Multicast techniques for hybrid RF/FSO DTNs

Increasing the contact bandwidth in delay tolerant networks (DTNs) via multicopy routing leads to a decrease in data delivery delay and an improvement in throughput. In DTNs, in which nodes have both radio frequency (RF) and free space optical (FSO) PHY layers, contact bandwidth can be increased by using the FSO PHY as the data channel and performing multicasting. As we show in this paper, due to the highly directional nature of FSO, a naïve broadcast strategy where the beam divergence includes all nodes in the broadcast set, it does not always result in the minimization of data delivery delay and the maximization of delivery probability. To this end, we develop multicast strategies for hybrid RF/FSO DTNs via an emulation of static conditions in mobile DTNs in which RF is primarily used for control. We show that the optimal multicast problem in static environments is an abstraction of the minimum weighted set cover problem, which is known to be NP-hard. To save on computation time, we propose a greedy local optimum heuristic. Performance of the various multicast techniques is comprehensively evaluated in a DTN simulator, using the Epidemic routing protocol. These evaluations show that our computationally cheap solution yields results identical to optimal while not compromising the performance of the DTN.