Application-Layer Multicast in MANETs: To Broadcast or not to Broadcast?

Mobile ad-hoc networks allow wireless devices to freely communicate without the need of any fixed infrastructure. While many applications rely on group communication, providing a multicast service on the network layer turns out to be difficult because of diverging application requirements. Research in the past year has thus focused on application-layer multicast protocols: These can easily be deployed among group members and flexibly be customized to meet an application´s requirements. As group members handle packet duplication they are required to repeatedly access the medium for packet forwarding. Especially in areas of increased group member density this process can result in heavy performance degradation: Indeed, as in this area the wireless medium will be accessed respectively often for forwarding one single packet, the achievable multicast throughput will drop. Broadcast transmissions can improve the situation since one single medium access will forward a data packet to an arbitrary number of group members located within transmission range. With common MAC layers such as IEEE 802.11, broadcasts, however, are not covered by retransmissions and thus show to be more prone to packet errors than unicasts. Depending on the nature of emitted traffic and a multicast group´s size, we in this paper analyze in which situations broadcasting data pays for application-layer multicast protocols and in which situations it hurts.