performance. In this thesis, we start from the path diversity characteristic of these architectures, in order to build a new framework, specific for media streaming in multipath networks. Within this framework we address important issues related to an efficient streaming process, namely path selection and rate allocation, forward error correction and packet scheduling over multiple transmission paths. First we consider a network graph between the streaming server and the client, offering multiple possible transmission paths to the media application. We are interested in finding the optimal subset of paths employed for data transmission, and the optimal rate allocation on these paths, in order to optimize a video distortion metric. Our in-depth analysis of the proposed scenario eventually leads to the derivation of three important theorems, which, in turn represent the basis for an optimal, linear time algorithm that finds the solution to our optimization problem. At the same time, we provide distributed protocols which compute the optimal solution in a distributed way, suitable for large scale network graphs, where a centralized solution is too expensive. Next, we address the problem of forward error correction for scalable media streaming over multiple network paths. We propose various algorithms for error protection in a multipath scenario, and we assess the opportunity of in-network error correction. Our analysis stresses the advantage of being flexible in the scheduling and error correction process on multiple network paths, and emphasizes the limitations of possible real systems implementations, where application choices are limited. Finally, we observe the improvements brought by in-network processing of transmitted media flows, in the case of heterogeneous networks, when link parameters vary greatly. Once the rate allocation and error correction issues are addressed, we discuss the packet scheduling problem over multiple transmission paths. We rely on a scalable bitstream packet model inspired from the media coding process, where media packets have different priorities and dependencies. Based on the concept of data pre-fetch, and on a strict time analysis of the transmission process, we propose fast algorithms for efficient packet scheduling over multiple paths. We ensure media graceful degradation at the client in adverse network conditions by careful load balancing among transmission paths, and by conservative scheduling which transparently absorb undetected network variations, or network estimation errors. The final part of this thesis presents a possible system for media streaming where our proposed mechanisms and protocols can be straightforwardly implemented. We describe a wireless setup where clients can access various applications over possibly multiple wireless services. In this setup, we solve the rate allocation problem with the final goal of maximizing the overall system performance. To this end, we propose a unifying quality metric which maps the individual performance of each application (including streaming) to a common value, later used in the optimization process. We propose a fast algorithm for computing a close to optimal solution to this problem and we show that compared to other traditional methods, we achieve a more fair performance, better adaptable to changing network environments.
EPFL2007
