While acoustic communications have been considered the prominent technologyto communicate under water for several years, other technologies are beingdeveloped based, e.g., on optical and radio-frequency electro-magnetic waves.Each technology has its own advantages and drawbacks: for example, acousticsignals achieve long communication ranges at order-of-kbit/s bit rate, whereasoptical signals offer order-of-Mbit/s transmission rates but only over shorttransmitter--receiver distances. Such a technological diversity can beleveraged by multi-modal systems, which integrate different technologies andprovide intelligence to decide which one should be used at any given time. Inthis paper, we address a fundamental part of this intelligence by proposing anovel routing protocol for networks of multi-modal nodes. The protocol makesdistributed decisions about the flow in each link and over each technology atany given time, in order to advance a packet towards its destination. Ourrouting protocol prevents bottlenecks and allocates resources fairly todifferent nodes. We analyze the performance of our protocol via simulations andin a field experiment. The results show that our protocol successfullyleverages all technologies to deliver data, even in the presence of imperfecttopology information. To permit the reproduction of our results, we share oursimulation code.
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