NEEWS

System Architecture: In collaboration with MSU's Computational Ecology and Visualization laboratory, we are developing a scalable ecological sensor network with sensor platforms, networks, field servers, regional and national repository servers, and web based client/application modules for end users in a fully customizable manner. An end-to-end system architecture that we propose to cater to such functions is depicted in the following Figure.

The lowest tier of the network contains homogeneous ultra-low power nodes that are used for sensing with spatial resolution of the order of 1m to 30m. Data collected at this tier is aggregated by the tier-2 nodes. Tier-2 nodes assume a dual role of aggregation and routing of the tier-1 data as well as performing their own data collection using sensors with spatial resolution of approximately 30m to 500m. Data collected at this tier and that aggregated from the lower tier is routed to a centralized server through the gateway nodes in the data aggregation layer. All sensor data must pass through these gateway nodes in which middleware software, supporting sensor node programming, is located. A sufficient number of redundant gateway nodes are used in order to accomplish reliability. From the network management server, administrators are able to program a sensor node or a group of sensors nodes with a specific sensing schedule. Once programmed, a sensor node can initiate data delivery to the gateway node middleware, which, after implementing required post-processing, can deliver the data to the appropriate repository servers.

System Development: As the tier-1 nodes we use off the shelf MICA2 MOTE processor/radio card and MTS310 MOTE sensor card (both are available from Crossbow Technologies). MICA2 card comprises an ultra low-power 8-bit microcontroller ATmega 128L that run TinyOS nano-kernel with only 4 KB onboard RAM and 128KB program flash memory. The most attractive feature of this card is that it draws only 8mA current in active mode and 15 uA in a supported sleep mode of operation. The card also supports a 400 MHz or 900 MHz band radio communication interface with a transmission power controlled range of up to few hundred feet. Sensor modalities supported by these tier-1 nodes include low resolution acoustics, temperature, light intensity, magnetic field intensity, and two-axis acceleration. While the tier-1 nodes are used for low resolution acoustic sampling and primarily for monitoring ancillary meteorological parameters, high-resolution audio sampling is performed at the tier-2 nodes. Considering the high data volume, on-board processing and heavy-weight communication requirements we decided to develop a high-performance yet relatively low power consuming sensor platform for the tier-2 nodes

As shown in the figure above, the tier-2 sensor platform is designed around the STARGATE single-board computer from Crossbow Technologies. STARGATE has a low-power 400MHz PXA255 processor, 64MB onboard SDRAM, 32Mb flash and general-purpose interface ports. In addition to the processor card, the system comprises several power related and peripheral components including a power supply (5V-12V), wireless card (802.11b), local flash storage (1 Gb), USB multiplexer, a camera, a microphone, a solar panel (18w) and a deep cycle marine 12V battery, weatherproof enclosure and a weatherproof battery box. A MICA2 MOTE processor/radio card is also connected through the 52-pin connector so that the data collected from the tier-1 nodes can be aggregated by the tier-2 habitat sensor platform over 400 MHz or 900 MHz links.

We employ the embedded Linux operating system on STARGATE. Other software modules in the tier-2 sensor include TCP/IP for networking, Telnet and FTP for remote access and diagnostics. Additional networking software for wireless ad hoc routing, clustering, and sensor programming are also implemented in this platform. Since the end-applications often require time-domain correlation analysis of the collected sensor data from multiple sensor platforms, it was mandatory to run a low resolution time synchronization protocol across the tier-2 nodes. We have implemented a millisecond resolution time synchronization protocol to accomplish this. Finally, a visualization tool for network status monitoring and sensor programming has also been developed for management purposes.

Sensor Deployment: To field test the developed sensor platforms, we deployed three site clusters of sensor networks at the MSU Kellogg Biological Station (see the figure below) within the LTER Site to test the described sensor system. Each site was chosen to include different land use types including forest, urban, grassland, agricultural, lake, and wetland. Each site contains several sensor platforms and an associated server that receives sensed data and uploads them to a centralized remote server. Two of the sites (LTER Main Site and Pond Lab Site) were within range of the MSU wireless network cloud and the other site (Bird Sanctuary Site) could not reach the wireless backbone. Data from the Bird Sanctuary Site is manually downloaded and periodically moved through the corresponding server.

Details about the ecological applications of the deployed sensor network can be found at the web site of MSU's Computational Ecology and Visualization laboratory.