Recent advances in the development of microsensors, microprocessors, information fusion algorithms, and ad hoc networking have led to increasingly capable wireless sensor networks. These networks, when deployed to monitor an urban area, show great promise in enhancing warfighter situational awareness. However, delivering the sensor network's collected information back to the proper decision makers is one network capability that still requires improvement. To bridge this gap between the tactical operations center and multiple wireless sensor networks distributed across a city, engineers must create a system-of-systems architecture. This architecture must permit a warfighter to receive near-real-time sensor information from an out-of-theater operating post, whether a mile or an ocean away. Research accomplished in efforts such as the Defense Advanced Research Projects Agency's (DARPA) Information Exploitation Office sponsored Networked Embedded Systems Technology (NEST) program has provided information gathering algorithms for wireless sensor networks that are independent of the hardware platform on which they run. Nevertheless, these networks have no means for publishing the massive amounts of information to the Global Information Grid (GIG). To address this publication requirement, AFRL engineers have begun integrating NEST technologies with the Joint Battlespace Infosphere (JBI).1, 2 They recently developed a proof-of-concept demonstration of this idea for Scientific Advisory Board (SAB) review. In this demonstration, they integrated a tracking application developed for the NEST program with the AFRL developed JBI Reference Implementation and showcased the resultant capability to connect low-level information gatherers to high-level information distributors.
To visually convey the benefits of this combined NEST/JBI architecture to the SAB, the AFRL engineers deployed a small wireless sensor network. The NEST application included a display screen showing the overhead layout of this network, and as a network node's sensors detected objects, the screen displayed the estimated location of each object. To reduce false alarm rates, the engineers divided the network into clusters and calculated a corresponding confidence level for each cluster. The team assigned one node from each cluster as a cluster master; the role of this designated node was to aggregate information from its neighboring nodes and report to the local data collection station the number of nodes in its cluster that sensed the intruding object. If the number of nodes detecting the intruder within a certain time period was above a predefined threshold, the system accepted the information from that cluster as being useful. The figure illustrates the network architecture and information flow established for the SAB review. As depicted, the engineers modified the NEST application (representing an in-field tactical sensor network) to publish network events, such as object detection, to the JBI. They configured another application to (1) duplicate the overhead of the network view, (2) connect to the JBI as a subscriber to the NEST application's published network events, and (3) display subscribed events to a remote operator monitoring the battlespace. As illustrated, a local data collection station is necessary in the deployment as a means of translating raw sensor messages into meaningful information objects for the JBI. Connectivity between this local station and the JBI server is subject to the constraints of the specific environment unique to each deployment. During the NEST/JBI demonstration, the engineers executed the tracking application and sent—via a wireless connection meeting the 802.11b communications standard—network events from the customized NEST application, through the JBI, and to the network monitoring application.
The number of messages passed from the sensor network to the JBI depends on numerous factors, as exemplified by the following real-world example. In December 2004, DARPA, the NEST program team, and The Ohio State University conducted the Extreme-Scale Wireless Networking (ExScal) demonstration, which consisted of roughly 1,000 sensor nodes in a nonaggregating, multitiered network.3 In the ExScal environment, sensor data is forwarded to a single network access point, which then receives and stores the information on a local computer. For a particularly large event, such as an earthquake, it is possible that every node in the network will send at least a single message. Assuming 100% sensor reliability, the earthquake would generate a throughput of 1,000 information objects (1 per node) in a very short period of time. Such volume easily overloads the computer and can lead to dropped messages.
Background on NEST and the JBI
Because of their inherently autonomous nature, wireless sensor networks allow the warfighter—from a standoff position—to maintain situational awareness and persistent monitoring of all aspects of an urban environment, made possible by the capability to "set and forget" thousands of low-cost, lowpower sensors across rooftops, in alleyways, and inside buildings. The goal of the NEST program is to coordinate the operation of these distributed embedded sensor networks by developing the necessary underlying information services. The NEST team has already undertaken to improve many of these services, pursuing such developments as algorithms for self-localization in a Global Positioning System deprived environment, efficient ad hoc routing, security, multitiered fault tolerance, and time synchronization. Most recently, the NEST team has begun to apply distributed sensors to support warfighter operational needs. The team created the largest wireless sensor network established to date—a 1,000-node, robust autonomous ad hoc sensor network, dispersed over an area approximately 1 × .25 km in an operationally relevant environment—to demonstrate monitoring and protection of long, linear areas. In another effort, NEST engineers developed algorithms to locate small-arms fire in high reverberation environments as an adjunct to the multipath locative method. Finally, to provide battlespace awareness to the participants of the Team Patriot 2005 exercise, the NEST team successfully integrated multiple, geographically dispersed sensor fields within a single small unmanned air vehicle. The use of distributed sensors to support warfighter operational needs is the common thread connecting these NEST accomplishments; however, even the best information is useless without an overarching information management system to allow its global accessibility. The JBI provides this access by aggregating, integrating, fusing, and intelligently disseminating relevant battlespace knowledge to those who need it most.
The JBI was born from the SAB vision to establish a flexible interoperability layer between information systems, as opposed to the traditional architecture comprising stovepiped legacy systems. The key to this envisioned interoperability is the use of the Common Applications Programming Interface, which presents client applications with a consistent interface to the services supplied by the JBI. A vital service offered by the JBI is the ability to connect information producers to information consumers via publish-and-subscribe mechanisms. These mechanisms allow the JBI to push new information, as soon as it arrives from publishers, to all subscribers registered to receive that information. Therefore, the information displayed by JBI-enabled remote network monitoring applications is always current, and the JBI user has a means to remotely monitor these networks to ensure their operability.
An essential JBI feature is its capacity to manage network information as distinct information object types. When a client application registers with the JBI as either a publisher or a subscriber, that client must specify the information object type it will be exchanging. This specification enables producers and consumers interested in the same information to be connected through the JBI. The JBI can also manage data from different wireless sensor network applications as distinct information types. Again, when registering with the JBI, network monitoring applications must subscribe to specific information types, allowing the remote monitor to receive updates only from those network applications it has interest in monitoring. This useful feature enables different monitors for the various sensor networks.
Another JBI service, the ability to archive published information and provide it to applications for later query, is important to NEST users. Remote network monitoring applications employ this "information persistence" to provide replay functionality and allow users to review network activity over a period of time.
Future Research Opportunities
Due to the challenges involved with large-scale deployment, AFRL engineers have not yet tested the NEST/JBI architecture against a field the size of the ExScal demonstration. However, the team plans to increase the number of sensors to stress-test the architecture. The engineers will then present the architecture at upcoming NEST meetings involving members of US Special Operations Command (USSOCOM), DARPA, and the Defense Intelligence Agency. Concurrent to completing these activities, the team will continue to develop and enhance NEST services in anticipation of the technology's upcoming transition to USSOCOM in Fiscal Year 2007.
Mr. Mark Oliver and Mr. Ryan Sites, of the Air Force Research Laboratory's Information Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn_index.asp. Reference document IF-H-06-05.
- Scientific Advisory Board. "Building the Joint Battlespace Infosphere." Technical Report SAB-TR-99-02, US Air Force, Dec 00.
- JBI Program Office. "Reference Implementation Quick-Start Guide." Core Services Reference Implementation Version 1.2.5, AFRL, Jul 05.
- Arora, A., Bapat, S., and Kulathumani, V. "Analyzing the Yield of ExScal, a Large- Scale Wireless Sensor Network Experiment." 13th IEEE International Conference on Network Protocols, vol 53, no 62 (Nov 05): 6-9.