Phoebus Chen

CITRIC: A low-bandwidth wireless camera network platform

In this paper, we propose and demonstrate a novel wireless camera network system, called CITRIC. The core component of this system is a new hardware platform that integrates a camera, a frequency-scalable (up to 624 MHz) CPU, 16MB FLASH, and 64MB RAM onto a single device. The device then connects with a standard sensor network mote to form a camera mote. The design enables in-network processing of images to reduce communication requirements, which has traditionally been high in existing camera networks with centralized processing. We also propose a back-end client/server architecture to provide a user interface to the system and support further centralized processing for higher-level applications. Our camera mote enables a wider variety of distributed pattern recognition applications than traditional platforms because it provides more computing power and tighter integration of physical components while still consuming relatively little power. Furthermore, the mote easily integrates with existing low-bandwidth sensor networks because it can communicate over the IEEE 802.15.4 protocol with other sensor network platforms. We demonstrate our system on three applications: image compression, target tracking, and camera localization.

Tracking and Coordination of Multiple Agents Using Sensor Networks: System Design, Algorithms and Experiments

This paper considers the problem of pursuit evasion games (PEGs), where the objective of a group of pursuers is to chase and capture a group of evaders in minimum time with the aid of a sensor network. The main challenge in developing a real-time control system using sensor networks is the inconsistency in sensor measurements due to packet loss, communication delay, and false detections. We address this challenge by developing a real-time hierarchical control system, named LochNess, which decouples the estimation of evader states from the control of pursuers via multiple layers of data fusion. The multiple layers of data fusion convert noisy, inconsistent, and bursty sensor measurements into a consistent set of fused measurements. Three novel algorithms are developed for LochNess: multisensor fusion, hierarchical multitarget tracking, and multiagent coordination algorithms. The multisensor fusion algorithm converts correlated sensor measurements into position estimates, the hierarchical multitarget tracking algorithm based on Markov chain Monte Carlo data association (MCMCDA) tracks an unknown number of targets, and the multiagent coordination algorithm coordinates pursuers to chase and capture evaders using robust minimum-time control. The control system LochNess is evaluated in simulation and successfully demonstrated using a large-scale outdoor sensor network deployment

A low-bandwidth camera sensor platform with applications in smart camera networks

Smart camera networks have recently emerged as a new class of sensor network infrastructure that is capable of supporting high-power in-network signal processing and enabling a wide range of applications. In this article, we provide an exposition of our efforts to build a low-bandwidth wireless camera network platform, called CITRIC, and its applications in smart camera networks. The platform integrates a camera, a microphone, a frequency-scalable (up to 624 MHz) CPU, 16 MB FLASH, and 64 MB RAM onto a single device. The device then connects with a standard sensor network mote to form a wireless camera mote. With reasonably low power consumption and extensive algorithmic libraries running on a decent operating system that is easy to program, CITRIC is ideal for research and applications in distributed image and video processing. Its capabilities of in-network image processing also reduce communication requirements, which has been high in other existing camera networks with centralized processing. Furthermore, the mote easily integrates with other low-bandwidth sensor networks via the IEEE 802.15.4 protocol. To justify the utility of CITRIC, we present several representative applications. In particular, concrete research results will be demonstrated in two areas, namely, distributed coverage hole identification and distributed object recognition.

Latency and connectivity analysis tools for wireless mesh networks

There has been a recent rise in interest in building networked control systems over a wireless network, whether they be for robot navigation, multi-robot systems, or traditional industrial automation. The wireless networks in these systems must deliver packets between the controller and the actuators/sensors reliably and with low latency. Furthermore, they should be amenable to modeling and characterization so they can be designed as part of a complete control system. Mesh networks are particularly suited for control applications because they provide greater reliability through path diversity. This paper introduces tools for characterizing the end-to-end connectivity of two points in a wireless mesh network as a function of latency. In particular, we use tools derived from Markov chain models to compare end-to-end connectivity in two routing protocols running on the Data Link/MAC layer provided by Dust Networks Time Synchronized Mesh Protocol (TSMP): Directed Staged Flooding (DSF) and Dust Networks Unicast Path Diversity (UPD). These models also allow us to calculate the traffic load, the sensitivity of end-to-end connectivity to link estimation error, and the robustness of the network to node failure. The paper gives an example of how these tools can be used to evaluate the feasibility of running control applications over sensor networks.

Inherent Security of Routing Protocols in Ad-Hoc and Sensor Networks

Many of the routing protocols that have been designed for wireless ad-hoc networks focus on energy-efficiency and guaranteeing high throughput in a non-adversarial setting. However, given that ad-hoc and sensor networks are deployed and left unattended for long periods of time, it is crucial to design secure routing protocols for these networks. Over the past few years, attacks on the routing protocols have been studied and a number of secure routing protocols have been designed for wireless sensor networks. However, there has not been a comprehensive study of how these protocols compare in terms of achieving security goals and maintaining high throughput. In this paper, we focus on the problem of analyzing the inherent security of routing protocols with respect to two categories: multi-path and single-path routing. Within each category, we focus on deterministic vs. probabilistic mechanisms for setting up the routes. We consider the scenario in which an adversary has subverted a subset of the nodes, and as a result, the paths going through these nodes are compromised. We present our findings through simulation results.

Performance analysis of different routing protocols in Wireless Sensor Networks for real-time estimation

In this paper we analyze the performance of two different routing protocols specifically designed for Wireless Sensor Networks (WSNs) for real-time estimation, control, and monitoring. These protocols are designed to compensate for the lossy nature of the wireless links and the delay from sending messages over multiple hops from the sensors to the controller. The routing protocols are designed to reduce packet delay and packet loss using either retransmissions or multicasting. For some routing topologies one protocol may be better than the other at reducing the worst case packet delay but may have a worse packet loss rate. Here, we apply mathematical tools to analytically compute the average real-time performance based on end-to-end packet delay statistics for two recently proposed routing strategies. We show that the performance is strongly related to the dynamics of the systems being estimated, and we construct a computationally efficient estimation strategy based on the delay statistics. This suggests that routing protocols are to be designed based on the specific real-time estimation and control application under consideration.

Pursuit Controller Performance Guarantees for a Lifeline Pursuit-Evasion Game over a Wireless Sensor Network

Pursuit-evasion games have been studied as a generalization of many control problems, and have been also used to study the viability of running control algorithms over sensor networks. We provide a method for computing performance bounds on a sample pursuit-evasion game, the classic lifeline game described by Rufus Isaacs. Given a routing topology and pairwise link probabilities, we compute an n-hop disk model abstraction of a sensor network to model delay and lost packets. Using this model, we then compute a probabilistic barrier that splits the state space of the game into an escape zone and a capture zone. This barrier and the corresponding optimal control laws are the solution to the game. The position of the barrier in the state space provides a sense of how well such a control application can perform over a sensor network