Stéphane Ubéda

Dual-mode real-time MAC protocol for wireless sensor networks: a validation/simulation approach

Several wireless sensor network applications are currently appearing in various domains. Their goal is often to monitor a geographical area. When a sensor detects a monitored event, it informs a sink node using alarm messages. The area surveillance application needs to react to such an event with a finite, bounded and known delay: these are real-time constraints. This work proposes a real-time MAC protocol with realistic assumptions for a random linear network, where sensors are deployed randomly along a line. We present a formal validation of this protocol both for initialization and run-time, and present simulation results on a realistic scenario.

QoS constrained wireless LAN optimization within a multiobjective framework

Wireless LANs have experienced great success in the past five years. This technology has been quickly adopted in private and public areas to provide convenient network access. The fast pace of development has often induced an uncoordinated deployment strategy where WLAN planning tools have barely been used. This article highlights the difficulty of planning such wireless networks in indoor environments. The first issue that must be faced in WLAN planning is accurate description of the quality of a network based on realistic propagation predictions. The second issue is to implement a search strategy that provides several alternative solutions. Thus, the radio engineer can choose the most promising one among them based on his/her experience and maybe some additional constraints. A description of already proposed planning strategies is given and opens out onto a new multiobjective planning formulation. This formulation evaluates coverage, interference level, and quality of service (in terms of data throughput per user) to measure the quality of a planning solution. A Tabu multiobjective algorithm is then implemented to search for the optimal set of non-dominated planning solutions, and a final selection process extracts the most significant solutions for the end user. This multiobjective QoS-oriented method is illustrated with a practical example that shows the performance of looking for several solutions, each expressing different trade-offs between the planning objectives

Survey and benchmark of stream ciphers for wireless sensor networks

For security applications in wireless sensor networks (WSNs), choosing best algorithms in terms of energy-efficiency and of small-storage requirements is a real challenge because the sensor networks must be autonomous. In [22], the authors have benchmarked on a dedicated platform some block-ciphers using severalmodes of operations and have deduced the best block cipher to use in the context of WSNs. This article proposes to study on a dedicated platform of sensors some stream ciphers. First, we sum-up the security provided by the chosen stream ciphers (especially the ones dedicated to software uses recently proposed in the European Project Ecrypt, workpackage eStream [27]) and presents some implementation tests performed on the platform [16].

Centroid virtual coordinates - A novel near-shortest path routing paradigm

Geographic routing has received increasing attention in the context of Wireless Sensor Networks since it frees the network from the energy-demanding task of building and maintaining a structure. It requires however each node to know its position, which may be a prohibitive assumption for many applications. To this end, some prior work has focused on inferring a nodes location from a set of location-aware anchor nodes. In this work, we free ourselves from positioning techniques and anchor nodes altogether, and introduce and analyze the concept of virtual coordinates. These coordinates are chosen randomly when a node is switched on, and are updated each time the node relays a packet. As this process goes on, the virtual coordinates of the nodes converge to a near-optimal state. When using a greedy geographic approach on top of these coordinates, we show that the number of hops to reach the destination exceeds the shortest path by a few percent only. Moreover, our approach guarantees delivery even when nodes appear/disappear in the network, and under realistic transmission models. We analytically prove the correctness of our protocol. Moreover, extensive simulations are used to show that our position-free solution outperforms existing geographic protocols - such as Greedy-Face-Greedy (GFG) or Greedy Perimeter Stateless Routing (GPSR) - in terms of energy-efficiency, path length and robustness.

Mono- and multiobjective formulations for the indoor wireless LAN planning problem

Wireless LANs (WLANS) experienced great success in the past five years. This technology has been quickly adopted in private and public areas to provide a convenient networking access. The fast pace of development has often induced an uncoordinated deployment strategy where WLAN planning tools have been barely used. This article highlights the difficulty of planning such wireless networks for indoor environments. The first issue the WLAN planning problem has to face is to accurately describe the quality of a network, based on realistic propagation predictions. The second issue is to implement a search strategy that provides efficient deployment strategies. This article is introduced by a description of previously proposed planning strategies. Their study opens out onto a problem formulation that accounts for coverage, interference level and quality of service (in terms of data throughput per user). This formulation is then introduced as either a mono- or a multiobjective (MO) optimization problem. In the first case, we propose to solve the mono-objective problem with a Tabu search metaheuristic minimizing a weighted sum of the planning criteria. Then, we compare the outcome of this strategy to the results of our previously proposed MO Tabu search strategy. We highlight the fact that efficient solutions are obtained quickly with the mono-objective approach if an appropriate set of weighting coefficients of the evaluation function is chosen. The main issue of mono-objective search is to determine these coefficients. It is a delicate task that often needs several runs of the algorithm. MO search is an interesting alternative heuristic as it directly provides a set of planning solutions that represent several trade-offs between the objectives. Our MO heuristic looks for a set of non-dominated solutions expected to converge to the Pareto front of the problem and selects the most significant ones for the end user. Both QoS-oriented planning methods are illustrated on a realistic environment representing a building floor of about 12600m^2. Results show the assets of both approaches but mainly emphasize the benefit of the MO search strategy that offers several alternative solutions to the radio engineer.

Parallel complexity of the medial axis computation

The main result of this paper shows that the block-based digital medial axis transform can be computed in parallel by a constant number of calls to scan (parallel prefix) operations. This gives time- and/or work-optimal parallel algorithms for the distance-based and the block-based medial axis transform in a wide variety of parallel architectures. In order to design such an algorithm, we first demonstrate that computing a block-based medial axis transform of a binary image reduces to computing the distance based medial axis transform of an oversampling of the image, by proving that their labelings are equivalent.

Computing the medial axis transform in parallel with eight scan operations

The main result of this paper shows that the block-based digital medial axis transform can be computed in parallel by a constant number of calls to scan (parallel prefix) operations. This gives time- and/or work-optimal parallel implementations for the distance-based and the block-based medial axis transform in a wide variety of parallel architectures. Since only eight scan operations plus a dozen local operations are performed, the algorithm is very easy to program and use. The originality of our approach is the use of the notion of a derived grid and the oversampling of the image in order to reduce the computation of the block-based medial axis transform in the original grid to the much easier task of computing the distance based medial axis transform of the oversampling of the image on the derived grid.

A trust protocol for community collaboration

In ambient environments, new security challenges that are not adequately addressed by existing security models appear. In such context, intelligent communication devices participate to spontaneous and self-organized networks where unexpected interactions with unknown devices take place. Without centralized organization, security turns in a risk management problem.

Identity-Based Cryptosystems for Enhanced Deployment of OSGi Bundles

The OSGi platform is designed to make Java software extensible at runtime. This undeniably presents a great interest in several domains like embedded platforms or enterprise application servers. However, securing the deployment of the OSGi components, or bundles, proves to be a major challenge. The current approach consists in digitally signing the bundles and certifying the signature through a public key infrastructure (PKI). We propose to replace this technology with an identity-based cryptosystem, which provides both better performances and simplified key management. We present an infrastructure for initialization and use of identity-based cryptography, and define the digital signature of bundles using such a cryptographic scheme. Based on our implementation, we provide a comparison between classical PKI management and identity-based PKI management. The proposed approach proves to support radical improvement in the key management process, especially in strongly asymmetric system such as OSGi-based home gateway, where a few providers publish services for millions of potential users.

Cryptographic protocol to establish trusted history of interactions

In the context of ambient networks, this article describes a cryptographic protocol called Common History Extraction (CHE) protocol implementing a trust management framework. All the nodes are supposed to share the same cryptographic algorithms and protocols. An entity called imprinting station provides them with two pairs of public/private keys derived from their identities. Also, two strange nodes wanting to initiate an interaction have to build a seed of trust. The trust between two nodes is based on a mutual proof of previous common met nodes.