Saurabh Ganeriwal

Reputation-based framework for high integrity sensor networks

The traditional approach of providing network security has been to borrow tools from cryptography and authentication. However, we argue that the conventional view of security based on cryptography alone is not sufficient for the unique characteristics and novel misbehaviors encountered in sensor networks. Fundamental to this is the observation that cryptography cannot prevent malicious or non-malicious insertion of data from internal adversaries or faulty nodes. We believe that in general tools from different domains such as economics, statistics and data analysis will have to be combined with cryptography for the development of trustworthy sensor networks. Following this approach, we propose a reputation-based framework for sensor networks where nodes maintain reputation for other nodes and use it to evaluate their trustworthiness. We will show that this framework provides a scalable, diverse and a generalized approach for countering all types of misbehavior resulting from malicious and faulty nodes. We are currently developing a system within this framework where we employ a Bayesian formulation, specifically a beta reputation system, for reputation representation, updates and integration. We will explain the reasoning behind our design choices, analyzing their pros & cons. We conclude the paper by verifying the efficacy of this system through some preliminary simulation results.

Optimizing sensor networks in the energy-latency-density design space

In wireless sensor networks, energy efficiency is crucial to achieving satisfactory network lifetime. To reduce the energy consumption significantly, a node should turn off its radio most of the time, except when it has to participate in data forwarding. We propose a new technique, called sparse topology and energy management (STEM), which efficiently wakes up nodes from a deep sleep state without the need for an ultra low-power radio. The designer can trade the energy efficiency of this sleep state for the latency associated with waking up the node. In addition, we integrate STEM with approaches that also leverage excess network density. We show that our hybrid wakeup scheme results in energy savings of over two orders of magnitude compared to sensor networks without topology management. Furthermore, the network designer is offered full flexibility in exploiting the energy-latency-density design space by selecting the appropriate parameter settings of our protocol.

Emerging techniques for long lived wireless sensor networks

In recent years, sensor networks have transitioned from being objects of academic research interest to a technology that is frequently being deployed in real-life applications and rapidly being commercialized. However, energy consumption continues to remain a barrier challenge in many sensor network applications that require long lifetimes. Battery-operated sensor nodes have limited energy storage capability due to small form-factors, or operate in environments that rule out frequent energy replenishment, resulting in a mismatch between the available energy budget for system operation and the required energy budget to obtain desired lifetimes. This article surveys several techniques that show promise in addressing and alleviating this energy consumption challenge. In addition to describing recent advances in energy-aware platforms for information processing and communication protocols for sensor collaboration, the article also looks at emerging, hitherto largely unexplored techniques, such as the use of environmental energy harvesting and the optimization of the energy consumed during sensing.

On selfish behavior in CSMA/CA networks

CSMA/CA protocols rely on the random deferment of packet transmissions. Like most other protocols, CSMA/CA was designed with the assumption that the nodes would play by the rules. This can be dangerous, since the nodes themselves control their random deferment. Indeed, with the higher programmability of the network adapters, the temptation to tamper with the software or firmware is likely to grow; by doing so, a user could obtain a much larger share of the available bandwidth at the expense of other users. We use a game-theoretic approach to investigate the problem of the selfish behavior of nodes in CSMA/CA networks, specifically geared towards the most widely accepted protocol in this class of protocols, IEEE 802.11. We characterize two families of Nash equilibria in a single stage game, one of which always results in a network collapse. We argue that this result provides an incentive for cheaters to cooperate with each other. Explicit cooperation among nodes is clearly impractical. By applying the model of dynamic games borrowed from game theory, we derive the conditions for the stable and optimal functioning of a population of cheaters. We use this insight to develop a simple, localized and distributed protocol that successfully guides multiple selfish nodes to a Pareto-optimal Nash equilibrium.

Aggregation in sensor networks: an energy-accuracy trade-off

Aggregation in Sensor Networks: An Energy- Accuracy Trade-off Athanassios Boulis, Saurabh Ganeriwal, and Mani B. Srivastava Networked and Embedded Systems Lab, EE Department, University of California at Los Angeles email: { boulis, saurabh, mbs }@ee.ucla.edu Abstract – Wireless ad hoc sensor networks (WASNs) are in need of the study of useful applications that will help the researchers view them as distributed physically coupled systems, a collective that estimates the physical environment, and not just energy- limited ad hoc networks. We develop this perspective using a large and interesting class of WASN applications called aggregation applications. In particular, we consider the challenging periodic aggregation problem where the WASN provides the user with periodic estimates of the environment, as opposed to simpler and previously studied snapshot aggregation problems. In periodic aggregation our approach allows the spatial-temporal correlation among values sensed at the various nodes to be exploited towards energy-efficient estimation of the aggregated value of interest. Our approach also creates a system level energy vs. accuracy knob whereby the more the estimation error that the user can tolerate, the less is the energy consumed. We present a distributed estimation algorithm that can be applied to explore the energy-accuracy subspace for a sub-class of periodic aggregation problems, and present extensive simulation results that validate our approach. The resulting algorithm, apart from being more flexible in the energy-accuracy subspace and more robust, can also bring considerable energy savings for a typical accuracy requirement (five -fold decrease in energy consumption for 5% estimation error) compared to repeated snapshot aggregations. Keywords: sensor networks, aggregation applications, distributed estimation, energy vs. accuracy trade-off. The original vision and promise of WASNs was that multiple nodes collectively perform the sensing task requested by the users and communicate the results to the users. However, most of the research so far has simply viewed WASNs as just another kind of wireless ad hoc networks, albeit one composed of nodes that are more energy- constrained and whose data sources are sensors. So, for example, much work has focused on issues such as energy- efficient MAC and ad hoc routing protocols to realize the needed point-to-point and point-to-multipoint communication patterns in WASNs. But, little has been done to develop an understanding of a WASN as a collective or an aggregate where sensor nodes collaborate to jointly estimate the desired answer about the sensed environment. In part this is because not many actual applications useful to the end-user have been studied. The only notable exception is the target-tracking problem, which has drawn attention from several r search e groups. Otherwise, the applications that have been examined are usually toy scenarios used to showcase the abilities of protocols and programming frameworks (e.g., [10]), or very specific applications examined for the sake of some energy- saving technique (e.g., [11]). In this research we have made a first attempt at exploring and understanding the performance of a WASN as a collective that performs a sensing task. We examine a general class of WASN applications that we call aggregation applications where the desired answer depends on the sensed value at multiple nodes. In particular, we explore the energy vs. accuracy subspace, i.e. how much energy savings can one get by relaxing some accuracy requirements and vice versa. We propose an algorithm that exploits this trade-off and jointly considers networking and signal processing issues to create a distributed estimation mechanism. A. Aggregation Applications Many of the examples and simple applications presented in WASN research are based around some kind of aggregation function. The most popular and simple examples of aggregation functions are maximum and average . That is, a user may be interested in knowing the max (or average) of a value in the WASN or in some restricted area of the WASN. If this function needs to be performed once, we refer to it as snapshot aggregation . If the user needs an update in periodic intervals we refer to it as periodic aggregation . The snapshot aggregation problem is trivial for a single static user. The user sends a request to flood the sensor network (or the area of interest). Upon reception of a request I. I NTRODUCTION The technological advances in embedded computers, sensors, and radios have led to the emergence of wireless ad- hoc sensor networks (WASNs) as a new class of system with uses in diverse and useful applications. Indeed, the early papers in the area [6][7][13][15] talk about the vision of cheap self-organizing ad-hoc networks that are able to perform a higher level sensing task through the collaboration of a large number of cheaper and resource constrained wireless sensor nodes. Leveraging numerous sensing devices placed close to the actual physical phenomena, the information that such networks can provide is more accurate and richer than the information provided by a system of few, expensive, state-of- the-art sensing devices. Since WASNs operate largely unattended, often i environments where the access cost of n deploying or maintaining nodes is high, a key problem in designing WASNs is how to prolong their useful lifetime by conserving energy. Consequently, a large fraction of research in WASNs has been dedicated to aspects of the energy- efficiency problem.Wireless ad hoc sensor networks (WASNs) are in need of the study of useful applications that will help the researchers view them as distributed physically coupled systems, a collective that estimates the physical environment, and not just energy-limited ad hoc networks. We develop this perspective using a large and interesting class of WASN applications called aggregation applications. In particular, we consider the challenging periodic aggregation problem where the WASN provides the user with periodic estimates of the environment, as opposed to simpler and previously studied snapshot aggregation problems. In periodic aggregation our approach allows the spatial–temporal correlation among values sensed at the various nodes to be exploited towards energy-efficient estimation of the aggregated value of interest. Our approach also creates a system level energy vs. accuracy knob whereby the more the estimation error that the user can tolerate, the less is the energy consumed. We present a distributed estimation algorithm that can be applied to explore the energy–accuracy subspace for a subclass of periodic aggregation problems, and present extensive simulation results that validate our approach. The resulting algorithm, apart from being more flexible in the energy– accuracy subspace and more robust, can also bring considerable energy savings for a typical accuracy requirement (fivefold decrease in energy consumption for 5% estimation error) compared to repeated snapshot aggregations. 2003 Elsevier B.V. All rights reserved.

Secure time synchronization service for sensor networks

In this paper, we analyze attacks on existing time synchronization protocols for wireless sensor networks. We propose a secure time synchronization toolbox to counter these attacks. This toolbox includes protocols for secure pairwise and group synchronization of nodes that lie in each others power ranges and of nodes that are separated by multiple hops. We provide an in-depth analysis of security and energy overhead of the proposed protocols.

Secure Time Synchronization in Sensor Networks

Time synchronization is critical in sensor networks at many layers of their design. It enables better duty-cycling of the radio, accurate and secure localization, beamforming, and other collaborative signal processing tasks. These benefits make time-synchronization protocols a prime target of malicious adversaries who want to disrupt the normal operation of a sensor network. In this article, we analyze attacks on existing time synchronization protocols for wireless sensor networks and we propose a secure time synchronization toolbox to counter these attacks. This toolbox includes protocols for secure pairwise and group synchronization of nodes that either lie in the neighborhood of each other or are separated by multiple hops. We provide an in-depth analysis of the security and the energy overhead of the proposed protocols. The efficiency of these protocols has been tested through an experimental study on Mica2 motes.

Estimating clock uncertainty for efficient duty-cycling in sensor networks

Radio duty cycling has received significant attention in sensor networking literature, particularly in the form of protocols for medium access control and topology management. While many protocols have claimed to achieve significant duty-cycling benefits in theory and simulation, these benefits have often not translated into practice. The dominant factor that prevents the optimal usage of the radio in real deployment settings is time uncertainty between sensor nodes which results in overhead in the form of long packet preambles, guard bands, and excessive control packets for synchronization. This paper proposes an uncertainty-driven approach to duty-cycling, where a model of long-term clock drift is used to minimize the duty-cycling overhead. First, we use long-term empirical measurements to evaluate and analyze in-depth the interplay between three key parameters that influence long-term synchronization: synchronization rate, history of past synchronization beacons, and the estimation scheme. Second, we use this measurement-based study to design a rate-adaptive, energy-efficient long-term time synchronization algorithm that can adapt to changing clock drift and environmental conditions, while achieving application-specific precision with very high probability. Finally, we integrate our uncertainty-driven time synchronization scheme with the BMAC medium access control protocol, and demonstrate one to two orders of magnitude reduction in transmission energy consumption with negligible impact on packet loss rate.

E/sup 2/WFQ: an energy efficient fair scheduling policy for wireless systems

As embedded systems are being networked, often wirelessly, an increasingly larger share of their total energy budget is due to the communication. This necessitates the development of power management techniques that address communication subsystems, such as radios, as opposed to computation subsystems, such as embedded processors, to which most of the research effort thus far has been devoted. In this paper, we present E2WFQ, an energy efficient version of the Weighted Fair Queuing (WFQ) algorithm for packet scheduling in communication systems. We employ a recently proposed radio power management technique, Dynamic Modulation Scaling (DMS), as a control knob to enable energy-latency tradeoffs during wireless packet scheduling. The use of E2WFQ results in an energy aware packet scheduler, which exploits the statistics of the input arrival pattern as well as the variability in packet lengths. Simulation results show that large savings in energy consumption can be obtained through the use of our scheduling scheme, compared to conventional WFQ, with only a small, bounded increase in worst case packet latency.

Poster abstract: density, accuracy, delay and lifetime tradeoffs in wireless sensor networks—a multidimensional design perspective

With the growing interest in wireless sensor networks, techniques for their systematic analysis design and optimization are essential. Despite numerous research efforts in optimizing hardware, algorithms and protocols for these networks, it remains largely unexplored how these innovations can be all tied together to design a sensor network for a specific practical application. We propose a methodology that starts from four independent quality of service (QoS) parameters and allows the user to completely and unambiguously describe the desired performance, without having to deal with the details of individual devices or protocols. By making appropriate choices in terms of device capabilities and run-time techniques, a design can be positioned in this four-dimensional QoS space. Furthermore, we describe a technique to explore the associated tradeoffs at design time, using both analytical expressions and simulations. To illustrate the benefits of our approach, a design example is worked out, which shows a five fold improvement in network operational lifetime by adapting the event reporting delay.