Abstract
Idleness has to be carefully exploited in wireless sensor networks (WSNs) to save power and to accumulate the energy possibly harvested from the environment. State-of-the-art microcontroller units provide a wide range of ultra-low-power inactive modes with sub-millisecond wakeup time that can be effectively used for this purpose. At the same time they are equipped with 16-bit RISC architectures clocked at tens of MHz, which make them powerful enough to run a Java-compatible virtual machine (VM). This makes it possible to bring the benefits of a virtual runtime environment into power-constrained embedded systems. VMs, however, risk to impair the effectiveness of dynamic power management as they are seen as always-active processes by the scheduler of the operating system in spite of the idleness of the threads running on top of them. Avoiding to keep sensor nodes busy when they could be idle is mandatory for the energetic sustainability of WSNs. While most of the tasks of a sensor node are inherently event-driven, the functioning of its hardware-software components is not, so that they require to be redesigned in order to exploit idleness. This paper presents VirtualSense, an open-hardware open-source ultra-low-power reactive wireless sensor module featuring a Java-compatible VM.
Highlights
The lifetime of a wireless sensor network (WSN) depends on the capability of its nodes to adapt to time-varying workload conditions by turning off unused components and by dynamically tuning the power-performance tradeoff of the used ones
Idleness is important in wireless sensor nodes, which spend most of their time waiting for external events or for monitoring requests, and which are often equipped with energy-harvesting modules which promise to grant them an unlimited lifetime [1] as long as their average power consumption is lower than the average harvested power
Virtualization adds to the simplicity and portability of applications for WSNs at the cost of increasing the distance between hardware and software, which might impair the effectiveness of Dynamic power management (DPM) both for the limited control of the underlying hardware offered by the virtual runtime environment, and for the limited visibility of the actual activity offered by the virtual machine (VM)
Summary
The lifetime of a wireless sensor network (WSN) depends on the capability of its nodes to adapt to time-varying workload conditions by turning off unused components and by dynamically tuning the power-performance tradeoff of the used ones. International Journal of Distributed Sensor Networks one is provided by full-fledged software stacks designed for power manageable sensor nodes in order to make it possible to take DPM decisions directly from the runtime environment and to grant to the OS scheduler full visibility of the idleness of the virtual tasks. In spite of the availability of ultra-low-power modes provided by the MCU, the effectiveness of DPM risks to be impaired by the paradigm adopted for inter node communication.
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