On demand multicast routing with unidirectional links

In wireless ad-hoc networks, unidirectional links occur for several reasons: nonuniform transmit power, nonuniform background noise, and external interference. Several researchers have addressed unidirectional links and the associated unidirectional routing problem. The main focus has been so far on "unicast" routing; the consensus is that unidirectional links should be detected and avoided. In this paper, we consider the multicast case and derive a different conclusion: namely, it pays to exploit unidirectional links rather then avoid them. To prove the point, we select a popular ad hoc multicast protocol, on-demand multicast routing protocol (ODMRP) and introduce a slightly modified version, ODMRP-ASYM, that can handle unidirectional links. Specifically, ODMRP-ASYM reroutes the join reply packet when a unidirectional link is detected on the join query path. The option is invoked only when a unidirectional link is detected. The main advantages are: control overhead comparable with ODMRP even in highly asymmetric topologies; virtually no performance degradation in the presence of unidirectional links (while ODMRP typically suffers up to 15% drop in delivery performance); 2-connectivity maintenance even if no bidirectional path exists between sender and receiver (in this case, unidirectional link avoidance strategies fail). Extensive simulation experiments demonstrate ODMRP-ASYM robustness to unidirectional links and superiority over conventional ODMRP.