1-D Coordinate Based on Local Information for MAC and Routing Issues in WSNs

More and more critical Wireless Sensor Networks (WSNs) applications are emerging. Those applications need reliability and respect of time constraints. The underlying mechanisms such as MAC and routing must handle such requirements. Our approach to the time constraint problem is to bound the hop-count between a node and the sink and the time it takes to do a hop so the end-to-end delay can be bounded and the communications are thus real-time. For reliability purpose we propose to select forwarder nodes depending on how they are connected in the direction of the sink. In order to be able to do so we need a coordinate (or a metric) that gives information on hop-count, that allows to strongly differentiate nodes and gives information on the connectivity of each node keeping in mind the intrinsic constraints of WSWs such as energy consumption, autonomy, etc. Due to the efficiency and scalability of greedy routing in WSNs and the financial cost of GPS chips, Virtual Coordinate Systems (VCSs) for WSNs have been proposed. A category of VCSs is based on the hop-count from the sink, this scheme leads to many nodes having the same coordinate. The main advantage of this system is that the hops number of a packet from a source to the sink is known. Nevertheless, it does not allow to differentiate the nodes with the same hop-count. In this report we propose a novel hop-count-based VCS which aims at classifying the nodes having the same hop-count depending on their connectivity and at differentiating nodes in a 2-hop neighborhood. Those properties make the coordinates, which also can be viewed as a local identifier, a very powerful metric which can be used in WSNs mechanisms.Comment: (2011

RTXP : A Localized Real-Time Mac-Routing Protocol for Wireless Sensor Networks

Protocols developed during the last years for Wireless Sensor Networks (WSNs) are mainly focused on energy efficiency and autonomous mechanisms (e.g. self-organization, self-configuration, etc). Nevertheless, with new WSN applications, appear new QoS requirements such as time constraints. Real-time applications require the packets to be delivered before a known time bound which depends on the application requirements. We particularly focus on applications which consist in alarms sent to the sink node. We propose Real-Time X-layer Protocol (RTXP), a real-time communication protocol. To the best of our knowledge, RTXP is the first MAC and routing real-time communication protocol that is not centralized, but instead relies only on local information. The solution is cross-layer (X-layer) because it allows to control the delays due to MAC and Routing layers interactions. RTXP uses a suited hop-count-based Virtual Coordinate System which allows deterministic medium access and forwarder selection. In this paper we describe the protocol mechanisms. We give theoretical bound on the end-to-end delay and the capacity of the protocol. Intensive simulation results confirm the theoretical predictions and allow to compare with a real-time centralized solution. RTXP is also simulated under harsh radio channel, in this case the radio link introduces probabilistic behavior. Nevertheless, we show that RTXP it performs better than a non-deterministic solution. It thus advocates for the usefulness of designing real-time (deterministic) protocols even for highly unreliable networks such as WSNs

Modeling Local Broadcast Behavior of Wireless Sensor Networks with Timed Automata for Model Checking of WCTT

International audienceWireless Sensor Networks (WSNs) are usually deployed in order to monitor parameters of an area. When an event occurs in the network an alarm is sent to a special node called the sink. In critical real-time applications, when an event is detected, the Worst Case Traversal Time (WCTT) of the message must be bounded. Although real-time protocols for WSNs have been proposed, they are rarely formally verified. The model checking of WSNs is a challenging problem for several reasons. First, WSNs are usually large scale so it induces state space explosion during the verification. Moreover, wireless communications produce a local broadcast behavior which means that a packet is received only by nodes which are in the radio range of the sender. Finally, the radio link is probabilistic. The modeling of those aspects of the wireless link is not straightforward and it has to be done in a way that mitigate the state space explosion problem. In this paper we particularly focus on the modeling of the local broadcast behavior with Timed Automata (TA). We use TA because they have sufficient expressiveness and analysis power in order to check time properties of protocols, as shown in the paper. Three ways of modeling local broadcast with synchronizations of TA are presented. We compare them and show that they produce different state space sizes and execution times during the model checking process. We run several model checking on a simple WSN protocol and we conclude that one model mitigate the state explosion problem better than the others. In the future, the next step will be to enhance this model with the probabilistic aspect of radio communications and to show it remains the best one

Formal Verification of Real-Time Wireless Sensor Networks Protocols with Realistic Radio Links

International audienceMany critical applications which rely on Wireless Sensor Networks (WSNs) are proposed. Forest fire detection, land- slide detection and intrusion detection are some examples. Critical applications require correct behavior, reliability, and the respect of time constraints. Otherwise, if they fail, con- sequences on human life and the environment could be catas- trophic. For this reason, the WSN protocols used in these applications must be formally verified. Unfortunately the radio link is unreliable, it is thus difficult to give hard guar- antees on the temporal behavior of the protocols (on wired systems the link error probability is very low [7], so they are considered reliable). Indeed, a message may experience a very high number of retransmissions. The temporal guaran- tee has thus to be given with a probability that it is achieved. This probability must meet the requirements of the applica- tion. Network protocols have been successfully verified on a given network topology without taking into account unre- liable links. Nevertheless, the probabilistic nature of radio links may change the topology (links which appear and dis- appear). Thus instead of a single topology we have a set of possible topologies, each topology having a probability to exist. In this paper, we propose a method that produces the set of topologies, checks the property on every topology, and gives the probability that the property is verified. This technique is independent from the verification technique, i.e. each topology can be verified using any formal method which can give a "yes" or "no" answer to the question: "Does the model of the protocol respect the property?". In this paper we apply this method on f-MAC [23] pro- tocol. F-MAC is a real-time medium access protocol for WSNs. We use UPPAAL model checker [10] as verification tool. We perform simulations to observe the difference be- tween average and worst case behaviors

On the Reliability of Wireless Sensor Networks Communications

International audienceMore and more Wireless Sensor Networks (WSNs) applica- tions and protocols are proposed. Notably, critical applications, which must meet time and reliability requirements. Works on the real-time ca- pability of WSNs have been proposed [1]. In this paper we propose to study the achievable reliability of WSNs, tacking into account the prob- abilistic nature of the radio link. We define the reliability of a WSN to be the probability that an end-to-end communication is successful (i.e. the packet is received by the sink). We propose a theoretical framework inspired by a reference model [5]. We use the framework to derive the reliability of two types of routing schemes: unicast-based and broadcast- based. We show that in the case of broadcast-based, the sink is a relia- bility bottleneck of the network. We also discuss the impact of the MAC scheme on the reliability

Formal QoS Validation Approach on a Real-Time MAC Protocol for Wireless Sensor Networks

Several wireless sensor network applications are currently popping up, in various domains. Their goal is often to monitor a geographic 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. The network being linear, routing becomes unnecessary. This work proposes a new real-time MAC protocol with realistic assumptions on sensor networks. We present a formal validation of this protocol, and explicit the worst case times for the services offered by the protocol (initialization and alarm transmission using different modes)

Application de Concepts de Routage Géographique au Routage par Gradient: une Étude Qualitative

Poster à l'occasion du colloque CFIP : 4 pagesLes réseaux de capteurs sans fil connaissent un fort engouement ces dernières années, à la fois dans le monde académique et dans le monde industriel, grâce notamment, au grand nombre d'applications envisagées, et à l'apparition de solutions commerciales viables. A cause de leurs contraintes propres, des mécanismes de routage spécifiques ont été proposés. Le routage géographique et le routage par gradient sont des candidats intéressants pour les réseaux de capteurs, puisqu'ils ne requièrent qu'une phase d'auto-organisation minimale. Dans cet article, nous montrons que les concepts du routage géographique sont applicables au routage par gradient. Nous mettons en évidence une faille dans la règle de la main droite, et nous proposons une solution sous la forme d'un protocole appliqué au routage géographique et par gradient

WiFly: experimenting with Wireless Sensor Networks and Virtual coordinates

Experimentation is important when designing communication protocols for Wireless Sensor Networks. Lower-layers have a major impact on upper-layer performance, and the complexity of the phenomena can not be entirely captured by analysis or simulation. In this report, we go through the complete process, from designing an energy-efficient self-organizing communication architecture (MAC, routing and application layers) to real-life experimentation roll-outs. The presented communication architecture includes a MAC protocol which avoids building and maintaining neighborhood tables, and a geographically-inspired routing protocol over virtual coordinates. The application consists of a mobile sink interrogating a wireless sensor network based on the requests issued by a disconnected base station. After the design process of this architecture, we verify it functions correctly by simulation, and we perform a temporal verification. This study is needed to calculate the maximum speed the mobile sink can take. We detail the implementation, and the results of the off-site experimentation (energy consumption at PHY layer, collision probability at MAC layer, and routing). Finally, we report on the real-world deployment where we have mounted the mobile sink node on a radio-controlled airplane

WiFly: experimenting with Wireless Sensor Networks and Virtual coordinates

Experimentation is important when designing communication protocols for Wireless Sensor Networks. Lower-layers have a major impact on upper-layer performance, and the complexity of the phenomena can not be entirely captured by analysis or simulation. In this report, we go through the complete process, from designing an energy-efficient self-organizing communication architecture (MAC, routing and application layers) to real-life experimentation roll-outs. The presented communication architecture includes a MAC protocol which avoids building and maintaining neighborhood tables, and a geographically-inspired routing protocol over virtual coordinates. The application consists of a mobile sink interrogating a wireless sensor network based on the requests issued by a disconnected base station. After the design process of this architecture, we verify it functions correctly by simulation, and we perform a temporal verification. This study is needed to calculate the maximum speed the mobile sink can take. We detail the implementation, and the results of the off-site experimentation (energy consumption at PHY layer, collision probability at MAC layer, and routing). Finally, we report on the real-world deployment where we have mounted the mobile sink node on a radio-controlled airplane

Mécanisme de compétition pour allocation localisée de ressources multiples

Session Réseaux de capteursInternational audienceL'endormissement cyclique permet l'élaboration de protocoles d'accès au canal peu consommants en énergie pour des réseaux de noeuds limités en ressources et devant opérer durant plusieurs années.Cependant le gain en durée de vie que confèrent ces protocoles est obtenu au détriment de la capacité du réseau et de son adaptabilité aux bouffées de trafic. Les mécanismes existants de gestion de la compétition ne permettent en effet pas d'élire et ordonnancer plusieurs transmissions de façon dynamique et localisée. Dans ce travail, nous considérons un médium sub-divisé en plusieurs canaux logiques (obtenus par exemple par un partage en temps/fréquence du medium de communication) et nous proposons d'allouer dynamiquement ces différentes ressources aux noeuds désirant émettre. Dans cette optique, nous proposons un mécanisme localisé de compéti- tion/ordonnancement, appelé Extended Slot Selection (ESS), qui permet l'allocation dynamique d'une pluralité de canaux logiques aux différentes paires émetteurs/récepteurs