Summary & Concluding Remarks

gateway designation and routing dimensions can be complementary when aiming at improving WSNs traffic schedulability at systems design time.

As seen in the prior chapter, a gateway designation with reduced overlapping degree among network flows can considerably benefit real-time network performance. This situation has been further demonstrated by our minimal-overlap routing method which is driven by the same in-tuition. In this chapter, we have clearly shown that a reduction on path overlapping leads to a reduction in worst-case conflicts delays, and this in turn, to significant benefits in terms of traf-fic schedulability. Notably, our minimal-overlap routing achieves this with marginal impact on wort-case contention delays, mostly, due to its (weighted) shortest-path restriction.

In the future, we aim to further explore the opportunities for this new routing scheme. For example, it is a clearly open question to be verified if a joint approach with both minimal-overlap methods will lead or not to further improvement in terms traffic schedulability. Similar with the case of multiple-gateway in which flows are typically already balanced among gateways. Another interesting direction to explore is related to its (worst-case) energy-consumption efficiency, which can be driven by alike foundational basis in terms of deterministic guarantees.

Finally, showing its practical usefulness in real-world deployments is a must direction. Not only in arbitrary mesh WSNs replicating realistic situations, but especially in overwater WSNs, often characterized for long-term dynamics. In this direction, we envision an adaptive version of our minimal-overlap routing able to counteract possible changes on the network topology due to tides, by providing source routing paths of minimal-overlap at each point of the tide.

Chapter 9

Conclusions & Future Directions

9.1 Summary

This dissertation has addressed some of the most relevant challenges of wireless network design when dealing with real-time and overwater wireless networked systems. We have covered physi-cal, data-link and network layer considerations, in overall, attempting to improve the performance of such kind of systems beyond the state of the art. Specifically, we have offered contributions with impact in terms of overwater RF propagation (physical), network schedulability (data-link) and routing (networking) under the viewpoint of providing novel solutions with cross-layer inci-dence.

We started designing each (physical) link of our networked systems taking into account the peculiarities of overwater propagation. In particular, we have addressed path loss performance only since directly related to signal strength, in turn, influencing practical metrics such as range and link stability. We also gave special emphasis to the impact of tides and related features such as intertidal zones, which although barely explored, can impact significantly impact link quality.

In general, we considered scenarios of short to medium range, i.e. between few tens of me-ters to (no more than) one kilometer, when operating with antennas close and/or very to the wa-ter surface. We justified and motivated those settings with two emerging but relevant use cases:

IoT-driven environmental monitoring in water environments, and remote control of autonomous surface vehicles at near-shore scenarios. With this in mind, we have provided novel modeling and link design methods that effectively predict and/or improve path loss performance in shore-to-shore and shore-to-vessel configurations. Notably, we have demonstrated part of our findings using commercial wireless technologies, i.e. LoRa and WiFi, targeting sensor nodes in estuaries surroundings, and autonomous underwater vehicles operating at the surface, respectively.

After properly designing the network from the physical point of view, e.g. mitigating the detrimental impact of tidal fading and/or nulls, we have tackled challenges with impact on the upper layers of wireless communication. Concretely, we first introduced the idea of gateway designation in wireless mesh networks for enhanced traffic schedulability. We addressed both single and multi-gateway designation problem by offering straightforward criteria for the selection

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of the gateway nodes, based on the notion of network centrality from graph mining. We first evaluate the use of classical metrics (e.g. eigenvector, betweenness, etc.) to then follow an akin insight by proposing a dominant metric, tailored engineered to improve real-time performance in mesh wireless sensor networks.

The novel gateway designation method leverage on the minimization of path node-overlaps, known to influence worst-case end-to-end delays and thus schedulability. The single metric takes advantage of this observation by choosing as gateway the node which leads to the least overall number of overlaps when routing paths are known. The same idea also shown to be useful when generalized to multiple gateways after a spectral clustering process is completed. The gateway designation then occurs at cluster level, offering a novel interpretation of cluster centrality for the proposed metric, as well as by offering a novel and fruitful combination for spectral clustering and centrality in the domain of real-time WSNs. A similar routing method was proposed to improve traffic schedulability by design, regardless of gateway designation. This method was also inspired by the idea of minimize overlaps but using a novel greedy search heuristic to generate source routes of minimal overlaps. Notably, offering complementary benefits in terms of real-time performance when used jointly with a centrality-driven gateway designation.