Masterarbeit
Multi-description Coding for Emergency Communications
Betreuer: Thomas Staub, Prof. Dr. Torsten Braun
Centralized communication infrastructures, e.g. GSM, often get overloaded or totally break down in emergency situations. Wireless mesh networks (WMNs) may provide communication facilities even in such situations. WMNs are based on multi-hop wireless communication, i.e. the individual nodes work as router and forward data packets for others. The nodes are building a mesh and data packets are transmitted using paths that are dynamically built up by an ad hoc routing protocol. Therefore, communication is even possible if some nodes are unavailable.
Real-time communications (IP telephony or video conferencing) require short delays and moderate packet loss in the network. Therefore, the main obstacle for exploiting WMNs as a communication infrastructure for real-time communication is the varying link and route quality in WMNs due to the unreliable and erroneous wireless medium. Routes may break due to node and link failures. Moreover, links and nodes may become congested, which leads to higher delays or packet loss rates. The end user may get bad quality in the received transmission, e.g. high rate of artefacts in the transmitted video sequence, stumbles or interruptions in the audio communication. Hence, real-time communication might be affected.
One solution is multi-path routing and multi-stream coding. As the characteristics of multiple paths are usually uncorrelated, transmission over multiple paths may compensate the unpredictable effects of the wireless medium (link breaks, delays, etc.). The communication is encoded on multiple streams, which are transmitted over different paths in the network. Some redundancy is introduced at this encoding step. The quality of the audio / video transmission at the destination depends on the number of received streams and the used encoding scheme. The audio / video quality gets better the more streams are received.
Different multi-stream coding mechanisms are available. We distinguish between layered coding (LC) and multi-description coding (MDC). In LC the source stream is encoded in one base layer and multiple refinement layers. The transmitted streams are combined at the destination. The successful reception of the base layer is required for reconstruction of the stream. Each received refinement layer improves the quality of the reconstructed stream. In MDC, all streams are equivalent. Any received combination of streams may be use for reconstruction of the source stream and any further received stream improves the quality.
ATOM (Adaptive Transport over Multiple Paths) is an architecture integrating multi-path routing and multi-stream coding. It is currently being developed in the PhD thesis of Thomas Staub. This Master project should contribute to this architecture by implementing and evaluating several multi-stream coding mechanisms.
The video streaming solution with several multi-streaming coding options has to be evaluated in a real network and over a virtual mesh network provided by VirtualMesh. The direct integration of the multi-streaming coding libraries into the network simulator OmNet++ has to be discussed during the project. The project focuses on MDC and compares different MDC codecs. In addition, it compares the approaches with an existing implementation of Layered Coding. The MDC source code will be made public available (GPL v2 licence).
This Master thesis is assigned to Roger Strähl.
Required skills: C++, basic knowledge in computer networks and statistics