Inaccessibility in wireless sensor networks

Tese de mestrado em Engenharia Informática, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2013As redes sem fios têm sido encaradas como as redes de comunicação do futuro, fornecendo capacidades de comunicação onde os cabos não podem de ser utilizados. As tecnologias sem fio permitem flexibilidade e mobilidade na rede como também reduzir o tamanho, peso e consumo energético (SWaP) dos dispositivos de comunicação. A norma IEEE 802.15.4 foi projetada para suportar a especificação de redes de sensores sem fio (WSNs) e redes de sensores e atuadores sem fios (WSANs), e a sua utilização está a emergir em ambientes com requisitos de tempo real, tais como o industrial e aeroespacial. A camada de controlo de acesso ao meio (MAC) é o alicerce de controlo dos serviços de comunicação da rede. Distúrbios no funcionamento desta camada podem levar a rede a entrar num estado apelidado de inacessibilidade, este caracteriza-se numa falta temporária de comunicação na rede, embora não se considere que a rede falhou. Exemplos de tais perturbações são ondas electromagnéticas, falhas no circuito de dispositivos sem fios, ou até mesmo obstáculos no caminho de comunicação. Um estudo teórico anterior indica a ocorrência de inacessibilidade como fontes de atraso portanto, falhas no cumprimento de prazos que podem comprometer propriedades de confiabilidade e pontualidade de todo o sistema. Assim, este trabalho tem como objetivo validar que o estudo anterior, utilizando o simulador de rede NS-2. O simulador de rede NS-2 é uma ferramenta amplamente utilizada no suporte a simulação de redes sem fio IEEE 802.15.4. No entanto, descobrimos que não se encontra totalmente em conformidade com a norma IEEE 802.15.4. Com o intuito de efetuar a validação dos modelos de inacessibilidade, novos mecanismos devem ser introduzidos no modelo de simulação referente ao IEEE 802.15.4. Estes melhoramentos compreendem: Suporte para transmissões de tempo real, através da incorporação do mecanismo de acesso livre de contenção (CFP) e do intervalo de tempo de acesso garantido (GTS); Desenvolver as operações de gestão normalizadas não concretizadas no modulo IEEE 802.15.4 presente na versão oficial do NS-2;Adição de novos recursos necessários para a avaliação da rede em condições de erro, mais especificamente, um injetor de faltas, e um módulo de contabilização temporal e energético.Wireless networks are seen as the communication networks of the future, providing communication capabilities where cables are not able to be used. Wireless technologies enable network flexibility and mobility, and reduce size, weight, and power consumption (SWaP) of communication devices. The IEEE 802.15.4 standard was designed to support the specification of wireless sensor networks (WSNs) and wireless sensor and actuator networks (WSANs), where is emerging their utilization within environments with real time requirements, such as industrial and aerospace. The medium access control (MAC) layer is the control foundation of the network communication services. Disturbances in the MAC layer operation may lead to a network inaccessibility scenario, which consists in a temporary absence of network communication although the network is not considered failed. Examples of such disturbances are electromagnetic noise interference, glitches in the wireless device circuitry, or even obstacles in the communication path. A previous theoretical study indicates the occurrence of periods of network inaccessibility as a source of MAC transmission protocol delays which may induce application deadline misses which that compromise the dependability and timeliness properties of the whole networked system. Thus, this work aims to validate that previous study using the network simulator NS-2. The NS-2 simulator is a widely used tool supporting the simulation of IEEE 802.15.4 wireless networks. However, we discovered that its compliance to the IEEE 802.15.4 standard is imperfect. In order to perform the validation of the theoretical characterization of network inaccessibility new mechanisms need to be introduced in the IEEE 802.15.4 simulation model. These improvements comprises: the support for real-time transmissions, through the incorporation of the contention free period (CFP) and of guaranteed time slot (GTS) ; IEEE 802.15.4 standard management operations not implemented in the official NS-2 release; A flexible tool capable of re-create the inaccessibility events and simulate different error conditions on the network, which include the Fault Injector and temporal and energetic analysis tool

Resource management algorithms for real-time wireless sensor networks with applications in cyber-physical systems

Wireless Sensor Networks (WSN) are playing a key role in the efficient operation of Cyber Physical Systems (CPS). They provide cost efficient solutions to current and future CPS re- quirements such as real-time structural awareness, faster event localization, cost reduction due to condition based maintenance rather than periodic maintenance, increased opportunities for real-time preventive or corrective control action and fine grained diagnostic analysis. However, there are several critical challenges in the real world applicability of WSN. The low power, low data rate characteristics of WSNs coupled with constraints such as application specified latency and wireless interference present challenges to their efficient integration in CPSs. The existing state of the art solutions lack methods to address these challenges that impediment the easy integration of WSN in CPS. This dissertation develops efficient resource management algorithms enabling WSNs to perform reliable, real-time, cost efficient monitoring. This research addresses three important problems in resource management in the presence of different constraints such as latency, precedence and wireless interference constraints. Additionally, the dissertation proposes a solution to deploy WSNs based real-time monitoring of critical infrastructure such as electrical overhead transmission lines. Firstly, design and analysis of an energy-aware scheduling algorithm encompassing both computation and communication subsystems in the presence of deadline, precedence and in- terference constraints is presented. The energy-delay tradeoff presented by the energy saving technologies such as Dynamic Voltage Scaling (DVS) and Dynamic modulation Scaling (DMS) is studied and methods to leverage it by way of efficient schedule construction is proposed. Performance results show that the proposed polynomial-time heuristic scheduling algorithm offers comparable energy savings to that of the analytically derived optimal solution. Secondly, design, analysis and evaluation of adaptive online algorithms leveraging run- time variations is presented. Specifically, two widely used medium access control schemes are considered and online algorithms are proposed for each. For one, temporal correlation in sensor measurements is exploited and three heuristics with varying complexities are proposed to perform energy minimization using DMS. For another, an adaptive algorithm is proposed addressing channel and load conditions at a node by influencing the selection of either low energy or low delay transmission option. In both cases, the simulation results show that the proposed schemes provide much better energy savings as compared to the existing algorithms. The third component presents design and evaluation of a WSN based framework to mon- itor a CPS namely, electrical overhead transmission line infrastructure. The cost optimized hybrid hierarchical network architecture is composed of a combination of wired, wireless and cellular technologies. The proposed formulation is generic and addresses constraints such as bandwidth and latency; and real world scenarios such as asymmetric sensor data generation, unreliable wireless link behavior, non-uniform cellular coverage and is suitable for cost minimized incremental future deployment. In conclusion, this dissertation addresses several challenging research questions in the area of resource management in WSNs and their applicability in future CPSs through associated algorithms and analyses. The proposed research opens up new avenues for future research such as energy management through network coding and fault diagnosis for reliable monitoring