On the performance limits of slotted CSMA/CA in IEEE 802.15.4 for broadcast transmissions in wireless sensor networks

The IEEE 802.15.4 has been adopted as a communication protocol standard for Low-Rate Wireless Private Area Networks (LRWPANs). While it appears as a promising candidate solution for Wireless Sensor Networks (WSNs), its adequacy must be carefully evaluated. In this paper, we analyze the performance limits of the slotted CSMA/CA medium access control (MAC) mechanism in the beacon-enabled mode for broadcast transmissions in WSNs. The motivation for evaluating the beacon-enabled mode is due to its flexibility and potential for WSN applications as compared to the non-beacon enabled mode. Our analysis is based on an accurate simulation model of the slotted CSMA/CA mechanism on top of a realistic physical layer, with respect to the IEEE 802.15.4 standard specification. The performance of the slotted CSMA/CA is evaluated and analyzed for different network settings to understand the impact of the protocol attributes (superframe order, beacon order and backoff exponent), the number of nodes and the data frame size on the network performance, namely in terms of throughput (S), average delay (D) and probability of success (Ps). We also analytically evaluate the impact of the slotted CSMA/CA overheads on the saturation throughput. We introduce the concept of utility (U) as a combination of two or more metrics, to determine the best offered load range for an optimal behavior of the network. We show that the optimal network performance using slotted CSMA/CA occurs in the range of 35% to 60% with respect to an utility function proportional to the network throughput (S) divided by the average delay (D).

On the use of the ZigBee protocol for wireless sensor networks

This project was developed within the ART-WiSe framework of the IPP-HURRAY group (http://www.hurray.isep.ipp.pt), at the Polytechnic Institute of Porto (http://www.ipp.pt). The ART-WiSe ��� Architecture for Real-Time communications in Wireless Sensor networks ��� framework (http://www.hurray.isep.ipp.pt/art-wise) aims at providing new communication architectures and mechanisms to improve the timing performance of Wireless Sensor Networks (WSNs). The architecture is based on a two-tiered protocol structure, relying on existing standard communication protocols, namely IEEE 802.15.4 (Physical and Data Link Layers) and ZigBee (Network and Application Layers) for Tier 1 and IEEE 802.11 for Tier 2, which serves as a high-speed backbone for Tier 1 without energy consumption restrictions. Within this trend, an application test-bed is being developed with the objectives of implementing, assessing and validating the ART-WiSe architecture. Particularly for the ZigBee protocol case; even though there is a strong commercial lobby from the ZigBee Alliance (http://www.zigbee.org), there is neither an open source available to the community for this moment nor publications on its adequateness for larger-scale WSN applications. This project aims at fulfilling these gaps by providing: a deep analysis of the ZigBee Specification, mainly addressing the Network Layer and particularly its routing mechanisms; an identification of the ambiguities and open issues existent in the ZigBee protocol standard; the proposal of solutions to the previously referred problems; an implementation of a subset of the ZigBee Network Layer, namely the association procedure and the tree routing on our technological platform (MICAz motes, TinyOS operating system and nesC programming language) and an experimental evaluation of that routing mechanism for WSNs.

Collision-free prioritized medium access control in wireless networks with hidden nodes

We propose a collision-free medium access control (MAC) protocol, which implements static-priority scheduling and works in the presence of hidden nodes. The MAC protocol allows multiple masters and is fully distributed; it is an adaptation to a wireless channel of the dominance protocol used in the CAN bus. But unlike that protocol, our protocol does not require a node having the ability to sense the channel while transmitting to the channel. Our protocol is collision-free even in the presence of hidden nodes and it achieves this without synchronized clocks or out-of-band busy tones. In addition, the protocol is designed to ensure that many non-interfering nodes can transmit in parallel and it functions for both broadcast and unicast transmissions.

Beacon scheduling in cluster-tree IEEE 802.15.4/ZigBee wireless sensor networks

The recently standardized IEEE 802.15.4/Zigbee protocol stack offers great potentials for ubiquitous and pervasive computing, namely for Wireless Sensor Networks (WSNs). However, there are still some open and ambiguous issues that turn its practical use a challenging task. One of those issues is how to build a synchronized multi-hop cluster-tree network, which is quite suitable for QoS support in WSNs. In fact, the current IEEE 802.15.4/Zigbee specifications restrict the synchronization in the beacon-enabled mode (by the generation of periodic beacon frames) to star-based networks, while it supports multi-hop networking using the peer-to-peer mesh topology, but with no synchronization. Even though both specifications mention the possible use of cluster-tree topologies, which combine multi-hop and synchronization features, the description on how to effectively construct such a network topology is missing. This report tackles this problem, unveils the ambiguities regarding the use of the cluster-tree topology and proposes two collisionfree beacon frame scheduling schemes.

Repeater-based hybrid wired/wireless PROFIBUS network simulator

This technical report describes the Repeater-Based Hybrid Wired/Wireless PROFIBUS Network Simulator that implements a simulation model of the repeater-based approach. This approach defines the mechanism to extend the PROFIBUS protocol to supprot wireless communication, in which the interconnection of the wired and wireless segments is done by a intermediate system operating at Physical Layer, as repeater.

A wireless sensor network platform for structural health monitoring: enabling accurate and synchronized measurements through COTS+custom-based design

Structural health monitoring has long been identified as a prominent application of Wireless Sensor Networks (WSNs), as traditional wired-based solutions present some inherent limitations such as installation/maintenance cost, scalability and visual impact. Nevertheless, there is a lack of ready-to-use and off-the-shelf WSN technologies that are able to fulfill some most demanding requirements of these applications, which can span from critical physical infrastructures (e.g. bridges, tunnels, mines, energy grid) to historical buildings or even industrial machinery and vehicles. Low-power and low-cost yet extremely sensitive and accurate accelerometer and signal acquisition hardware and stringent time synchronization of all sensors data are just examples of the requirements imposed by most of these applications. This paper presents a prototype system for health monitoring of civil engineering structures that has been jointly conceived by a team of civil, and electrical and computer engineers. It merges the benefits of standard and off-the-shelf (COTS) hardware and communication technologies with a minimum set of custom-designed signal acquisition hardware that is mandatory to fulfill all application requirements.

Pose Estimation using a C-Arm Fluoroscope

Os m��todos cl��nicos que s��o realizados com recurso a tecnologias de imagiologia t��m registado um aumento de popularidade nas ��ltimas duas d��cadas. Os procedimentos tradicionais usados em cirurgia t��m sido substitu��dos por m��todos minimamente invasivos de forma a conseguir diminuir os custos associados e aperfei��oar factores relacionados com a produtividade. Procedimentos cl��nicos modernos como a broncoscopia e a cardiologia s��o caracterizados por se focarem na minimiza����o de ac����es invasivas, com os arcos em ���C��� a adoptarem um papel relevante nesta ��rea. Apesar de o arco em ���C��� ser uma tecnologia amplamente utilizada no aux��lio da navega����o em interven����es minimamente invasivas, este falha na qualidade da informa����o fornecida ao cirurgi��o. A informa����o obtida em duas dimens��es n��o �� suficiente para proporcionar uma compreens��o total da localiza����o tridimensional da regi��o de interesse, revelando-se como uma tarefa essencial o estabelecimento de um m��todo que permita a aquisi����o de informa����o tridimensional. O primeiro passo para alcan��ar este objectivo foi dado ao definir um m��todo que permite a estimativa da posi����o e orienta����o de um objecto em rela����o ao arco em ���C���. De forma a realizar os testes com o arco em ���C���, a geometria deste teve que ser inicialmente definida e a calibra����o do sistema feita. O trabalho desenvolvido e apresentado nesta tese foca-se num m��todo que provou ser suficientemente sustent��vel e eficiente para se estabelecer como um ponto de partida no caminho para alcan��ar o objectivo principal: o desenvolvimento de uma t��cnica que permita o aperfei��oamento da qualidade da informa����o adquirida com o arco em ���C��� durante uma interven����o cl��nica.

WeSenseNET - Plataforma de Visualiza����o, Gest��o Remota e Agrega����o M��ltipla de Dados

Nos ��ltimos anos, o avan��o da tecnologia e a miniaturiza����o de diversos componentes de electr��nica associados a novos conceitos t��m permitido nascer novas ideias e projectos, que at�� h�� alguns anos n��o passariam de fic����o cient��fica. Talvez o exemplo mais acabado seja actualmente o smartphone, um pequeno bloco de hardware e software, com capacidade de processamento que ultrapassa v��rias vezes o dos computadores com uma d��zia de anos. Estas capacidades t��m sido utilizadas em comunica����es, blocos de notas, agendas e at�� entretenimento. No entanto, podem ser reutilizadas para ajudar a resolver algumas limita����es/constrangimentos da actualidade. Dentro destes destacam-se a gest��o de recursos escassos. Com efeito, o consumo de energia el��ctrica tem aumentado como consequ��ncia directa do desenvolvimento global e aumento do n��mero de aparelhos el��ctricos. Uma percentagem significativa de energia el��ctrica tem sido produzida atrav��s de recursos n��o-renov��veis de energia. No entanto, a depend��ncia energ��tica, associada �� subida de pre��os e a redu����o das emiss��es de gases do efeito estufa, estimula o desenvolvimento de novas solu����es que permitam lidar com esta situa����o. O desempenho energ��tico por sua vez depende n��o s�� das caracter��sticas da estrutura, mas tamb��m do comportamento do utilizador. O desempenho energ��tico dos edif��cios �� muito importante, uma vez que os respectivos consumos s��o respons��veis por mais de metade do total da energia produzida. Desta forma, a fim de alcan��ar um melhor desempenho �� importante n��o s�� considerar o desempenho de estrutura, mas tamb��m monitorizar o comportamento do utilizador. Esta ��ltima quest��o coloca v��rias limita����es, uma vez que depende muito do tipo de utilizador. Um dos conceitos actuais emergentes s��o as chamadas redes de sensores sem fio. Com esta tecnologia, pequenos m��dulos podem ser desenvolvidos com muitas possibilidades de conectividade, com elevado poder de processamento e com grande autonomia, sem serem excessivamente caros. Isto proporciona os meios para implementar v��rios dispositivos em toda a instala����o, para recolher uma variedade de dados, sendo posteriormente armazenados num servidor. Os blocos fundamentais da infra-estrutura de sensores do projecto foram concebidos na Evoleo Technologies em simult��neo com o decorrer do est��gio. Estes blocos recolhem dados espec��ficos na instala����o, e periodicamente enviam para o servidor central os valores recolhidos, onde s��o armazenados e colocados �� disposi����o do utilizador. Os dados recolhidos podem ent��o ser apresentados ao utilizador, proporcionando um registo de consumo de energia associado a um dado per��odo de tempo. Uma vez que todos os dados s��o armazenados no servidor, podem ser efectuados estudos para determinar o uso t��pico, poss��veis problemas em aparelhos, a qualidade da energia el��ctrica, etc., permitindo determinar onde a energia est�� a ser eventualmente desperdi��ada e fornecendo dados ao utilizador para que este possa proceder a altera����es, tendo por base dados recolhidos num dado per��odo. O objectivo principal deste trabalho passa por estabelecer a liga����o entre o n��vel m��quina e o n��vel de utilizador, isto ��, uma plataforma de interac����o entre dispositivos e administrador da instala����o. Fornecer os dados de uma forma f��cil e sem necessidade de instala����o de software espec��fico em cada dispositivo que se pretenda utilizar para monitorizar foi uma das principais preocupa����es das fases de concep����o do projecto.

Fast mobility support in low-power wireless networks: smart-HOP over RPL/6LoWPAN

With the emergence of low-power wireless hardware new ways of communication were needed. In order to standardize the communication between these low powered devices the Internet Engineering Task Force (IETF) released the 6LoWPAN stand- ard that acts as an additional layer for making the IPv6 link layer suitable for the lower-power and lossy networks. In the same way, IPv6 Routing Protocol for Low- Power and Lossy Networks (RPL) has been proposed by the IETF Routing Over Low power and Lossy networks (ROLL) Working Group as a standard routing protocol for IPv6 routing in low-power wireless sensor networks. The research performed in this thesis uses these technologies to implement a mobility process. Mobility management is a fundamental yet challenging area in low-power wireless networks. There are applications that require mobile nodes to exchange data with a xed infrastructure with quality-of-service guarantees. A prime example of these applications is the monitoring of patients in real-time. In these scenarios, broadcast- ing data to all access points (APs) within range may not be a valid option due to the energy consumption, data storage and complexity requirements. An alternative and e cient option is to allow mobile nodes to perform hand-o s. Hand-o mechanisms have been well studied in cellular and ad-hoc networks. However, low-power wireless networks pose a new set of challenges. On one hand, simpler radios and constrained resources ask for simpler hand-o schemes. On the other hand, the shorter coverage and higher variability of low-power links require a careful tuning of the hand-o parameters. In this work, we tackle the problem of integrating smart-HOP within a standard protocol, speci cally RPL. The simulation results in Cooja indicate that the pro- posed scheme minimizes the hand-o delay and the total network overhead. The standard RPL protocol is simply unable to provide a reliable mobility support sim- ilar to other COTS technologies. Instead, they support joining and leaving of nodes, with very low responsiveness in the existence of physical mobility.

A survey of wireless technologies coexistence in WBAN: analysis and open research issues

Wireless Body Area Network (WBAN) is the most convenient, cost-effective, accurate, and non-invasive technology for e-health monitoring. The performance of WBAN may be disturbed when coexisting with other wireless networks. Accordingly, this paper provides a comprehensive study and in-depth analysis of coexistence issues and interference mitigation solutions in WBAN technologies. A thorough survey of state-of-the art research in WBAN coexistence issues is conducted. The survey classified, discussed, and compared the studies according to the parameters used to analyze the coexistence problem. Solutions suggested by the studies are then classified according to the followed techniques and concomitant shortcomings are identified. Moreover, the coexistence problem in WBAN technologies is mathematically analyzed and formulas are derived for the probability of successful channel access for different wireless technologies with the coexistence of an interfering network. Finally, extensive simulations are conducted using OPNET with several real-life scenarios to evaluate the impact of coexistence interference on different WBAN technologies. In particular, three main WBAN wireless technologies are considered: IEEE 802.15.6, IEEE 802.15.4, and low-power WiFi. The mathematical analysis and the simulation results are discussed and the impact of interfering network on the different wireless technologies is compared and analyzed. The results show that an interfering network (e.g., standard WiFi) has an impact on the performance of WBAN and may disrupt its operation. In addition, using low-power WiFi for WBANs is investigated and proved to be a feasible option compared to other wireless technologies.

Cloud-assisted Wireless Body Area Networks

Wireless Body Area Networks (WBANs) have emerged as a promising technology for medical and non-medical applications. WBANs consist of a number of miniaturized, portable, and autonomous sensor nodes that are used for long-term health monitoring of patients. These sensor nodes continuously collect information of patients, which are used for ubiquitous health monitoring. In addition, WBANs may be used for managing catastrophic events and increasing the effectiveness and performance of rescue forces. The huge amount of data collected by WBAN nodes demands scalable, on-demand, powerful, and secure storage and processing infrastructure. Cloud computing is expected to play a significant role in achieving the aforementioned objectives. The cloud computing environment links different devices ranging from miniaturized sensor nodes to high-performance supercomputers for delivering people-centric and context-centric services to the individuals and industries. The possible integration of WBANs with cloud computing (WBAN-cloud) will introduce viable and hybrid platform that must be able to process the huge amount of data collected from multiple WBANs. This WBAN-cloud will enable users (including physicians and nurses) to globally access the processing and storage infrastructure at competitive costs. Because WBANs forward useful and life-critical information to the cloud ��� which may operate in distributed and hostile environments, novel security mechanisms are required to prevent malicious interactions to the storage infrastructure. Both the cloud providers and the users must take strong security measures to protect the storage infrastructure.

Reliable and Fast Hand-Offs in Low-Power Wireless Networks

Hand-off (or hand-over), the process where mobile nodes select the best access point available to transfer data, has been well studied in wireless networks. The performance of a hand-off process depends on the specific characteristics of the wireless links. In the case of low-power wireless networks, hand-off decisions must be carefully taken by considering the unique properties of inexpensive low-power radios. This paper addresses the design, implementation and evaluation of smart-HOP, a hand-off mechanism tailored for low-power wireless networks. This work has three main contributions. First, it formulates the hard hand-off process for low-power networks (such as typical wireless sensor networks - WSNs) with a probabilistic model, to investigate the impact of the most relevant channel parameters through an analytical approach. Second, it confirms the probabilistic model through simulation and further elaborates on the impact of several hand-off parameters. Third, it fine-tunes the most relevant hand-off parameters via an extended set of experiments, in a realistic experimental scenario. The evaluation shows that smart-HOP performs well in the transitional region while achieving more than 98 percent relative delivery ratio and hand-off delays in the order of a few tens of a milliseconds.

Celorico Wireless

Mestrado em Engenharia Inform��tica

Une langue ��trag��re, un ordinateur, une image: c'est simple comme bonjour!

Este artigo surgiu na sequ��ncia de um atelier ���Une langue ��trang��re, un ordinateur, une image: c���est simple comme bonjour!���, desenvolvido no ��mbito do XXI Congresso da Associa����o Portuguesa dos Professores de Franc��s, Images et imaginaires pour agir. Teve como prop��sito divulgar, experimentar e refletir sobre recursos digitais que podem dar um bom contributo ao processo de ensino e aprendizagem do Franc��s L��ngua Estrangeira (FLE). Evidencia-se o poder da imagem na constru����o do conhecimento, desafiando a criatividade e novos modos de ensinar a aprender. Verificou-se que os professores se interessaram pelas ferramentas digitais e evidenciaram a sua import��ncia e a sua aplicabilidade nos contextos educativos. Neste sentido, o artigo divulga ferramentas inform��ticas focadas no desenvolvimento da oralidade/leitura/escrita do franc��s l��ngua estrangeira, refere boas pr��ticas de utiliza����o em contexto de sala de aula, constituindo uma contribui����o para a renova����o da escola.