Supporting Real-time Communication in Wireless Mesh Networks Thomas Staub, Torsten Braun Institute of Computer Science and Applied Mathematics, University of Bern, Neubrückstrasse 10, 3012 Bern, Switzerland Tel: +41 31 5112637/2631, Email: {staub|braun}@iam.unibe.ch Abstract: Wireless Mesh Networks are a key technology for providing wireless broadband access to communities and rural development areas, as they do not require any expensive wired backbone. They offer increased network resilience to network failures due to self-organization and redundancy in the network. But the noisy and erroneous wireless communication channel affects the quality of real-time communication. User- received transmission quality may become unacceptable due to artefacts, stumbles, interruptions etc. We propose a new architecture that exploits path diversity and multi-stream coding to cope with link quality variations and to reduce the effects of the wireless medium. Moreover, the architecture dynamically adapts to changes in the network. 1. Introduction Wireless Mesh Networks (WMNs) caught on as a technology for increased network coverage [1]. Several research networks (MIT Roofnet [2], Berlin Roofnet [3]) as well as deployments in cities as a public service [4, 5] exist. Connections inside WMNs are automatically established over wireless links, which eases the deployment and reduces the costs. The mesh nodes build up an ad-hoc network over wireless links, which is adapted by the network’s self- organisation capabilities. As all communication runs over wireless links, no wired backbone is required as for conventional access networks. These facts make WMNs easy to deploy and maintain. Although WMNs provide increased network resilience due to self-organisation and redundancy in the network, the performance of real-time application suffers from quality variations of the wireless links. The noisy and erroneous wireless communication channel degenerates the quality of the real-time communication by bit errors or link failures. The bit errors in the data transmission require either additional redundancy or retransmissions and therefore reduce available bandwidth. Interference or other environmental changes may further cause link failures. High delays or packet loss are the consequence. The user received transmission quality becomes unacceptable due to high rate of artefacts, stumbles, or even interruptions. Hence the deployment of real-time applications in WMNs is challenging. We propose to use path diversity and multi-stream coding to cope with the link quality variations in the network. The combination of both techniques may drastically reduce the effects of the wireless medium to the transmission. Multiple paths are usually not affected by the same errors, delays, jitter, and loss rates at the same time (see Figure 1 for an illustration). Their error characteristics are mostly uncorrelated. Therefore, the transmission over multiple paths (path diversity) adds redundancy and therewith lowers the influence of the unreliable wireless medium. The degree of redundancy can be optimised by using advanced multi- stream coding schemes such as layered (LC) or multi-description coding (MDC) [6, 7, 8]