Distributed Media Systems
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Nowadays, several applications require the transmission of multimedia content to clients, possibly sparse all over the world. While multimedia data delivery presents very stringent requirements in terms of bandwidth, delay and jitter, today’s networks often fail to provide the necessary “quality of service”. Congestion and network failures could cause severe degradation in the quality perceived by the final users. In particular, the delivery of video content over an error-prone network poses some important challenges due to the predictive structure of compressed signal. In this case, an error affecting one image usually is usually propagated over several consequent pictures due to temporal prediction. The activity of this group is focused on the development of novel techniques for the transmission of compressed video to a large number of users either over the Internet or a wireless local area network.
Group communication from one source to many destinations is often involved in the transmission of video over the Internet. In this scenario, the classical client-server architecture fails to scale with the number of clients attached to the system, mostly because of the fixed amount of outgoing bandwidth supported by the server. In this case, a peer-to-peer (P2P) network can be exploited to increase the performance. In fact, P2P networks provide a “zero-cost” and extremely flexible infrastructure for the delivery of multimedia content, since the amount of bandwidth available to each client allows the relay of the data to other peers in a distributed fashion. Obviously, the dynamics of P2P networks, join and leave frequency and the heterogeneity of the connections technologies still require a lot of research to develop stable solutions. Improvements may be introduced in the communication protocol, designing algorithms that ensure the connectivity without an excessive increase in terms of control overhead.
Another interesting scenario is the transmission over a wireless network. In this case, the end-to-end delay can be low enough to allow to exploit network feedback. Information from lower levels in the network protocol stack can be used to dynamically adapt working parameters for both the source and the network coder. For instance, a network-aware rate control can be used to react to network congestion. These techniques are referred as “Cross-Laye” design, since they exploit the exchange of information among different layers in the network protocol stack. Finally, some novel solutions can also be introduced in the source coder to provide more “robust” representations of the signal. Techniques such as Forward Error Correction or Multiple Description Coding increase the resilience of the signal, providing a graceful degradation of the perceived video quality with worsening channel conditions.