Supporting MAC layer multicast in IEEE 802.11 based MANETs: issues and solutions

In IEEE 802.11 based mobile ad hoc networks (MANET) multicast packets are generally forwarded as one hop broadcast; mainly to reach all the multicast members in the neighborhood in a single transmission. Because of the broadcast property of the forwarding, packets suffer from increased instances of the hidden terminal problem. Mobility of nodes makes things more difficult, and unlike unicast transmissions where MAC can detect the movement of a nexthop by making several retries, it is not possible in case of multicast forwarding. To address these issues, we propose a multicast aware MAC protocol (MMP) for MANET. The basic objective of MMP is to provide a MAC layer support for multicast traffic. This is done by attaching an extended multicast header (EMH) by the multicast agent, which provides the address of the nexthop nodes that are supposed to receive the multicast packet. The MAC layer in MMP uses the EMH field to support an ACK based data delivery. After sending the data packet, the transmitter waits for the ACK from each of its destinations in a strictly sequential order. A retransmission of the multicast packet is performed only if the ACK from any of the nodes in EMH is missing. We compare MMP with IEEE 802.11 and results show that MMP substantially improves the performance of multicast packet delivery in MANET without creating much MAC overhead. In addition, MMP provides a better mechanism to detect the movement of its nexthop members.