A Java Library for Event-Driven Communicationin Power-Manageable Reactive Sensor Nodes

The energy efficiency of wireless sensor networks strongly depends on the possibility of exploiting the idleness of their nodes. In principle, idle periods could be fully exploited by making use of ultra low power micro controller units (MCUs) and power manageable network interfaces which provide a wide range of sleep states with sub micro Watt power consumption. One of the key issues, however, is to avoid to keep sensor nodes busy when they could be idle, thus reducing the opportunity of dynamic power management. This issue is particularly serious in case of sensor nodes running a virtual runtime environment, since the virtual machine (VM) is seen by the scheduler of the underlying operating system (OS) as a process which is always active in spite of the idleness of the threads running on top of it. On the other hand, the benefits of virtualization in terms of abstraction and usability motivates the development of sensor nodes with power manageable virtual runtime environments. Promising results have been recently achieved in this direction by using a modified version of the Darjeeling VM on top of Contiki OS. This paper moves a step forward by introducing VirtualSense, an event-driven communication library for the Darjeeling VM which exhibits two distinguishing features. First, it is general enough to enable the implementation of advanced communication protocols in Java. Second, its event-driven nature makes it possible for a Java thread to react to incoming messages without keeping the MCU busy while waiting.