Vinodkrishnan Kulathumani

ExScal: elements of an extreme scale wireless sensor network

Project ExScal (for extreme scale) fielded a 1000+ node wireless sensor network and a 200+ node peer-to-peer ad hoc network of 802.11 devices in a 13km by 300m remote area in Florida, USA during December 2004. In comparison with previous deployments, the ExScal application is relatively complex and its networks are the largest ones of either type fielded to date. In this paper, we overview the key requirements of ExScal, the corresponding design of the hardware/software platform and application, and some results of our experiments.

Analyzing the yield of ExScal, a large-scale wireless sensor network experiment

Recent experiments have taken steps towards realizing the vision of extremely large wireless sensor networks, the largest of these being ExScal, in which we deployed about 1200 nodes over a 1.3 km by 300 m open area. Such experiments remain especially challenging because of: (a) prior observations of failure of sensor network protocols to scale, due to network faults and their spatial and temporal variability, (b) complexity of protocol interaction, (c) lack of sufficient data about faults and variability, even at smaller scales, and (d) current inadequacy of simulation and analytical tools to predict sensor network protocol behavior. In this paper, we present detailed data about faults, both anticipated and unanticipated, in ExScal. We also evaluate the impact of these faults on ExScal as well as the design principles that enabled it to satisfy its application requirements despite these faults. We describe the important lessons learnt from the ExScal experiment and suggest services and tools as a further aid to future large scale network deployments.

A Fault-Local Self-Stabilizing Clustering Service for Wireless Ad Hoc Networks

We present a fast, local clustering service, FLOC, that partitions a multihop wireless network into nonoverlapping and approximately equal-sized clusters. Each cluster has a clusterhead such that all nodes within unit distance and some nodes within distance m of the clusterhead belong to the cluster. We show that, by asserting a stretch factor m ges 2, FLOC achieves locality of clustering and fault-local self-stabilization: the effects of cluster formation and faults/changes at any part of the network are contained within at most m + 1 units. Through simulations and experiments with actual deployments, we analyze the trade-offs between clustering time and the quality of clustering and suggest suitable parameters for FLOC to achieve a fast completion time without compromising the quality of the resulting clustering

Differential games in large-scale sensor-actuator networks

Surveillance systems based on sensor network technology have been shown to successfully detect, classify and track targets of interest over a large area. State information collected via the sensor network also enables these systems to actuate mobile agents so as to achieve surveillance goals such as target capture and asset protection. But satisfying these goals is complicated by the fact that track information in a sensor network is routed to mobile agents through multi-hop communication links and is thus subject to delays and losses. In addition, as the sensor network is scaled in size, high throughput rates for all pursuers cannot be sustained at all times, which necessitates a network communication strategy that adapts to pursuer information requirements. In this paper, we concentrate on the formulation of optimal pursuit control strategies in the presence of network effects, assuming that target track information has been established locally in the sensor network. We adapt ideas from the theory of differential games to networked games-including ones involving non-periodic track updates, message losses and message delays-to derive optimal strategies, bounds on the information requirements, and scaling properties of these bounds. Moreover, we present a specific network communication protocol which has the required scalable information characteristics and conclude with the results of experimental studies

Trail: A distance-sensitive sensor network service for distributed object tracking

Distributed observation and control of mobile objects via static wireless sensors demands timely information in a distance-sensitive manner: Information about closer objects is required more often and more quickly than that of farther objects. In this article, we present a wireless sensor network protocol, Trail, that supports distance-sensitive tracking of mobile objects for in-network subscribers upon demand. Trail achieves a find time that is linear in the distance from a subscriber to an object, via a distributed data structure that is updated only locally when the object moves. Notably, Trail does not partition the network into a hierarchy of clusters and clusterheads, and as a result Trail has lower maintenance costs, is more locally fault tolerant, and it better utilizes the network in terms of load balancing and minimizing the size of the data structure needed for tracking. Moreover, Trail is reliable and energy efficient, despite the network dynamics that are typical of wireless sensor networks. Trail can be refined by tuning certain parameters, thereby yielding a family of protocols that are suited for different application settings such as rate of queries, rate of updates, and network size. We evaluate the performance of Trail by analysis, simulations in a 90 × 90 sensor network, and experiments on 105 Mica2 nodes in the context of a pursuer-evader control application.

Trail: a distance sensitive WSN service for distributed object tracking

Distributed observation and control of mobile objects via static wireless sensors demands timely information in a distance sensitive manner: information about closer objects is required more often and more quickly than that of farther objects. In this paper, we present a wireless sensor network protocol, Trail, that supports distance sensitive tracking of mobile object by in-network subscribers upon demand. Trail achieves a find time that is linear in the distance from the subscriber to the object, via a distributed data structure that is updated only locally when objects move. Trail seeks to minimize the size of the data structure. Moreover, Trail is reliable, fault-tolerant and energy-efficient, despite the network dynamics that are typical of wireless sensor networks. We evaluate the performance of Trail by simulations in a 90-by-90 sensor network and report on 105 node experiments in the context of a pursuer-evader control application.

RAMP: accelerating wireless sensor hardware design with a reconfigurable analog/mixed-signal platform

The requirements of many wireless sensing applications approach, or even exceed, the limited hardware capabilities of energy-constrained sensing platforms. To achieve such demanding requirements, some sensing platforms have included low-power application-specific hardware---at the expense of generality---to pre-process the sensor data for reduction to only the relevant information. While this additional hardware can save power by reducing the activity of the microcontroller and radio, a unique hardware solution is required for each application, which presents an unrealistic burden in terms of design time, cost, and ease of integration. To diminish these burdens, we present a reconfigurable analog/mixed-signal sensing platform in this work. At the hardware-level, this platform consists of a reconfigurable integrated circuit containing many commonly used signal-processing blocks and circuit components that can be connected in any configuration. At the software level, this platform provides a framework for abstracting this underlying hardware. We demonstrate how to quickly develop new applications on this platform, ranging from standard sensor interfacing techniques to more complicated intelligent pre-processing and wake-up detection. We also demonstrate how to integrate this platform with commonly used wireless sensor nodes and embedded-system platforms.

ON THE EFFECT OF FAULTS IN VIBRATION CONTROL OF FAIRING STRUCTURES

Sensor-actuator networks are increasingly being used in distributed control applications. The cost of sensors and actuators is dropping substantially and hence control by a large number of these components is now feasible. One such application is the damping of acoustic and structural vibration associated with the launch of a rocket. Reliability in the presence of faults is critical for such mission systems. These faults could be broken components, insecure or compromised components offering erroneous data to the control. The network itself could add unpredictable delays and data drop outs that could affect the control in potentially unanticipated ways. In this paper, we consider the Boeing Open Experimental Platform fairing control application for acoustic and structural vibration damping and study the effect of component level and network level faults. We identify several scenarios under which control performance is intolerable. This leads us to design an alternative control scheme. We design the application using a purely local on-off control scheme and compare its performance with that of the original system.Copyright

Reliable Control System Design Despite Byzantine Actuators

Sensor-actuator networks are increasingly being used in distributed control of large scale systems. Often these applications are mission-critical and are required to maintain satisfactory performance in the presence of component failures. On the one hand, sensor-actuator network components are becoming inexpensive but they also tend to be unreliable, especially when deployed in harsh or unpredictable environments. The various component failures can manifest themselves in the form of arbitrary actuator behavior in which case their effect on the underlying systems can be severe. In this paper we focus our attention on applications of sensor networks in control of linear systems and show how to deal with Byzantine faults of actuators. We first describe a fault-tolerant control scheme using locally redundant actuators. We then relax the requirement on actuators to be at the same location and design a fault-tolerant scheme where the actuator redundancies are further reduced as well. We demonstrate our methodologies using a beam vibration control application as a case study.Copyright

EZ-AG: structure-free data aggregation in MANETs using push-assisted self-repelling random walks

This paper describes EZ-AG, a structure-free protocol for duplicate insensitive data aggregation in MANETs. The key idea in EZ-AG is to introduce a token that performs a self-repelling random walk in the network and aggregates information from nodes when they are visited for the first time. A self-repelling random walk of a token on a graph is one in which at each step, the token moves to a neighbor that has been visited least often. While self-repelling random walks visit all nodes in the network much faster than plain random walks, they tend to slow down when most of the nodes are already visited. In this paper, we show that a single step push phase at each node can significantly speed up the aggregation and eliminate this slow down. By doing so, EZ-AG achieves aggregation in only O(N) time and messages. In terms of overhead, EZ-AG outperforms existing structure-free data aggregation by a factor of at least log(N) and achieves the lower bound for aggregation message overhead. We demonstrate the scalability and robustness of EZ-AG using ns-3 simulations in networks ranging from 100 to 4000 nodes under different mobility models and node speeds. We also describe a hierarchical extension for EZ-AG that can produce multi-resolution aggregates at each node using only O(NlogN) messages, which is a poly-logarithmic factor improvement over existing techniques.