In wireless sensor networks (WSNs), most time synchronization protocols allow synchronizing the different nodes' clocks with the clock of a reference node called ROOT. The position of the ROOT determines the accuracy of time synchronization. Indeed, the closer the ROOT is to the other nodes in the network, better is the accuracy. In this paper, we propose a new fully distributed method to select the ROOT in WSNs. The proposed method is based on a heuristic developed as a result of extensive simulations and also real experiments on a dedicated IoT prototyping platform. According to the simulation results, on average, our method successfully selects the ROOT from the 5 % of best-positioned nodes.

Web applications use authentication mechanisms to provide user-friendly content to users. However, some dangerous techniques like session fixation attacks target these mechanisms, by making the legitimate user use a session identifier that is controlled by the attacker. In this way, he can then impersonate the legitimate user without the need to know his credentials. In this paper, we present SAWFIX, a PHP static analyzer that checks web applications for session fixation vulnerabilities. To the best of our knowledge, SAWFIX is the first analyzer that checks exhaustively for this type of vulnerabilities, while the other methods only ensure partial correctness that is limited to a fraction of possible executions. SAWFIX is based on abstract interpretation, which is a theory for approximating the semantics of programs and allows designing static analyzers that are fully automatic and sound by construction. We implemented a prototype of our approach and tested it on several complex web applications. We obtained promising results in terms of detection accuracy and processing time, which reflects the efficiency of our system.

The wireless shared medium used by Wireless Sensor Networks applications causes a problem related to interference, specially with dense deployment that characterizes the WSNs. This requires the use of congestion control scheme to avoid interferences and buffer overflow that degrade the application reliability. REFIACC (Reliable, Efficient, fair and Interference aware Congestion control) schedule scheme is a cross layer congestion control protocol that avoids the aforementioned problems while maximizing throughput and fairness. In our previously studies, we have validated REFIACC using extensive simulations. In this study, REFIACC is modeled using Stochastic Automata Networks (SAN). In fact, SAN presents a good tool to avoid state-space explosion of Markov chains.

The Consensus Time Synchronization (CTS) overcomes the shortcoming of centralized time synchronization in terms of scalability and robustness to node failure. However, CTS leads to slow convergence rate, high communication traffic and the inability to provide synchronization to an external time source. This paper proposes a novel distributed time synchronization protocol for WSNs, the Consensus-based Multi-hop Time Synchronization (CMTS) protocol. CMTS combines the benefits of consensus-based scheme, multi-level topology, synchronization by overhearing, master node synchronization, and MAC-layer timestamping. Simulations are performed to validate the effectiveness of CMTS. The results show that CMTS achieves high accuracy and improves the convergence time compared to competing schemes in the literature.

In this paper, we propose a new Medium Access Control (MAC) protocol for Wireless Sensor Networks (WSNs) called MMSMAC (Multi-Mode Sensor MAC protocol), which can operate and switch among three modes: synchronous, asynchronous, and hybrid, according to the application requirements. In the synchronous mode, MMSMAC organizes the sensor nodes under even and odd clusters. Each sensor node has its own active/sleep and send/receive periods according to its cluster identifier, which ensures better load balancing among nodes. In the asynchronous mode, sensor nodes communicate freely without the utilization of even and odd clusters. In this mode, we also propose another mechanism to circumvent the hidden host problem. In the hybrid mode, the features of synchronous and asynchronous modes are combined. Simulation results and analysis show that each of the MMSMAC modes shows convincing performance gains and outperforms B-MAC and CSMA/TDMA protocols.

The Internet of Things is the natural continuity of the Ambient Intelligence where smart and ambient environments are built mainly by integrating a large number of interconnected smart objects (sensors, actuators, Smartphone, appliances, etc.) with heterogeneous capabilities abstracted as software services. These services can be composed on the fly and provided, all the time and everywhere, to assist users in their daily activities. A key issue in user-centered services composition is to intelligently and effectively discover and select the most relevant services that best match the users' requirements and closely meet the specified quality-of-service level. Monitoring seamlessly the provided services and enhancing their quality, is still a challenging issue due mainly to the dynamicity and uncertainty characterizing ambient environments. In this paper, we propose a new service-oriented, user-centered and event-aware Framework capable of performing services monitoring to handle automatically events that may occur in ambient environments. This monitoring is based on a dynamic services discovery and selection process to enhance self-adaptation to unpredicted changes, and ensure services continuity with best quality. The overall proposed Framework has been implemented and validated through a scenario dedicated to daily activity recognition in an Ambient-Assisted Living environment. In addition, the obtained performances from extensive tests show clearly the efficiency and feasibility of the proposed approach in the case of a large-scale environment.

Optimizing energy consumption and end-to-end (e2e) packet delay in energy-constrained, delay-sensitive wireless sensor networks is a conflicting multiobjective optimization problem. We investigate the problem from a game theory perspective, where the two optimization objectives are considered as game players. The cost model of each player is mapped through a generalized optimization framework onto protocol-specific MAC parameters. From the optimization framework, a game is first defined by the Nash bargaining solution (NBS) to assure energy consumption and e2e delay balancing. Secondy, the Kalai-Smorodinsky bargaining solution (KSBS) is used to find an equal proportion of gain between players. Both methods offer a bargaining solution to the duty-cycle MAC protocol under different axioms. As a result, given the two performance requirements (i.e., the maximum latency tolerated by the application and the initial energy budget of nodes), the proposed framework allows to set tunable system parameters to reach a fair equilibrium point that dually minimizes the system latency and energy consumption. For illustration, this formulation is applied to six state-of-the-art wireless sensor network (WSN) MAC protocols: B-MAC, X-MAC, RI-MAC, SMAC, DMAC, and LMAC. The article shows the effectiveness and scalability of such a framework in optimizing protocol parameters that achieve a fair energy-delay performance trade-off under the application requirements.

The aim of this work is to investigate main techniques and technologies enabling user's mobility in wearable health monitoring systems. For this, design requirements for key enabling mechanisms are pointed out, and a number of conceptual and technological recommendations are presented. The whole is schematized and presented into the form of a design framework taking in consideration patient context constraints. This work aspires to bring a further contribution for the conception and possibly the evaluation of health monitoring systems with full support of mobility offering freedom to users while enhancing their life quality.

Failure detection in distributed systems under MANET environment is a hard task to achieve. Indeed, due to mobility and resource constraints, traditional protocols record a high number of false suspicions. We propose in this paper a new failure detection protocol for MANETs that achieves a better compromise in terms of accuracy and completeness, by introducing new mechanisms to correct false suspicions. Simulation results are reported to compare the performances of our protocol with other approaches.

We consider networks where mobile sensor/actor nodes move to specific locations in order to conduct data collection or deliver a response to an event. The challenge is to find the best tour for the mobile nodes in order to visit the given set of locations. In this paper, the objective of the optimization is to extend the node lifetime by emphasizing both path efficiency and balanced energy consumption when identifying and assigning tours to mobile nodes. Compared to existing schemes in the literature, we consider the initial position of mobile sensors when determining the tours. We formulate the optimization as a balanced multi-salesman travel problem and propose a solution based on a two-step approach. First, we determine the shortest tour that includes all event locations by forming the Hamiltonian cycle. Then, we formulate the optimal partitioning of such a cycle as a linear program (LP) where the objective is to reduce the tour length while minimizing the maximum tour a node has to be make. For scalability and to expedite convergence, we propose a method for solving the LP formulation based on Branch & Price algorithm. The simulation results confirm the effectiveness of our optimization formulation and the advantage of our solution compared to competing schemes.

Area-based localization is a simple and efficient approach, where each node estimates its position based on proximity information to some special nodes with known location, called anchors. Based on the anchors' coordinates, each node first determines its residence area and then approximates its position as the centroid of that area. Therefore, the accuracy of the estimated position depends on the size of the residence area; the smaller the residence area is, the better the accuracy is likely to be. Because the size of the residence area mainly depends on the number and the positions of anchor nodes, their deployment should be carefully considered in order to achieve a better accuracy while minimizing the cost. For this purpose, in this paper we conduct a theoretical study on anchor placement for a very popular area based localization approach. We determine the optimal anchor placement pattern for increased accuracy and how to achieve a particular accuracy goal with the least anchor count. Our analytical results are further validated through simulation.

In this paper, we deal with failure detection in distributed systems under mobile environment constraints. For this effect, we propose a new protocol, called AFDAN (Accurate Fault Detection Protocol for Ad hoc Network), that is in charge of monitoring the distributed application against any node failure. The simulation results of our protocol show good performances in terms of accuracy and message overhead comparing to other protocols dedicated for MANETs.

Area-based localization algorithms use only the position of some reference nodes, called anchors, to estimate the residence area of the remaining nodes. Existing algorithms use a triangle, a ring or a circle as the geometric shape that defines the node's residence area. However, existing algorithms suffer from two major problems: (1) in some cases, they might make wrong decisions about a node presence inside a given area, or (2) they require high anchor density to achieve a low location estimation error and high ratio of localizable nodes. In this paper, we overcome these shortcomings by introducing a new approach for determining the node's residence area that is geometrically shaped as a half-symmetric lens. A novel half symmetric lens based localization algorithm (HSL) is proposed. HSL yields smaller residence areas, and consequently, better location accuracy than contemporary schemes. HSL further employs Voronoi diagram in order to boost the percentage of localizable nodes. The performance of HSL is validated through mathematical analysis, extensive simulations experiments and prototype implementation. The validation results confirm that HSL achieves better location accuracy and higher ratio of localizable nodes compared to competing algorithms.

This article considers the data aggregation scheduling problem, where a collision-free schedule is determined in a distributed way to route the aggregated data from all the sensor nodes to the base station within the least time duration. The algorithm proposed in this article (Distributed algorithm for Integrated tree Construction and data Aggregation (DICA)) intertwines the tree formation and node scheduling to reduce the time latency. Furthermore, while forming the aggregation tree, DICA maximizes the available choices for parent selection at every node, where a parent may have the same, lower, or higher hop count to the base station. The correctness of the DICA is formally proven, and upper bounds for time and communication overhead are derived. Its performance is evaluated through simulation and compared with six delay-aware aggregation algorithms. The results show that DICA outperforms competing schemes. The article also presents a general hardware-in-the-loop framework (DAF) for validating data aggregation schemes on Wireless Sensor Networks (WSNs). The framework factors in practical issues such as clock synchronization and the sensor node hardware. DICA is implemented and validated using this framework on a test bed of sensor motes that runs TinyOS 2.x, and it is compared with a distributed protocol (DAS) that is also implemented using the proposed framework.

Wireless sensors and actors networks (WSANs) have the capacity for not only monitoring some phenomena through sensor nodes but also performing appropriate actions. Most of the contemporary WSAN management solutions focus on defining communication path among sensors and actors and on tasking appropriate actors to handle the detected events. In this paper we classify events based on how they evolve over time into continuous and discrete and categorize the WSAN management strategies accordingly. Unlike discrete events, a continuous event spreads quickly and becomes more serious as time passes. Such a characteristic introduces more challenges and motivates a non-conventional management strategies. This paper presents an approach for Sensor-Actuator Coordination for Handling Spreading events (SACHS). SACHS opts to enable the network to respond quickly in order to avoid the event from growing in scope, e.g., prevent a fire from spreading, while reducing the energy overhead due to the coordination messages and due to actor's relocation to the event region. SACHS limits sensor-actor and actor-actor interactions and exploits local sensor-sensor communication to determine the scope of the event, define spots for actors to position at, and schedule the actors' response. The simulation results confirm the performance advantage of SACHS compared to competing schemes.

This paper proposes an Enhanced Secure Pairwise Broadcast Time Synchronization (E-SPBS) protocol that allows authenticated MAC-layer timestamping on high-data rate radio interfaces. E-SPBS ensures the security of the Receiver-Only synchronization approach using a Public-Key-based Cryptography authentication scheme. The robustness and accuracy of E-SPBS were evaluated through simulations and experiments on a MICAz platform. Both simulation and experimental results demonstrate that E-SPBS achieves high robustness to external and internal attacks with low energy consumption. However, while the simulation results indicate that E-SPBS can achieve an average accuracy of less than 1μ s, the experimental results show that the synchronization error is higher and not stable. This comparison gives us a good indication on how much confidence can be put into simulation results.

Optimizing energy consumption and end-to-end (e2e) packet delay in energy constrained distributed wireless networks is a conflicting multi-objective optimization problem. This paper investigates this trade-off from a game-theoretic perspective, where the two optimization objectives are considered as virtual game players that attempt to optimize their utility values. The cost model of each player is mapped through a generalized optimization framework onto protocol specific MAC parameters. A cooperative game is then defined, in which the Nash Bargaining solution assures the balance between energy consumption and e2e packet delay. For illustration, this formulation is applied to three state-of-the-art wireless sensor network MAC protocols; X-MAC, DMAC, and LMAC as representatives of preamble sampling, slotted contention-based, and frame-based MAC categories, respectively. The paper shows the effectiveness of such framework in optimizing protocol parameters for achieving a fair energy-delay performance trade-off, under the application requirements in terms of initial energy budget and maximum e2e packet delay. The proposed framework is scalable with the increase in the number of nodes, as the players represent the optimization metrics instead of nodes.

In wireless sensor networks, the sensor data are often aggregated en-route to the base-station in order to eliminate redundancy and conserve the network resources. The basestation not only acts as a destination for the upstream data traffic, but it also configures the network by transmitting commands downstream to nodes. The data delivery latency is a critical performance metric in time-sensitive applications and is considered by a number of data aggregation schemes in the literature. However, to the best of our knowledge, no solution has considered the scheduling of downstream packets, originated from the base-station, in conjunction with upstream data aggregation traffic. This paper fills such a gap and proposes MASAUD, which intertwines the medium access schedule of upstream and downstream traffic in order to reuse time slots in a non-conflicting manner and reduce delay. MASAUD can be integrated with any scheme for data aggregation scheduling. The simulation confirms the effectiveness of MASAUD.

The real-world node deployment aspect is investigated, while considering cost minimization for resolving the energy hole around the sink, which represents a serious problem in typical sensor networks with uniform distribution. A novel strategy is proposed that is based on the use of two sinks and a few extra relay nodes close to the sinks' areas. The traffic is then alternatively sent to the sinks in every other cycle. As a second contribution, an efficient data collection mechanism has been developed to determine the optimal data rate that meets delay requirements of individual sensor reports and improves the network lifetime. The comparison of the proposed node deployment strategy with uniform, non-uniform geometric and linear increase node distributions demonstrates that the cost of the proposed solution is very close to that of the uniform distribution and much lower than all the others, while achieving a load balancing at the same order of the state-of-the-art solutions.

In wireless sensor networks, in-network data aggregation filters out redundant sensor readings in order to reduce the energy and bandwidth consumed in disseminating the data to the base-station. In this paper, we investigate the problem of reliable collection of aggregated data with minimal latency. The aim is to form an aggregation tree such that there are k disjoint paths from each node to the base-station and find a collision-free schedule for node transmissions so that the aggregated data reaches the base-station in minimal time. We propose a novel algorithm for Reliable and Timely dissemination of Aggregated Data (RTAD). RTAD intertwines the formation of the aggregation tree and the allocation of time slots to nodes, and assigns parents to the individual nodes in order to maximize time slot reuse. The simulation results show that RTAD outperforms competing algorithms in the literature.

Minimizing the energy consumption of battery-powered sensors is an essential consideration in sensor network applications like coverage, and sleep/wake scheduling mechanism has been proved to an efficient approach to handling this issue. Nevertheless, the frequent switching between states, during scheduling, leads also to significant energy consumption. In this article, a coverage-guaranteed distributed sleep/wake scheduling scheme is presented with the purpose of prolonging network lifetime while guaranteeing network coverage. Our scheme mitigates scheduling process to be more stable by avoiding useless transitions between states without affecting the coverage level required by the application. The simulation results illustrate that out scheme outperforms some other existed algorithms in terms of coverage guarantee, energy conservation and stability.

We present Duo-MAC, an asynchronous cascading wake-up scheduled MAC protocol for heterogeneous traffic forwarding in low-power wireless networks. Duo-MAC deals with energy-delay minimization problem and copes with transmission latency encountered by Today's duty-cycled protocols when forwarding heterogeneous traffic types. It switches, according to the energy and delay requirements, between Low Duty cycle (LDC) and High Duty Cycle (HDC) operating modes, and it quietly adjusts the wake-up schedule of a node according to (i) its parent's wake-up time and (ii) its estimated load, using an effective real-time signal processing linear traffic estimator. As a second contribution, Duo-MAC, proposes a service differentiation through an improved contention window adaptation algorithm to meet delay requirements of heterogeneous traffic classes. Duo-MAC's efficiency stems from balancing between the two traffic award operation modes. Implementation and experimentation of Duo-MAC on a MicaZ mote platform reveals that the protocol outperforms other state-of-the-art MAC protocols from the energy-delay minimization perspective.

In-network data aggregation is often pursued to remove redundancy and correlate the data en-route to the base-station in order to save energy in wireless sensor networks (WSNs). In this paper, we present a novel cross-layer approach for reducing the latency in disseminating aggregated data to the base-station over multi-frequency radio links. Our approach forms the aggregation tree with the objective of increasing the simultaneity of transmissions and reducing buffering delay. Aggregation nodes are picked and time-slots are allocated to the individual sensors so that the most number of ready nodes can transmit their data without delay. Colliding transmissions are avoided by the use of different radio channels. Our approach is validated through simulation and is shown to outperform previously published schemes.

Ensuring area coverage is one of the key requirements of wireless sensor networks (WSNs). When nodes are randomly placed in the area of interest, redundancy is often provisioned in order to lower the probability of having voids, where part of the area is not within the detection range of any sensor. To extend the lifetime of the network, a duty cycle mechanism is often applied in which only a subset of the nodes are activated at a certain time while the other nodes switch to low-power mode. The set of active nodes are changed over time in order to balance the load on the individual sensors. The selection of active nodes is subject to meeting the coverage requirement. Assessing the coverage of a sensor is based on knowing its position. However, localization schemes usually yield a margin of errors which diminishes the coverage fidelity. Conservative approaches for mitigating the position inaccuracy assume the worst-case error across the network and end up activating excessive number of nodes and reduces the network lifetime. In this paper, we present an approach for estimating a bound on the maximum error for the position of each sensor and propose a distributed algorithm for achieving high fidelity coverage while engaging only a subset of the sensors. The simulation results confirm the performance advantages of our approach.

This paper focuses on data aggregation scheduling problem in wireless sensor networks (WSNs), to minimize time latency. Prior works on this problem have adopted a structured approach, in which a tree-based structure is used as an input for the scheduling algorithm. As the scheduling performance mainly depends on the supplied aggregation tree, such an approach cannot guarantee optimal performance. To address this problem, we propose approaches based on Semi-structured Topology (DAS-ST) and Unstructured Topology (DAS-UT). The approaches are based on two key design features, which are: (1) simultaneous execution of aggregation tree construction and scheduling, and (2) parent selection criteria that maximize the choices of parents for each node and maximize time slot reuse. We prove that the latency of DAS-ST is upper-bounded by ([2π/arccos(1/1+ϵ)]+4)R+Δ-4, where R is the network radius, Δ is the maximum node degree, and 0.05 <; ϵ ≤ 1. Simulations results show that DAS-UT outperforms DAS-ST and four competitive state-of-the-art aggregation scheduling algorithms in terms of latency and network lifetime.

This paper considers slotted duty-cycled medium access control (MAC) protocols, where sensor nodes periodically and synchronously alternate their operations between active and sleep modes to save energy. Communications can occur only when nodes are in active mode. The synchronous feature makes these protocols more appropriate for delay-sensitive applications than asynchronous protocols. With asynchronous protocols, additional delay is needed for the sender to meet the receiver's active period. This is eliminated with synchronous approaches, where nodes sleep and wake up all together. Moreover, the contention-based feature makes the protocols - considered in this paper - conceptually distributed and more dynamic compared to TDMA protocols. Duty cycling allows obtaining significant energy saving vs. full duty cycle (sleepless) protocols. However, it may result in significant latency. Forwarding a packet over multiple hops often requires multiple operational cycles (sleep latency), i.e. nodes have to wait for the next cycle to forward data at each hop. Timeliness issues of slotted contention-based MAC protocols are dealt with in this paper, where a comprehensive review and taxonomy is provided. The main contribution is to study and classify the protocols from the delay-efficiency perspective.

The area-based localization algorithms use only the location information of some reference nodes, called anchors, to give the residence area of the remaining nodes. The current algorithms use triangle, ring or circle as a geometric shape to determine the sensors' residence area. Existing works suffer from two major problems: (1) in some cases, they might issue wrong decisions about nodes' presence inside a given area, or (2) they require high anchor density to achieve a low location estimation error. In this paper, we deal with the localization problem by introducing a new way to determine the sensors' residence area which shows a better accuracy than the existing algorithms. Our new localization algorithm, called HSL (Half Symmetric Lens based localization algorithm for WSN), is based on the geometric shape of half-symmetric lens. We also uses the Voronoi diagram in HSL to mitigate the problem of unlocalizable sensor nodes. Finally, we conduct extensive simulations to evaluate the performance of HSL. Simulation results show that HSL has better locatable ratio and location accuracy compared to representative state-of-the-art area-based algorithms.

Mobile devices are now equipped with multiple sensors and networking capabilities. They can gather information about their surrounding environment and interact both with nearby nodes, using a dynamic and self-configurable ad-hoc network, and with distant nodes via the Internet. While the concept of mobile agent is appropriate to explore the ad-hoc network and autonomously discover service providers, it is not suitable for the implementation of strong distributed synchronization mechanisms. Moreover, the termination of a task assigned to an agent may be compromised if the persistence of the agent itself is not ensured. In the case of a transactional mobile agent, we identify two services, Availability of the Source and Atomic Commit, that have to be supplied by more powerful entities located outside the ad-hoc network, in a remote network, called the support network. We propose a solution where the two services are successively provided by an active entity called the leader. In the support network, crash failures may also occur. To guarantee reliability, the main actions performed by the leader are the subject of a consensus with all the other potential leaders. The proposed solution relies on a single agreement protocol that orders continuously all the new actions.

The mobile agent is a computer program that can move between different hosts in heterogeneous networks. This paradigm is advantageous for distributed systems implementation, especially in mobile computing application characterized by low bandwidth, high latency and unreliable networks connections. Mobile agent is also attractive for distributed transactions applications. Although mobile agent has been studied for twenty years for some good reasons, it is not largely used in developing distributed systems for simple reasons: important issues like security and fault tolerance are not solved in effective way. In this paper we address the issue of fault tolerance in mobile agent systems and transactional support. We present the agent system design and describe the protocol of our approach in which we treat infrastructure failures to prevent a partial or complete loss of mobile agent and deal with semantic failures to ensure atomic execution and transactional support for mobile agent.

International audience

Fault-tolerance is an important function in target tracking application using wireless sensor networks. We propose in this paper, an efficient fault-tolerant approach for target tracking that prevents the loss of the target. Instead of using a single prediction mechanism, our approach uses a multi-level incremental prediction technique that adjusts the prediction precision of the target movement. The responsible node of target detection uses multiple historical information pieces to calculate multi-level predictions which have different precision levels according to the number of information pieces used. Thanks to our parametric prediction model, our approach increases the prediction success rate and decreases the target loss frequency compared to basic approaches that use simple prediction models.

With the growing emergence of ubiquitous computing and networked systems, ubiquitous robotics is becoming an active research domain. The issue of services composition to offer seamless access to a variety of complex services has received widespread attention in recent years. The majority of the proposed approaches have been inspired from the research undertaken jointly on Workflow and AI-based classical planning techniques. However, the traditional AI-based methods assume that the environment is static and the invocation of the services is deterministic. In ubiquitous robotics, services composition is a challenging issue when the execution environment and services are dynamic and the knowledge about their state and context is uncertain. The services composition requires taking into account the parameters of quality of service (QoS) to adapt the composed service to context of the user and the environment, in particular, dealing with failures such as: service invocation failures, network disconnection, sensor failures, context change due to mobility of objects (robots, sensors, etc.), service discovery failures and service execution failures. In this paper, we present a framework which gives ubiquitous robotic system the ability to dynamically compose and deliver ubiquitous services, and to monitor their execution. The main motivation behind the use of services composition is to decrease time and costs to develop integrated complex applications using robots by transforming them from a single task issuer to smart services provider and human companion, without rebuilding each time the robotic system. To address these new challenges, we propose in this paper a new framework for services composition and monitoring, including QoS estimation and Bayesian learning model to deal with the dynamic and uncertain nature of the environment. This framework includes three levels: abstract plan construction, plan execution, and services discovery and re-composition. This approach is tested under USARSim simulator on a prototype of ubiquitous robotic services for assisting an elderly person at home. The obtained results from extensive tests demonstrate clearly the feasibility and efficiency of our approach.

The wireless sensor network is characterized by its density in number of deployed sensors. In reality, sensors in the same region collect and forward, in general, the same information. In order to conserve nodes energy, it is necessary to keep active a set of nodes while others can sleep. In this paper, we present a new topology control protocol (GECP) for wireless sensor network that extends the network lifetime by minimizing the energy consumption as well as ensuring the network connectivity. GECP exploits the sensor redundancy in the same region by dividing the network into a set of groups and allowing only to one sensor node to be active in each group. The group formation process is based only on exchanged messages between one-hop neighbors.

Because of the lack of a global naming scheme, routing protocols in sensor networks usually use flooding to select paths and deliver data. This process although simple and effective, is very costly in terms of energy consumption, an important design issue in sensor networks routing protocols. Cluster-based routing is one solution to save energy. In this paper, we propose a combination of an improved clustering algorithm and directed diffusion, a well-known data-centric routing paradigm in sensor networks. Our aim is to prolong the network lifetime by modifying passive clustering rules for building/maintaining the topology so an energy load balancing is achieved among the network nodes. We performed extensive computer simulations and showed that our solution outperforms original directed diffusion as well as when it is combined to passive clustering without energy considerations.

To bypass voids in sensor networks, most existing geographic routing protocols tend to route packets along the boundary nodes. Generally, a packet will be either forwarded along a void boundary by the right-hand rule or pushed back to find another route when it encounters the void. The two techniques consume more energy of boundary nodes, drop many packets and may incur data collisions if multiple communication sessions share the same boundary nodes. We propose in this paper an alternative and efficient void avoidance scheme. The proposed on-demand scheme consists of void discovery, void announce and packet rerouting steps. After discovering a void, a sender node inside the void's announce-area reroutes all data packets to get around the void in advance by selecting one appropriate forwarding side. The double objective of our scheme is to prevent data packets from traveling along the boundaries of voids and to avoid them the concave zones of voids. By achieving this objective we can reduce the energy consumption of boundary nodes and data collisions in these nodes. We can also reduce the packets rerouting overhead and the number of packets dropped by nodes on the boundaries of voids. Simulation showed the efficiency of our scheme.

Group communication is an abstraction which deals with multicasting a message from a source process to a group of processes. In Group Communication Systems (GCS), causal message ordering is an essential tool to ensure interaction among group members in a consistent way. In this paper, we propose a simple and optimal causal multicast protocol which copes with the dynamically changing groups in mobile environments. The protocol presents an optimal communication overhead without causing inhibition effect in the delivery of messages. The group membership management depends on a simple, yet powerful idea. This original idea consists in considering the join and leave requests as data messages, and then will be ordered with other messages. This makes no need to a coordination phase in the installation of a new view. Our protocol requires minimal resources on mobile hosts and wireless links and scales well with large groups.

Communication voids (i.e. holes) have negative impact on real-time routing protocols. To decrease both routing distance and energy consumption in large-scale sensor networks for real time applications, we propose in this paper a novel routing mechanism to make data packets avoid meeting voids in advance. After discovering boundary nodes of a void, our mechanism calculates and announces the center of the void to all nodes inside a circular area around the void. A sender node inside the announce area, at n-hops far from the boundary nodes, uses this information to obtain the appropriate forwarding region and starts countering the void in advance. For that, the sender node selects its forwarding candidate neighbors according to its already obtained forwarding region. The proposed mechanism is fairly simple to implement and saves sensor network resources.

To avoid the negative impact of void areas on real-time routing efficiency, we propose in this paper an oriented void avoidance scheme for wireless sensor networks. To select the forwarding region around a void (either clockwise or anticlockwise), the proposed scheme is guided by the target location with respect to the center of the void. Our scheme uses the right-hand rule to discover boundary nodes of the void and geometric formulas to obtain the forwarding region of a sender node located at 1-hop near the void. This node reduces its set of forwarding candidate nodes according to its already obtained forwarding region. Proposed approach is simple to implement, economic and could incorporate various other optimizations studies. Simulation showed the importance of this module and gives better performance compared to traditional schemes.

In Group Communication Systems (GCS), causal message ordering is an essential tool to ensure interaction among group members in a consistent way. In several group-based applications, the exchanged information is often diffused to all members. Using a multicast protocol to ensure this broadcast should make the definition of data structures not optimal for this kind of applications. In this paper, we propose a simple and optimal causal broadcast protocol which copes with the dynamically changing groups in mobile environments. The protocol depends on two simple, yet powerful ideas. The first depends on the use of the immediate dependency relationship in the construction of control information, resulting in O(1) message overhead. When the second original idea depends on considering the join and leave requests as data messages. This ensures a consistent perception of the communication done in the group and makes no need to a coordination phase in the installation of a new view.

In this paper, we propose a simple and optimal causal broadcast protocol which copes with the dynamically changing groups in mobile environments. The protocol depends on two simples, yet powerful ideas. The first consists in the use of the immediate dependency relationship (IDR) in the construction of control information (CI), resulting in O(1) message overhead. When the second original idea consists in considering the join/leave requests as data messages. This ensures a consistent perception within a group and makes no need to a coordination phase in the installation of a view.

Ensuring security in embedded systems translates into several design challenges, imposed by the unique features of these systems. These features make the integration of conventional security mechanisms impractical, and require a better understanding of the whole security problem. This paper provides a unified view on security in embedded systems, by introducing first the implied design and architectural challenges. It then surveys and discusses the currently proposed security solutions that address these challenges, drawing from both current practices and emerging research, and identifies some open research problems that represent the most interesting areas of contribution.

The classical definition of a self-stabilizing algorithm assumes generally that there are no faults in the system long enough for the algorithm to stabilize. Such an assumption cannot be applied to ad hoc mobile networks characterized by their highly dynamic topology. In this paper, we propose a self-stabilizing leader election algorithm that can tolerate multiple concurrent topological changes. By introducing the time-interval-based computation concept, the algorithm ensures that a network partition can within a finite time converge to a legitimate state even if topological changes occur during the convergence time. Our simulation results show that our algorithm can ensure that each node has a leader over 99 percent of the time. We also give an upper bound on the frequency at which network components merge to guarantee the convergence.

Abstract Since mobile ad hoc networks (MANETs) are infrastructureless and multi‐hop by nature, transmitting packets from any node to another usually relies on services provided by intermediate nodes. This reliance introduces a new vulnerability; one node could launch aBlack Hole DoS attackby participating in the routing protocol and including itself in routes, then simply dropping packets it receives to forward. Another motivation for dropping packets in self‐organized MANETs is resource preservation. Some solutions for detecting and isolating packet droppers have been recently proposed, but almost all of them employ the promiscuous mode monitoring approach (watchdog (WD)) which suffers from many problems, especially when employing the power control technique. In this paper we propose a novel monitoring approach that overcomes some WD's shortcomings, and improves the efficiency in detection. To overcome false detections due to nodes mobility and channel conditions we propose a Bayesian technique for the judgment, allowing node redemption before judgment. Finally, we suggest a social‐based approach for the detection approval and isolation of guilty nodes. We analyze our solution and asses its performance by simulation. The results illustrate a large improvement of our monitoring solution in detection versus the WD, and an efficiency through our judgment and isolation techniques as well. Copyright © 2007 John Wiley & Sons, Ltd.

In this paper, we define a new T-time Petri net extension model, called Pre-emptive Time Petri Nets with synchronizing transitions (STPTPN) devoted to the specification of multimedia applications. In this model, resource-pre-empting behaviours are modelled using a new mechanism called “pre-emptor hyperarc”, which lets a transition be: “resource strongly-enabled”, “resourceviolated” or “resource-violator”. Further, we consider in the model two additional mechanisms: A time suspension mechanism by using inhibitor arcs associated with stopwatches, and synchronization mechanisms by firing simultaneously a set of transitions, called Rendezvous. Compared to other existing models, our model is provided with an adapted semantic, designed to represent clearly and accurately time requirements, as well as complex resource-pre-empting mechanisms of multimedia systems.

We discuss in this paper an adaptive proxy Pre-fetch scheme, such that the proxy jitter induced when delivering the different media required in a SMIL presentation can be reduced under an acceptable threshold. Taking into account the media characteristics, their time requirements and the synchronization relations too; we show how to compute a semantic based request pattern, to set an adapted scheduling scheme, such that the bandwidth management as well as the SMIL presentation QoS can be improved. First experimental results are shown in order to validate the proposed scheme.

In mobile ad hoc networks, as nodes move freely, network partitions occur frequently, which significantly degrades the performance of data access. Data replication is a promising approach to improve data availability in mobile ad hoc networks. In this paper, we propose a localized hybrid data delivery scheme that combines the push-based and the pull-based approaches. This scheme is constructed by implementing a localized clustering algorithm that constructs groups, in which each node is at most K hops away from a group leader. This construction permits to bind the query access delay by K hops. The localized clustering algorithm dynamically creates groups to adapt to topology changes. It can also predict when the group will partition. So, it can replicate data items on nodes of the future separate group before the partitioning occurs. The proposed localized scheme helps to increase data availability while improving query delay and update cost

We discuss in this paper an adapted proxy working scheme for SMIL presentation delivery and show how to exploit the causal relations, the time requirements as well as the media characteristics, to deduce a semantic based request pattern. The latter is used to efficiently schedule the pre-fetch requests, as well as the send-back delivery, such that the imposed synchronizations constraints can be met while maintaining the proxy jitter under an acceptable threshold. First results towards validating the pre-fetch scheme are reported.

The proliferation of portable computing devices like laptops and PDAs within the past few years has led to an increase in the mobility of such devices and the consequent introduction of problems not encountered with static computing. However, the merging of mobile devices needs a new mechanism to handle the nodes mobility. In this paper, the author has studied and discussed the behavior of IP micromobility protocols for various TCP protocols (Tahoe, Reno, NewReno and Sack). The author chose to compare the most known micromobility protocol (CIP, HAWAII and HMIP) for some versions of the TCP protocols. It was shown that with the same hierarchical topology for the three IP micromobility and the same mobile node movement scenario, HAWAII performs better than HMIP and CIP for most TCP protocols. However, In TCP Tahoe and NewReno, HMIP performs quite similar to HAWAII. In addition, simulations have shown that CIP has the worst behavior with the various TCP protocols.

This paper considers the problem of fault-tolerance and built-in robustness in key agreement for dynamic peer groups. A fault-tolerant key establishment protocol is developed by extending the group Diffie-Hellman key agreement protocol to support asynchronous settings and faulty participants. The protocol uses recent results on failure detection in asynchronous distributed systems. Simulation results show that the key agreement protocol augmented with failure detection increases significantly the number of group members that participate in the computation of the group key while introducing a low message overhead.

Smooth handoff was introduced in mobile IP to overcome this problem of packet loss during handoff. However, smooth handoff causes packets sequence disruption during packet forwarding procedure, which may result in degradation of network performance in higher layer protocol. In this paper, we discuss the impact of receiving out-of-sequence packets by the mobile node on TCP and UDP applications and we propose a technique, which minimizes the arrival of out-of-sequence packets to the mobile node. This technique anticipates forwarding of packets from the current foreign agent to the new one while the mobile node initiates its handoff. To evaluate our solution, we use the unstable time period (UTP) when the packet sequence could be mis-ordered. We show that the unstable period in our solution is very low than the unstable time period in classical smooth handoff. Furthermore, we also show that our solution reduces the considerably out-of-sequence packets generated by smooth handoff.

It is now well recognised that the consensus problem is a fundamental problem when one has to implement fault-tolerant distributed services. We extend the consensus paradigm to asynchronous distributed mobile systems prone to disconnection and process crash failures. The paper, first, shows that a consensus problem between mobile hosts is reducible to two agreement problems (a consensus problem and a group membership problem) between fixed hosts. Then, following an approach investigated by Guerraoui and Schiper (see IEEE Transactions on Software Engineering, vol.27, no.1, p.29-41, 2001), the paper uses a genetic consensus service as a basic building block to construct a modular and simple solution.

Atomic broadcast or non-blocking atomic commitment are classic agreement problems encountered when designing fault-tolerant distributed systems. Specific protocols that solve such agreement problems can be designed based on a common building block, namely the consensus service. Unfortunately, the consensus problem has no deterministic solution in an asynchronous distributed system that is subject to even a single process crash failure. Among the solutions proposed to circumvent this impossibility result, the concept of unreliable failure detectors proposed by Chandra and Toueg is particularly attractive. They have defined a protocol that solves the consensus problem when the assumption that the underlying failure detector belongs to the class /spl square/S holds true. This paper presents an extension of their protocol that allows to solve this fundamental problem in a mobile environment. In such an environment, the problem is more challenging: based on their initial states, a set of mobile hosts must agree on a common decision, despite disconnections, changes of location and failures of mobile/fixed hosts.