Optimal Multicasting in Hybrid RF/FSO DTNs

TL;DR

This paper presents an optimal multicast algorithm for hybrid RF/FSO Delay Tolerant Networks, improving data delivery delay and throughput by efficiently managing directional FSO links with a greedy heuristic.

Contribution

It formulates the multicast problem as an NP-hard set cover abstraction and proposes a computationally efficient greedy heuristic for practical solutions.

Findings

The heuristic outperforms naive broadcast in delay and throughput.

It reduces computation time by 95% compared to exact algorithms.

Performance is comparable to optimal solutions in extensive evaluations.

Abstract

The multi-copy routing paradigm in Delay Tolerant Networks (DTNs) implies that increasing contact bandwidth leads to a decrease in data delivery delay and an improvement in throughput. With Hybrid Radio Frequency/Free Space Optical (RF/FSO) PHY layers, the high data rate FSO links can be used to increase the contact bandwidth. However, due to the highly directional nature of FSO links, broadcasting is difficult. A naive broadcast strategy where the beam divergence angle is increased to include many nodes in the broadcast set results in low data rate, and does not always result in the minimum achievable delay. In this work we develop an optimal multicast algorithm for hybrid RF/FSO networks. We show that the problem is an abstraction of the minimum weight set cover problem which is known to be NP-hard. A computationally cheap greedy local optimum heuristic is proposed. A comprehensive evaluation using delay, throughput and computation time as metrics is performed using various solutions. These extensive evaluations show that our solution outperforms both naive broadcast and multiple unicast, taking 95% less time as compared to the exact algorithm, while providing comparable performance.