Supporting real-time speech on wireless ad hoc networks: inter-packet redundancy, path diversity, and multiple description coding

We consider the problem of supporting real-time traffic over packetized wireless ad hoc networks. Our specific emphasis is on speech, since this is a critical application in many scenarios such as emergency deployment of ad hoc networks. Standard retransmission-based Medium Access Control (MAC) strategies are poorly matched to speech applications, because the payload size for speech as well as for MAC-layer acknowledgements (ACKs) is small compared to the packet header, which contains a large synchronization preamble. In this paper, we show that inter-packet redundancy is significantly more efficient than traditional MAC layer retransmissions, in terms of both network capacity and end-to-end delay. The key observations regarding our design and results are as follows. Because of the small payloads, introducing redundancy across packets only increases the packet transmission time slightly, and hence has negligible impact on the packet collision rate. Thus, we obtain large gains from redundant transmission essentially "for free." Because of the large packet header, elimination of ACKs leads to substantial bandwidth savings. Overall, a combination of inter-packet redundancy (at the MAC layer), path diversity (at the network layer), and multiple description source coding (at the application layer), is shown to provide significant improvements in bandwidth efficiency and delay.