eChallenges e-2010 Conference ProceedingsPaul Cunningham and Miriam Cunningham (Eds)IIMC International Information Management Corporation, 2010

ISBN: 978-1-905824-21-2

MURPESS - MUlti Radio Pedestrian Energy Scavenging Sensor Network

Ioannis CHATZIGIANNAKIS1, Ugo COLESANTI3, Spyros KONTOGIANNIS1, Guy LESHEM2, Alberto MARCHETTI-SPACCAMELA3, Jan MEHLER4 ,

Giuseppe PERSIANO5, Paul SPIRAKIS1, Andrea VITALETTI3

1RACTI, Patras, Rion 26500, GreeceEmail: ichatz@cti.gr, kontog@cs.uoi.gr, spirakis@hermes.cti.gr2Ben Gurion University of the Negev, Beer-Sheva, 84105, Israel

Email: leshemg@cs.bgu.ac.il3Sapienza University of Rome, via Ariosto 25, 00185 Roma, Italy

Email: {colesanti,alberto,vitale}@dis.uniroma1.it4University of Paderborn, Furstenallee 11, Paderborn, 33102, Germany

Email: Jan.Mehler@uni-padeborn.de5University of Salerno, via Ponte Don Melillo, 84081 Fisciano (SA), Italy

Email: giuper@dia.unisa.it

Abstract: We present and discuss challenges and solutions posed by the design of an adaptable network infrastructure of tiny artifacts. Such artifacts are characterized by severe limitations in computational power, communications capacity and energy; nevertheless they must realize a communication infrastructure able to deliver services to the end-users in a very dynamic and challenging environment. Namely we present one unifying scenario for the activities of the FRONTS project (www.fronts.cti.gr). The aim of the unifying scenario is to show how the results achieved in the project can be exploited to build such a communication infrastructure.

IntroductionIn the near future, new types of systems will appear, designed or emerged, of massive scale, expansive and permeating their environment, of very heterogeneous nature, and operating in a constantly changing networked environment. Such systems are expected to operate even beyond the complete understanding and control of their designers, developers, and users. Although they will be perpetually adapting to a constantly changing environment, they will have to meet their clearly-defined objectives and provide guarantees about certain aspects of their own behavior.

We expect that most such systems will have the form of a large society of tiny artifacts. Each such artifact will be unimpressive: small, with limited sensing, signal processing, and communication capabilities, and usually of limited energy. Yet by cooperation, they will be organized in large societies to accomplish tasks that are difficult or beyond the capabilities of today’s conventional centralized systems.

These systems or societies should have particular ways to achieve an appropriate level of organization and integration. This organization should be achieved seamlessly and with appropriate levels of flexibility, in order to be able to achieve their global goals and objectives. For this reason, they definitely need to adapt to the changes in their environment and change their internal organization by communicating, cooperating, and forming goal-driven sub-organizations.

In this paper we present and discuss challenges and solutions posed by the design of

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an adaptable network infrastructure of tiny artifacts characterized by severe limitations in computational power, communications capacity and energy; nevertheless they must realize a communication infrastructure able to deliver services to the end-users in a very dynamic and challenging environment. Our scenario is formed by artifacts that we name MURPESS (MUlti Radio Pedestrian Energy Scavenging Sensor) that, besides the usual characteristics of tiny wireless devices such as limited bandwidth, storage, computational power and energy supply, support some distinguish features: the co-existence of active and passive radio technologies, the exploitation of energy scavenging techniques. A network of MURPESSes devices is an example of opportunistic network of resource constrained devices. The realization of such networks poses challenging research issues whose solution requires the development of new algorithmic solutions and the use of new tools.

The purpose of the paper is to present these challenges and to propose solutions developed by the partners of the FET proactive project FRONTS (www.fronts.cti.gr).

Technology DescriptionA MURPESS is a tiny battery powered device equipped with heterogeneous sensors, that can be embedded in shoes soles and can scavenge energy from pedestrians' steps [18], enabling mobile users to collect data while moving around. MURPESSes are equipped with multiple radio technologies: passive ones, such as in RFID tags, and active ones, (e.g. 802.15.4, Bluetooth, NFC). When the level of energy in the battery is sufficiently high, a MURPESS can exploit active radios. When the battery is low and the energy scavenging mechanism is recharging it, only passive communications are possible. The resulting networking scenario, consists of a considerable number of MURPESSes carried by users that occasionally interact both among them and with a fixed infrastructure consisting of heterogeneous technologies such as RFID devices and WSNs. As an example, figure 1 illustrates a first active interaction between two MURPESSes (user A and WSN) and a second passive interaction involving the MURPESS carried by user B which stores a piece of information on the MURPESS of user C that in turn walks to PLANE whose MURPESS reads the carried information.

Figure 1: The networking architecture. Observe that Murpess To Murpess (M2M) communications can be either active or passive. In the latter, one MURPESS acts as a reader and the other as a passive device.

In passive communications, the device draws power from the radio waves emitted by another device (i.e. the reader). However, the energy provided by the reader, is barely sufficient to perform minimal computations and to possibly answer to reader-initiated

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queries. Furthermore, the distance between the reader and the tag is at most few meters (ISO 18000-6), but it is typically only a few centimeters (ISO 14443). Passive communications can thus be exploited to support stigmergy based services and/or localization ones, as well as form of delay tolerant and opportunistic communications. In the latter case, RFIDs act as mediators where data a (up to few Kb) are temporarily stored. In opportunistic networking [12], where mobile nodes are enabled to communicate with each other even in the absence of a route connecting them. Paths are built dynamically in a decentralized fashion and any node can be opportunistically used as next hop, provided it is likely to bring the message closer to the final destination.

Objectives The objective is to show how, despite above severe limitations, a MURPESS network will be used to deliver services to end-users as well as to monitor the environment and to realize social interactions among users. A network of MURPESSes will support the development of applications in many scenarios such as opportunistic monitoring [21,12], Mobile-commerce [22], social networks [19,20] etc. In [1] we discuss a possible application to recommendation systems that are widely deployed in a variety of e-commerce and social networking applications showing how, despite the limited memory resources, MURPESSes are able to provide effective recommendation to their users while visiting a shop.

Our approach is based on a usable quantitative theory of networked adaptation based on rigorous and measurable gains by studying specific issues in the realization of such network. We also intend to apply our models, methods, and results to the scrutiny of large scale simulations and experiments from which we expect to obtain valuable feedback.

The main objective of this paper is to show how MURPESSes are able to construct a dynamic network that is able to deliver the collected information to the destination using passive mobility and taking into account limited available computational and communication capabilities and the severe limitations on energy. In particular, the network should be able to maintain a communication infrastructure, to allow users to efficiently share information and to enforce a cooperative behavior to avoid cases in which a MURPESS refuses to cooperate in order to save its own energy.

Other important issues are given by the need to protect user privacy, while tracing MURPESS attached to humans, and the need to guarantee security coping with physical capture of such devices.

MethodologyThe realization of the above scenario requires to develop different methodologies from distributed computing, game theory, security, communications protocol, analysis of algorithms to address the main challenges. Namely, we identify the need of A self-stabilizing communication infrastructure

We need to adapt the communication structure under dynamic changes [23], induced by mobility or by entities entering or leaving the system in unpredictable ways, and in conditions of scarcity of (computational and communication) resources. Several solutions are known to achieve a lightweight communication infrastructure such as spanning trees, random overlays and backbones. We remark that all known techniques to dynamically maintain these combinatorial structures lack locality, self-organizational properties, and are not suitable for deployment in a highly dynamic situation of immense scale. Recently, the concept of self-organization has been widely mentioned in the scope of distributed computing and peer to peer networks. In [13] a framework for self-organization is proposed, including formal definitions of the self-organization concept and complementary proof techniques which can be used to prove that

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algorithms are indeed self-organizing. However proposed solutions are not suitable with the strict technological constraints and new solutions should be devised.

Algorithms and protocols to efficiently share information One of the most crucial services used in every distributed system is a transparent access to information shared by the nodes of the network. We consider the scenario of a database stored on a stationary server that should be accessible by MURPESSes that might not be directly connected to it. Whenever a request can not be fulfilled locally, data has to be transferred between the server and the involved MURPESS. The problem of on-line data management has been thoroughly investigated for static networks (see the survey [14]), whereas very little work has yet been done in dynamic and mobile networks. A first step towards data management in dynamic networks was accomplished by Bienkowski et al. [15], assuming a single copy of the data.

Algorithms and tools to enforce a cooperative behaviorIn a dynamic network comprising of many mobile sensing devices (MURPESSes) and a few fixed active elements (readers) the issue of cooperation vs. antagonism is crucial in accomplishing community-oriented goals. For example, for the consumption of sensors' energy, each (possibly selfish, or altruistic) sensor may wish to save its own energy for conducting direct communication operations for itself, rather than providing indirect service to the community (e.g. acting as an intermediary node for transferring some other moving node's information to the closest static reader). This goes far beyond the traditional model of a sensor network with routing to sink nodes or overlay structures. We plan to apply models and techniques of game theory to the study of networks of artifacts. The study of algorithmic game theory has achieved significant results in the last few years in the study of pervasive networks like internet [16, 17]. We remark that most proposed solutions are not distributed and/or require significant computational power that is not available in our scenario.

Algorithms and protocols to protect user privacy The MURPRESS system must guarantee that users will not be traced while wearing shoes with small devices and that information will be released only to authorized parties [9,10]. The small components embedded in the shoes will carry information to identify the object that will be used, for example, for the management of post-sale services.

Algorithms and protocols to cope with physical capture Another important issue is that embedded small devices present the additional security threat of physical capture which, for example, is not possible when dealing with Internet security; in fact, the devices can be physically stressed and reset so to perform the same protocol[11]. Therefore, the security guarantee offered by traditional models are void and new models (and protocols) must be presented.

ResultsThe above scenario poses problems that require the design and test of new algorithms. We use different methodologies from communications protocol, distributed computing, game theory, security to solve the main challenges. We now report the main interim results obtained in the FRONTS project.

5.1 A Self-Stabilizing Communication Infrastructure

Mobile devices are characterized by different speeds: there are 'fast moving' and 'slow moving' MURPESSes. The devices in the slow moving group are used as anchors (e.g., used to store data and provide services) for the fast moving devices. A suitable network infrastructure is built only for the slow moving devices, while the fast moving devices occasionally connect to this network. In any case, the network of slow-moving devices

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should rapidly react to topology changes supporting some forms of self-organization.An algorithm is self-organizing if it converges in sub-linear time and reacts “fast” to

topology changes; the self-organization property can then be used for gaining, in sub-linear time, global properties and reacting to changes.

In [2] we have proposed and analysed algorithms that are based on a new basic self-stabilizing snapshot algorithm, designed for message passing systems in which a distributed spanning tree is defined and in which nodes communicate using bounded links capacity. The combination of the self-stabilizing and self-organizing distributed hierarchy construction and the snapshot algorithm form an efficient self-stabilizer transformer: given a distributed algorithm for a specific task, the system is able to convert the algorithm into a self-stabilizing algorithm for the same task with an expected convergence time that is logarithmic in the number of devices.The hierarchy of subsystems is defined by partitioning the communication graph into small clusters, after which clusters are merged to form bigger clusters and so on. The partition can be done according to a designer's input, using an automatic on-line clustering algorithm or even an on-line clustering algorithm that reflects the system's current behavior. We propose a randomized self-stabilizing and self-organizing partition that is based on the periodical collection of local topology.

5.2 Algorithms and Protocols to Efficiently Share Information

We consider the scenario of a database stored on a stationary server that is connected to the network of MURPESSes via a set of wireless access points; however the network topology is extremely dynamic and the paths used for communication between nodes may change with time. If a MURPESSes is interested in accessing the database it issues read/write requests. If there is only one copy of the database, data has to be transferred between the server and the involved MURPESS through a multi-hop path at the cost of power consumption in every involved MURPESS.

The goal of a data management system is to minimize the overall energy consumption of the MURPESSes. Therefore it can create and delete copies of the database and place on the MURPESSes: a MURPESS which holds a copy can read the database for free, but, on the other hand, every update of the database has to be propagated to all other copies. Furthermore, the transmission of the whole database consumes significantly more energy than transferring a single entry. Thus, the data management system has to create/delete copies according to the current request emergence to balance the costs involved.

We have investigated the online version of the problem, when the data management system does not know beforehand which requests are issued. In [3] we have given an input sequence that gives a lower bound of the competitive ratio of such demand-driven online algorithms; furthermore, we have analyzed the demand-driven algorithm Count [4] that creates copies only on nodes which recently issued a request. We have also simulated Count and other online algorithms using the wireless sensor network simulator Shawn [5]. The simulations suggest that the given lower bound is very pessimistic and that Count performs much better on realistic input sequences.

5.3 Algorithms and Tools to Enforce a Cooperative Behavior

In an ad-hoc network of many moving nodes and a few static nodes, each node's behavior should be described by some sort of simple protocol determining its behavior within the community. The social goal of the whole community might indicate that each node abides with some prescribed protocol. Nevertheless, since selfishness might lead the community members to defect in favor of their own (private) interest, the community as a whole would

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also like to be able to react (but without any notion of centralized coordination) against possible deviations from the anticipated behavioral patterns.

In particular we want to enforce rules that prevent some subset of the sensors to join forces and comprise a selfish coalition (rather than acting solely), rules that allow to continuously adapt to the evolution of the network itself (learning how to behave) and rules to avoid the take-over of the whole network by some hostile and powerful malware. From the methodological point of view the main difficulties lies in the fact that the behavior of the network can be modeled through a infinitely repeated game; it is well known from game theory that equilibrium and solutions for such infinite games are in general much more difficult that the solution for the corresponding (one-shot) game. Our findings are reported in [6, 7, 8].

5.4 Algorithms to Protect User Privacy

The MURPRESS system must guarantee that users will not be traced while wearing shoes with small devices and that information will be released only to authorized parties. The small components embedded in the shoes will carry information to identify the object that will be used, for example, for the management of post-sale services. Obviously, it is expected that encryption is used for storing information on the tag so that only legitimate users can access the stored data.

Encryption though does not seem to solve of user tracing. Specifically, RFID tags can be read by anyone and the string stored on a tag, even though it is a ciphertext, can be used to trace the tag and, in the case the tag is attached to a personal object, to trace the owner of the tag. We thus envision a system in which the environment helps in alleviating this problem: as tags move in the environment they are read by special devices called the randomizers which guarantee anonymity. In particular anonymity can be achieved by randomizing the information stored in the tag; this operation should guarantee the user’s privacy and at the same time satisfy security requirement of the system. A preliminary construction of a cryptosystem that can be re-randomized has been presented in [9]. Problems specific to location privacy and new notions of security are also explored in [10].

5.5 Algorithms and Tools to Cope with Physical Capture

Another important issue is that MURPESSes present the additional security threat of physical capture which, for example, is not possible when dealing with Internet security. In this case the devices can be physically stressed and reset so to perform the same protocol twice using the same random coin tosses. In this case the security guarantee offered by traditional models are void and new models (and protocols) must be presented.

Typically, small devices can be used by the user to gain access to restricted area (e-passports constitute a notable application in this field) and must be able to perform identification protocols. The main threat derives from physical capture and from transferability: once a user has performed successfully a protocol, the transcript of the protocol itself can be used to convince a third party that the protocol has been executed (from which information on user localization can be inferred). In [11], we have presented efficient protocols for allowing identification by small devices that are robust against physical capture.

ConclusionsThe proposed scenario is an example of opportunistic networking, where mobile nodes are enabled to communicate with each other even in the absence of a route connecting them. Paths are built dynamically in a decentralized fashion and any node can be opportunistically

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be used as next hop, provided it is likely to bring the message closer to the final destination.In the paper we discuss the required methodologies and the challenges that need to

be solved in order to support the development and provisioning of new services in the above depicted scenario.

We have also discussed how the results of project FRONTS answers such challenges; the results of the project are promising and we expect, by the end of the project, to provide a set of tools and algorithms that are tested and simulated to be effectively deployed in the realization of opportunistic networks of devices with limited energy and computational powera set of recommendations for the realization of such opportunistic networks.

The MUPRESS scenario is an example of the integration of a variety of technologies and algorithms for the purpose of information delivery in an opportunistic network of tiny devices. We have already indicated a set of issues (such as self-organization, enforcement, privacy, information sharing etc.) and ways to confront each one of them.

In the future we need tests in real environments and experiments in order to evaluate the effort of integration of those techniques. In this way we expect to learn lessons about side-effects due to interaction of methods, and measure the overhead caused by such protocols working together. Specific targets to measure are energy requirements and efficient delivery of information. Robustness of such nets can be evaluated by creating adverse scenario (due to faults and/or to malicious devices) and see how the net responds to the challenges posed.

AcknowledgementsThis work has been partially supported by ICT programme of the European Union under contract number ICT-2008-215270 (FRONTS).

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