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Abstract-Distributed time synchronization is an important part of a sensor network where sensing and actuation must be coordinated across multiple nodes. Several time synchronization protocol that maximize accuracy and energy conservation have been developed, including FTSP, TPSN, and RBS. All of these assume nearly instantaneous wireless communication between sensor nodes; each of them work well in today's RF-based sensor networks. We are just beginning to explore underwater sensor networks where communication is primarily via acoustic telemetry. With acoustic communication, where the propagation speed is nearly five orders of magnitude slower than RF, assumptions about rapid communication are incorrect and new approaches to time synchronization are required. We present Time Synchronization for High Latency (TSHL), designed assuming such high latency propagation. We show through analysis and simulation that it achieves precise time synchronization with minimal energy cost. Although at very short distances existing protocols are adequate, TSHL shows twice the accuracy at 500m, demonstrating the need to model both clock skew and propagation latency.

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The goal of this paper is to gain deep understanding of how location-dependent propagation latency affects medium access control (MAC) by using ALOHA as a case study. MAC protocols in underwater acoustic networks suffer from latency that is five orders-of-magnitude larger than that in radio networks. Existing work on analyzing MAC throughput in RF networks, where the propagation latency is negligible, generally makes assumptions that render propagation latency irrelevant. As a result, only transmit time is considered as being uncertain in contention-based protocols. We introduce the spatial dimension of uncertainty that is inherent to varying locations of transmitters, resulting in unequal propagation latency to a receiver, where collision occurs. We show through simulation that the benefit of synchronization in slotted ALOHA is lost due to such latency. We propose a modification that adds guard bands to transmission slots to handle spatial uncertainty. We then perform simulation and first order analysis on this modified MAC to find its optimal operating parameters.

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Abstract-This paper introduces T-Lohi, a new class of distributed and energy-efficient media-access protocols (MAC) for underwater acoustic sensor networks (UWSN). MAC design for UWSN faces significant challenges. For example, acoustic communication suffers from latencies five orders-of-magnitude larger than radio communication, so a naive CSMA MAC would require very long listen time resulting in low throughput and poor energy efficiency. In this paper, we first identify unique characteristics in underwater networking that may affect all MACs, such as space-time uncertainty and deafness conditions. We then develop T-Lohi employing a novel tone-based contention resolution mechanism that exploits space-time uncertainty and high latency to detect collisions and count contenders, achieving good throughput across all offered loads. T-Lohi exploits a low-power wake-up receiver to significantly reduce energy consumption. We evaluate design choices and protocol performance through extensive simulation. Finally, we compare T-Lohi against a few canonical MAC protocols. The results show that the energy cost of packet transmission is within 3-9% of optimal, and that Lohi achieves good channel utilization, within 30% utilization of the theoretical maximum. We also show that Lohi is stable and fair under both low and very high offered loads. Finally, we compare Lohi with other alternatives, including TDMA, CSMA, and ALOHA. Except for TDMA under heavy load, Lohi provides the best utilization in all cases, and it is always the most energy efficient.

Advances in micro-electronics and miniaturized mechanical systems are redefining the scope and extent of the energy constraints found in battery-operated wireless sensor networks (WSNs). On one hand, ambient energy harvesting may prolong the systems’ lifetime or possibly enable perpetual operation. On the other hand, wireless energy transfer allows systems to decouple the energy sources from the sensing locations, enabling deployments previously unfeasible. As a result of applying these technologies to WSNs, the assumption of a finite energy budget is replaced with that of potentially infinite , yet intermittent , energy supply, profoundly impacting the design, implementation, and operation of WSNs. This article discusses these aspects by surveying paradigmatic examples of existing solutions in both fields and by reporting on real-world experiences found in the literature. The discussion is instrumental in providing a foundation for selecting the most appropriate energy harvesting or wireless transfer technology based on the application at hand. We conclude by outlining research directions originating from the fundamental change of perspective that energy harvesting and wireless transfer bring about.

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