Wireless Sensor Networks

Covers UNIVERSIDAD POLITÉCNICA DE MADRID ESCUELA TÉCNICA SUPERIOR DE INGENIEROS DE TELECOMUNICACIÓN METHODOLOGY FOR IMPLEMENTATION OF SYNCHRONIZATION STRATEGIES FOR WIRELESS SENSOR NETWORKS DOCTORAL THESIS Francisco Tirado Andrés TELECOMMUNICATION ENGINEER 2020 Cover d etai ls UNIVERSIDAD POLITÉCNICA DE MADRID ESCUELA TÉCNICA SUPERIOR DE INGENIEROS DE TELECOMUNICACIÓN METHODOLOGY FOR IMPLEMENTATION OF SYNCHRONIZATION STRATEGIES FOR WIRELESS SENSOR NETWORKS A dissertation by FRANCISCO TIRADO ANDRÉS under the supervision of DR. ALVARO ARAUJO PINTO submitted in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY by Universidad Politécnica de Madrid Madrid, 2020 iii How to cite How to cite this work: F. Tirado-Andrés. “Methodology for implementation of synchronization strategies for wireless sensor networks” Ph. D dissertation. Escuela Técnica Superior de Ingenieros de Telecomunicación (ETSIT), Universidad Politécnica de Madrid, Spain, 2020. Correspondence: [email protected] This work was performed in: B105 Electronic Systems Lab b105.upm.es Escuela Técnica Superior de Ingenieros de Telecomunicación www.etsit.upm.es Universidad Politécnica de Madrid www.upm.es This work is licensed under a Creative Commons Attribution 4.0 (CC BY) International License. Any further distribution of this work must maintain the attribution to the author and the title of the work iv Summary Entre las dificultades se esconde la oportunidad Albert Einstein Over the last few years, we have seen and are experiencing how wireless communication devices are changing from being something exceptional to being commonplace in our daily lives. Advances in wireless communications, the miniaturization of electronic devices and the increasing capabilities of these devices have made wireless networks a very common resource in the industry and consumer electronics. Wireless Sensor Networks (WSNs) are spatially distributed sensors that monitor physical or environmental conditions and share their data over the network. WSNs are also characterized by the limitation of their resources as the need for low power consumption as they usually run on batteries, the need for low cost, or its low data throughput, among others. That is why in WSNs the resolution of some problems already solved in wired networks becomes a challenge, such as the implementation of time synchronization strategies. This time synchronization is necessary for the successful operation of many applications based on WSNs, but each node is independent and has its own clock source. Therefore, it is not enough to synchronize the clocks of the different nodes at the beginning of the application because many factors can cause them to be out of sync, for example, the frequency of oscillation of the clock sources may be slightly different, temperature changes, battery status, etc. Besides, the level of demand on time synchronization in WSNs depends on the type of application. There is a wide variety of applications in WSNs: applications that attempt to merge data from all nodes to obtain an overall picture of the scenario, applications that seek to develop communication strategies by making use of the medium access control layer, or applications that need to coordinate actions among all nodes to carry out cooperative tasks. Each application may have different requirements in terms of time synchronization that may range from very lax requirements, where several seconds offsets are allowed, too much more demanding requirements. It should also be noted that each developer may use a different hardware platform with different resources. When developers of applications for WSNs want to implement a synchronization strategy for their system they face all the above issues when making their decision. Today this decision was made by developing their own synchronization protocol or by adapting an already created protocol to their needs. After the state-of-the-art review carried out, it has been proven that no methodology helps the developer to decide to choose a synchronization protocol for WSNs that is adapted to the specific needs of each case. That is why this Doctoral Thesis has been developed in order to implement a methodology for the implementation of synchronization strategies for wireless sensor networks. v Taxonomy has been developed to organize and generate a structure that accommodates not only existing synchronization strategies but also those that may appear in the future. The development of this specific taxonomy has been done to facilitate and assist in the development of the methodology. The taxonomy is based on nine groups where each synchronization strategy must be cataloged. The methodology is based on a flow chart that guides the user through the ten most important themes and parameters regarding time synchronization in WSNs. These blocks to analyze are objective, power consumption, monetary cost, security, network topology, message distribution, accuracy, stability, and software, and hardware abstraction. Each of the blocks has a level scale that allows its comparison and its graphic representation. With this methodology, the aim is that any user, regardless of their level of knowledge in the subject of time synchronization in WSNs, has the possibility of selecting a synchronization strategy that adapts to their specific needs and requirements, reducing the time of their selection. In addition to the methodology, we also present a Time Synchronization Search Tool (TSST). This tool is composed of several blocks: the administration block, the user block and the database that interrelates both. In the administration block, the experts in synchronization strategies parameterize each strategy before incorporating it into the database. In the user block, users looking for a synchronization strategy that suits their needs to make use of a parameterized filter. This selection filter shows all those synchronization strategies that meet the requirements and needs that the user has imposed. The tool displays the results in table form and graphic form making the display and comparison of the strategies faster. Once the methodology and the tool have been developed, it is necessary to validate them to verify their correct operation. The validation has been done through two use cases with different levels of exigency concerning time synchronization. The applications chosen as use cases are structure health monitoring and environmental monitoring. For the monitoring of structures, a theoretical study and simulations have been carried out with real data on how time synchronization affects the identification of the frequencies and modal forms of the structures. Also, tests have been carried out on a bridge in northern Spain. In the application of environmental monitoring, a study and real tests have been conducted on the behavior of different clock sources in the presence of temperature changes. This has allowed us to see how decision making in the early stages of design can help us to minimize the problem of time synchronization in WSNs. After the good results obtained in the different studies, simulations, and experiments performed in the validation, it is concluded that the developed methodology works and helps in the selection of time-synchronization strategies for WSNs. vi Resumen Nada tarda tanto como aquello que no se empieza Alain Durante los últimos años hemos visto y estamos experimentando como los dispositivos con comunicaciones inalámbricas pasan de ser algo excepcional a ser algo habitual en nuestra vida cotidiana. Los avances en comunicaciones inalámbricas, la miniaturización de los dispositivos electrónicos y el aumento de las capacidades de estos dispositivos han hecho que las redes inalámbricas se conviertan en un recurso muy común en la industria y la electrónica de consumo. El subgrupo de las redes de sensores inalámbricas, en inglés Wireless Sensor Networks (WSNs) se caracterizan por ser sensores espacialmente distribuidos que monitorizan condiciones físicas o ambientales y comparten sus datos a través de la red. Las WSNs también se caracterizan por la limitación de sus recursos como necesidad de bajo consumo ya que suelen funcionar con baterías, la necesidad de que sean de bajo coste, o su baja tasa de transmisión de datos entre otros. Es por ello por lo que en WSN la resolución de algunos problemas ya resueltos en redes cableadas se convierte en un reto, como la implementación de estrategias de sincronización temporal. Esta sincronización temporal es necesaria para el correcto funcionamiento de muchas aplicaciones basadas en WSNs, pero cada nodo es independiente y tiene su propia fuente de reloj. Por tanto, no basta con sincronizar los relojes de los diferentes nodos al inicio de la aplicación porque son muchos los factores que pueden hacer que se desincronicen, por ejemplo, la frecuencia de oscilación de las fuentes de reloj puede ser ligeramente diferente, los cambios de temperatura, el estado de la batería, etc. A esto hay que añadir problemas heterogéneos de diferente naturaleza que van desde utilizar diferentes plataformas hardware con diferentes recursos hasta el desarrollo de diferentes aplicaciones con diferentes requisitos. Cuando los desarrolladores de aplicaciones para WSNs desean implementar una estrategia de sincronización para su sistema se enfrentan a todos los problemas anteriores a la hora de tomar su decisión. Hoy en día esta decisión pasaba por desarrollar su propio protocolo de sincronización o por adaptar un protocolo ya creado a sus necesidades.

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