Loren Schwiebert

Power efficient topologies for wireless sensor networks

Wireless sensor networks have become possible because of the on-going improvements in sensor technology and VLSI. One issue in smart sensor networks is achieving efficient operation because of the limited available power. For important classes of sensor networks, such as biomedical sensors, the locations of the sensing nodes are fixed and the placement can be pre-determined. In this paper, we consider the topology that best supports communication among these sensor nodes. We propose a power-aware routing protocol and simulate the performance, showing that our routing protocol adapts routes to the available power. This leads to a reduction in the total power used as well as more even power usage across nodes. We consider different routes and topologies, demonstrating the difference in performance and explaining the underlying causes.

Preserving source location privacy in monitoring-based wireless sensor networks

While a wireless sensor network is deployed to monitor certain events and pinpoint their locations, the location information is intended only for legitimate users. However, an eavesdropper can monitor the traffic and deduce the approximate location of monitored objects in certain situations. We first describe a successful attack against the flooding-based phantom routing, proposed in the seminal work by Celal Ozturk, Yanyong Zhang, and Wade Trappe. Then, we propose GROW (Greedy Random Walk), a two-way random walk, i.e., from both source and sink, to reduce the chance an eavesdropper can collect the location information. We improve the delivery rate by using local broadcasting and greedy forwarding. Privacy protection is verified under a backtracking attack model. The message delivery time is a little longer than that of the broadcasting-based approach, but it is still acceptable if we consider the enhanced privacy preserving capability of this new approach. At the same time, the energy consumption is less than half the energy consumption of flooding-base phantom routing, which is preferred in a low duty cycle, environmental monitoring sensor network

Communication scheduling to minimize thermal effects of implanted biosensor networks in homogeneous tissue

A network of biosensors can be implanted in a human body for health monitoring, diagnostics, or as a prosthetic device. Biosensors can be organized into clusters where most of the communication takes place within the clusters, and long range transmissions to the base station are performed by the cluster leader to reduce the energy cost. In some applications, the tissues are sensitive to temperature increase and may be damaged by the heat resulting from normal operations and the recharging of sensor nodes. Our work is the first to consider rotating the cluster leadership to minimize the heating effects on human tissues. We explore the factors that lead to temperature increase, and the process for calculating the specific absorption rate (SAR) and temperature increase of implanted biosensors by using the finite-difference time-domain (FDTD) method. We improve performance by rotating the cluster leader based on the leadership history and the sensor locations. We propose a simplified scheme, temperature increase potential, to efficiently predict the temperature increase in tissues surrounding implanted sensors. Finally, a genetic algorithm is proposed to exploit the search for an optimal temperature increase sequence.

On tree-based convergecasting in wireless sensor networks

A wireless sensor network (WSN) consists of sensors implanted in an environment for collecting and transmitting data regarding changes in the environment based on the requests from a controlling device (called base station) using wireless communication. WSNs are being used in medical, military, and environment monitoring applications. Broadcast (dissemination of information from a central node) and convergecast (gathering of information towards a central node) are important communication paradigms across all application domains. Most sensor applications involve both convergecasting and broadcasting. The time taken to complete either of them has to be kept minimal. This can be accomplished by constructing an efficient tree for both broadcasting as well as convergecasting and allocating wireless communication channels to ensure collision-free communication. There exist several works on broadcasting in multihop radio networks (a.k.a. ad hoc networks), which can also be used for broadcasting in WSNs. These algorithms construct a broadcast tree and compute a schedule for transmitting and receiving for each node to achieve collision-free broadcasting. In this paper, we show that we need a new algorithm for applications, which involve both convergecasting and broadcasting since the broadcast tree may not be efficient for convergecasting. So we propose a heuristic algorithm (convergecasting tree construction and channel allocation algorithm (CTCCAA)), which constructs a tree with schedules assigned to nodes for collision free convergecasting. The algorithm is capable of code allocation (direct sequence spread spectrum (DSSS)/ frequency hopping spread spectrum (FHSS)), in case multiple codes are available, to minimize the total duration required for convergecasting. We also show that the same tree can be used for broadcasting and is as efficient as a tree exclusively constructed for broadcasting.

Towards a propagation model for wireless biomedical applications

Propagation model plays a very important role in designing wireless communication systems. Current advances in semiconductor technology has made it possible to implant a network of bio-sensors inside the human body for health monitoring purposes [C. Furse, H.K. Lai, C. Estes, A. Mahadik, A. Duncan, 1999], [C. Furse, R. Mohan, A. Jakayar, S. Karidehal, B. McCleod, S. Going, 2001], [L. Schwiebert, S.K.S. Gupta, P.S.G. Auner, G. Abrams, R. Lezzi, P. McAlister, 2002]. For wireless communication inside the human body, the tissue medium acts as a channel through which the information is sent as electromagnetic (EM) radio frequency (RF) waves. A propagation model is necessary to determine the losses involved in the form of absorption of EM wave power by the tissue. Absorption of EM waves by the tissue body, which consists of mostly saline water, accounts for a major portion of the propagation loss. In this paper we present a propagation loss model (PMBA) for homogeneous tissue bodies. We have verified the model for the frequency range of our interest (900 MHz to 3 GHz) using a 3D EM simulation software, HFSS/spl trade/, and experimental measurements using saturated salt water.

TARA: thermal-aware routing algorithm for implanted sensor networks

Implanted biological sensors are a special class of wireless sensor networks that are used in-vivo for various medical applications. One of the major challenges of continuous in-vivo sensing is the heat generated by the implanted sensors due to communication radiation and circuitry power consumption. This paper addresses the issues of routing in implanted sensor networks. We propose a thermal-aware routing protocol that routes the data away from high temperature areas (hot spots). With this protocol each node estimates temperature change of its neighbors and routes packets around the hot spot area by a withdraw strategy. The proposed protocol can achieve a better balance of temperature rise and only experience a modest increased delay compared with shortest hop, but thermal-awareness also indicates the capability of load balance, which leads to less packet loss in high load situations.

Enforcing Privacy Using Symmetric Random Key-Set in Vehicular Networks

Vehicular networks have attracted extensive attentions in recent years for their promises in improving safety and enabling other value-added services. Security and privacy are two integrated issues in the deployment of vehicular networks. Privacy-preserving authentication is a key technique in addressing these two issues. We propose a random keyset based authentication protocol that preserves user privacy under the zero-trust policy, in which no central authority is trusted with the user privacy. We show that the protocol can efficiently authenticate users without compromising their privacy with theoretical analysis. Malicious user identification and key revocation are also described

Role-based hierarchical self organization for wireless ad hoc sensor networks

Efficiently self organizing a network hierarchy with specific assignment of roles (or tasks) to sensors based on their physical wireless connectivity and sensing characteristics is an important and challenging problem. In this paper, we extend the hierarchical connected dominating set (CDS) construction algorithm, proposed by Jie Wu, to develop our role-based hierarchical self organization algorithm for wireless sensor networks. The resulting self organized sensor network establishes a network-wide infrastructure consisting of a hierarchy of backbone nodes, and sensing zones that include sensor coordinators, and sensing collaborators (or sensing zone members). Our paper identifies the need for organizing a sensor network according to the tasks appropriate for each sensor node based on their initial deployment in the network. Past research in group-based (or hierarchical) sensor networks have ignored the possibility of utilizing both the physical communication and sensing characteristics to assign roles to sensor nodes. We demonstrate the effectiveness of our design, which considers both, through simulations.

A Necessary and Sufficient Condition for Deadlock-Free Wormhole Routing

An important open problem in wormhole routing has been to find a necessary and sufficient condition for deadlock-free adaptive routing. Recently, Duato has solved this problem for a restricted class of adaptive routing algorithms. In this paper, a necessary and sufficient condition is proposed that can be used for any adaptive or nonadaptive routing algorithm for wormhole routing, as long as only local information is required for routing. The underlying proof technique introduces a new type of dependency graph, thechannel waiting graph, which omits most channel dependencies that cannot be used to create a deadlock configuration. The necessary and sufficient condition can be applied in a straightforward manner to most routing algorithms. This is illustrated by proving deadlock freedom for a partially adaptive nonminimal mesh routing algorithm that does not require virtual channels and a fully adaptive minimal hypercube routing algorithm with two virtual channels per physical channel. Both routing algorithms are more adaptive than any previously proposed routing algorithm with similar virtual channel requirements.

Adaptive Privacy-Preserving Authentication in Vehicular Networks

Vehicular networks have attracted extensive attentions in recent years for their promises in improving safety and enabling other value-added services. Most previous work focuses on designing the media access and physical layer protocols. Privacy issues in vehicular systems have not been well addressed. We argue that privacy is a user-specific concept, and a good privacy protection mechanism should allow users to select the degrees of privacy they wish to have. To address this requirement, we propose an adaptive privacy-preserving authentication mechanism that can trade off the privacy degree with computational and communication overheads (resource usage). This mechanism, to our knowledge, is the first effort on adaptive privacy-preserving authentication. We present analytical and preliminary simulation results to show that the proposed protocol is not only adaptive but also scalable.