Sensor Web Adaptation to Dynamic Sensor Networks

As nodes appear and disappear in a Wireless Sensor Network (WSN), communication protocols acting in the data link, network and transport layers adapt dynamically to the new network structure. We present an extension of the dynamic nature of WSNs to the web via an adaptive communication protocol called the adaptive Sensor Web Language (SWL). Adaptive SWL provides a web application with a reliable mechanism for automatically tracking and displaying changes in sensor network architecture. New nodes automatically appear in web-based applications. A color fading mechanism is also provided to differentiate sensor nodes which have not communicated within the expected time frame. Two new message types, Request Announce and Announce are added to SWL to support this adaptation. All software layers, including sensor nodes, gateway, base station (including the database) and the web applications were updated. Two web applications were implemented to clearly demonstrate web application adaptation to dynamic WSNs. The Open Geospatial Consortium´s (OGC) standard Sensor Observation Service (SOS) was integrated with Google maps to show the spatial context of changing WSN structure. A test network was established using 6 sensor nodes and 10 sensors (6 battery voltage sensors, 2 air temperature sensors and 2 solar radiation sensors), and 1 gateway. Each node was added one by one over 4 hours in the network, then removed one by one from the network over 4 hours. Testing indicates appearance of a node in the web application within about 13 seconds of being added to the WSN with a system latency of 47.5 seconds averaged over 40 tests.