Improving the reliability of WiDom in a single broadcast domain

http://sies2009.epfl.ch/WiDom is a previously proposed prioritized medium access control protocol for wireless channels. We present a modification to this protocol in order to improve its reliability. This modification has similarities with cooperative relaying schemes, but, in our protocol, all nodes can relay a carrier wave. The preliminary evaluation shows that, under transmission errors, a significant reduction on the number of failed tournaments can be achieved

Efficient aggregate computations in large-scale dense wireless sensor networks

Tese de doutoramento em InformáticaAssuming a world where we can be surrounded by hundreds or even thousands of inexpensive computing nodes densely deployed, each one with sensing and wireless communication capabilities, the problem of efficiently dealing with the enormous amount of information generated by those nodes emerges as a major challenge. The research in this dissertation addresses this challenge. This research work proves that it is possible to obtain aggregate quantities with a timecomplexity that is independent of the number of nodes, or grows very slowly as the number of nodes increases. This is achieved by co-designing the distributed algorithms for obtaining aggregate quantities and the underlying communication system. This work describes (i) the design and implementation of a prioritized medium access control (MAC) protocol which enforces strict priorities over wireless channels and (ii) the algorithms that allow exploiting this MAC protocol to obtain the minimum (MIN), maximum (MAX) and interpolation of sensor values with a time-complexity that is independent of the number of nodes deployed, whereas other state-of-the-art approaches have a time-complexity that is dependent on the number of nodes. These techniques also enable to efficiently obtain estimates of the number of nodes (COUNT) and the median of the sensor values (MEDIAN). The novel approach proposed to efficiently obtain aggregate quantities in large-scale, dense wireless sensor networks (WSN) is based on the adaptation to wireless media of a MAC protocol, known as dominance/binary countdown, which existed previously only for wired media, and design algorithms that exploit this MAC protocol for efficient data aggregation. Designing and implementing such MAC protocol for wireless media is not trivial. For this reason, a substantial part of this work is focused on the development and implementation of WiDom (short for Wireless Dominance) - a wireless MAC protocol that enables efficient data aggregation in large-scale, dense WSN. An implementation of WiDom is first proposed under the assumption of a fully connected network (a network with a single broadcast domain). This implementation can be exploited to efficiently obtain aggregated quantities. WiDom can also implement static priority scheduling over wireless media. Therefore, a schedulability analysis for WiDom is also proposed. WiDom is then extended to operate in sensor networks where a single transmission cannot reach all nodes, in a network with multiple broadcast domains. These results are significant because often networks of nodes that take sensor readings are designed to be large scale, dense networks and it is exactly for such scenarios that the proposed distributed algorithms for obtaining aggregate quantities excel. The implementation and test of these distributed algorithms in a hardware platform developed shows that aggregate quantities in large-scale, dense wireless sensor systems can be obtained efficientlly.É possível prever um mundo onde estaremos rodeados por centenas ou até mesmo milhares de pequenos nós computacionais densamente instalados. Cada um destes nós será de dimensões muito reduzidas e possui capacidades para obter dados directamente do ambiente através de sensores e transmitir informação via rádio. Frequentemente, este tipo de redes são denominadas de redes de sensores sem fio. Perante tal cenário, o problema de lidar com a considerável quantidade de informação gerada por todos estes nós emerge como um desafio de grande relevância. A investigação apresentada nesta dissertação atenta neste desafio. Este trabalho de investigação prova que é possível obter quantidades agregadas com uma complexidade temporal que é independente do número de nós computacionais envolvidos, ou cresce muito lentamente quando o número de nós aumenta. Isto é conseguido através uma co-concepção dos algoritmos para obter quantidades agregadas e do sistema de comunicação subjacente. Este trabalho descreve (i) a concepção e implementação de um protocolo de acesso ao meio que garante prioridades estáticas em canais de comunicação sem fio e (ii) os algoritmos que permitem tirar partido deste protocolo de acesso ao meio para obter quantidades agregadas como o mínimo (MIN), máximo (MAX) e interpolação de valores obtidos a partir de sensores ambientais com uma complexidade que é independente do número de nós computacionais envolvidos. Estas técnicas também permitem obter, de forma eficiente, estimativas do número de nós (COUNT) e a mediana dos valores dos sensores (MEDIAN). A abordagem inovadora, proposta para obter de forma eficiente quantidades agregadas em redes de sensores sem fio de larga escala, é baseada na adaptação para meios de comunicação sem fio de um protocolo de acesso ao meio anteriormente apenas existente em sistemas cablados, e na concepção de algoritmos que tiram partido deste protocolo para agregação de dados eficiente. A concepção e implementação de tal protocolo de acesso ao meio não é trivial. Por esta razão, uma parte substancial deste trabalho é focada no desenvolvimento e implementação de um protocolo de acesso ao meio que permite agregação de dados eficiente em redes de sensores sem fio densas e de larga escala. Esta implementação é denominada de WiDom. A implementação do WiDom apresentada foi inicialmente desenvolvida assumindo que a rede é totalmente ligada (uma transmisão de um nó alcança todos os outros nós). Esta implementação pode ser explorada para obter quantidades agregadas de forma eficiente. Adicionalmente, o protocolo WiDom pode implementar escalonamento utilizando prioridades fixas, permitindo a proposta de uma análise de resposta temporal. Neste trabalho, o WiDom é também estendido para funcionar em redes onde a transmissão de um nó não pode alcançar todos os outros nós. Os resultados apresentados neste trabalho são relevantes porque as redes de sensores sem fio são frequentemente concebidas para serem densas e de larga escala. É exactamente nestes casos que os algoritmos propostos para obter quantidades agregadas de forma eficiente apresentam maiores vantagens. A implementação e teste destes algoritmos distribuídos numa plataforma especialmente desenvolvida para o efeito demonstra que de facto podem ser obtidas quandidades agregadas de forma eficiente, mesmo em redes de sensores sem fio densas e de larga escala.This research was partially developed at the Real-Time Computing System Research Centre (CISTER), from the School of Engineering of the Polytechnic of Porto (ISEP/IPP

Efficient implementation of a dominance protocol for wireless medium access

Mestrado em Engenharia Electrotécnica e de ComputadoresEmbedded computing systems went through extraordinary evolutions during the past two decades, representing nowadays one of the most promising technologies for improving a wide range of application areas such as energy/resource management, safety, health or entertainment. New sensors and actuators are leading to an unprecedented level of interaction between computing systems and their surrounding physical environment. These embedded computers tend to be networked, often wirelessly, and they are becoming denser, of larger scale and more pervasively deployed. Since the wireless channel is a "natural resource" which must be shared between this large number of embedded computers, the medium access control (MAC) protocol significantly influences the performance of the entire system. In particular, satisfying real-time requirements — something that is needed for a computer to tightly interact with its physical environment — plays an important role. One solution was recently proposed by Pereira, Andersson and Tovar. It was a prioritized and collision-free MAC protocol belonging to a family of protocols called dominance/binary countdown protocols. This solution was implemented in commercial-off-the-shelf (COTS) wireless sensor networks (WSN) platforms and the implementation was demonstrated to be working. Unfortunately, those platforms had (for the MAC protocol) unfavourable characteristics which lead to limited efficiency and excessive overhead of the MAC protocol. This work presents a new hardware platform, in the form of a network adapter for common WSN platforms, that allows an efficient implementation of dominance protocols for wireless medium access, allowing the medium access to be performed in less than 5 ms for 216 priority levels, which represents an overhead reduction of more than ten times as compared to the protocol implementation in COTS WSN platforms. Additionally, the overall energy consumption was reduced by approximately 45 % when compared to the theoretical best-case performance of the protocol implementation in COTS WSN platforms. iv This work also allowed, for the first time ever, an aggregate computation scheme for WSN to work exploiting the new efficient implementation of a binary/dominance countdown protocol.No decurso das duas últimas décadas, os sistemas de computação embebidos sofreram uma extraordinária evolução, representando hoje em dia uma das mais promissoras tecnologias para possibilitar melhorias em áreas de aplicação tão diversas como a gestão de energia/recursos, segurança, saúde ou entretenimento. Novos sensores e actuadores conduzem a interacção entre os sistemas de computação e o ambiente físico onde se encontram inseridos até um nível sem precedentes. Estes computadores embebidos tendem a encontrar-se interligados (frequentemente recorrendo a ligações sem fios), sendo instalados de forma intensiva, e cada vez em maior escala. Sendo o meio de transmissão sem fios um “recurso natural” que deve ser partilhado entre este número elevado de computadores embebidos, o protocolo de controlo de acesso ao meio (do Inglês Medium Access Control - MAC) influencia significativamente o desempenho de todo o sistema. Em particular, a satisfação de requisitos temporais — algo que é necessário para uma estreira interacção entre um computador e o meio físico onde se insere — desempenha um papel muito importante. Uma solução foi recentemente proposta por Pereira, Andersson e Tovar, consistindo num protocolo MAC que implementa escalonamento baseado em prioridades e livre de colisões. Esta solução foi implementada em plataformas comercias de redes de sensores sem fios (do Inglês Wireless Sensor Networks - WSN), demonstrando-se funcional. Infelizmente, para a implementação do protocolo MAC, tais plataformas possuem características desfavoráveis, que conduzem a uma eficiência limitada do mesmo. Este trabalho apresenta uma nova platforma de hardware, sob a forma de um adaptador de rede, para platformas de WSN disponíveis comercialmente, que permite que a realização do acesso ao meio decorra em menos de 5 ms (para 216 níveis de prioridades). Tal representa uma redução superior a dez vezes no custo da execução do protocolo, quando comparada com a anterior implementação em platformas comercias de WSN. Adicionalmente, o consumo global de energia foi reduzido em aproximadamente 45 %, quando comparado com o melhor desempenho teórico possível da implementação em plataformas comerciais de WSN. vi Este trabalho permitiu ainda que, pela primeira vez, um esquema de computação agregada para WSN operasse tirando partido da implementação eficiente de um protocolo MAC apresentada neste trabalho

Load Balancing in Tree-based IP Micro-Mobility Domains

Nowadays the penetration of wireless access is continuously increasing. Additionally, the mobile users become more and more dependent on data. The IP-based (Internet Protocol) Internet was designed for data transmission and has become the most ubiquitous wired internetwork. According to these trends the next generation networks (and already 3G networks also include IP-based parts) are designed as a combination of these two types of networks (mobile and IP-based). The Mobile IP protocol handles mobility in the IP layer globally, but it is not well-adopted to local coverage areas. Within such access networks the micro-mobility proposals enhance the performance of Mobile IP. In this paper we propose a solution for improving the performance of tree-based micro-mobility protocols by rearranging their capacity using additional links. Based on analytical considerations we obtain a formula to determine the optimal link size in particular cases. The method is also examined with our simulation testbed, the results show improvement in the performance of the domain

The uncertain representation ranking framework for concept-based video retrieval

Concept based video retrieval often relies on imperfect and uncertain concept detectors. We propose a general ranking framework to define effective and robust ranking functions, through explicitly addressing detector uncertainty. It can cope with multiple concept-based representations per video segment and it allows the re-use of effective text retrieval functions which are defined on similar representations. The final ranking status value is a weighted combination of two components: the expected score of the possible scores, which represents the risk-neutral choice, and the scores’ standard deviation, which represents the risk or opportunity that the score for the actual representation is higher. The framework consistently improves the search performance in the shot retrieval task and the segment retrieval task over several baselines in five TRECVid collections and two collections which use simulated detectors of varying performance

Whom to Ask? Jury Selection for Decision Making Tasks on Micro-blog Services

It is universal to see people obtain knowledge on micro-blog services by asking others decision making questions. In this paper, we study the Jury Selection Problem(JSP) by utilizing crowdsourcing for decision making tasks on micro-blog services. Specifically, the problem is to enroll a subset of crowd under a limited budget, whose aggregated wisdom via Majority Voting scheme has the lowest probability of drawing a wrong answer(Jury Error Rate-JER). Due to various individual error-rates of the crowd, the calculation of JER is non-trivial. Firstly, we explicitly state that JER is the probability when the number of wrong jurors is larger than half of the size of a jury. To avoid the exponentially increasing calculation of JER, we propose two efficient algorithms and an effective bounding technique. Furthermore, we study the Jury Selection Problem on two crowdsourcing models, one is for altruistic users(AltrM) and the other is for incentive-requiring users(PayM) who require extra payment when enrolled into a task. For the AltrM model, we prove the monotonicity of JER on individual error rate and propose an efficient exact algorithm for JSP. For the PayM model, we prove the NP-hardness of JSP on PayM and propose an efficient greedy-based heuristic algorithm. Finally, we conduct a series of experiments to investigate the traits of JSP, and validate the efficiency and effectiveness of our proposed algorithms on both synthetic and real micro-blog data.Comment: VLDB201

Data sharing in DHT based P2P systems

International audienceThe evolution of peer-to-peer (P2P) systems triggered the building of large scale distributed applications. The main application domain is data sharing across a very large number of highly autonomous participants. Building such data sharing systems is particularly challenging because of the "extreme" characteristics of P2P infrastructures: massive distribution, high churn rate, no global control, potentially untrusted participants... This article focuses on declarative querying support, query optimization and data privacy on a major class of P2P systems, that based on Distributed Hash Table (P2P DHT). The usual approaches and the algorithms used by classic distributed systems and databases forproviding data privacy and querying services are not well suited to P2P DHT systems. A considerable amount of work was required to adapt them for the new challenges such systems present. This paper describes the most important solutions found. It also identies important future research trends in data management in P2P DHT systems

Distributed Real-time Systems - Deterministic Protocols for Wireless Networks and Model-Driven Development with SDL

In a networked system, the communication system is indispensable but often the weakest link w.r.t. performance and reliability. This, particularly, holds for wireless communication systems, where the error- and interference-prone medium and the character of network topologies implicate special challenges. However, there are many scenarios of wireless networks, in which a certain quality-of-service has to be provided despite these conditions. In this regard, distributed real-time systems, whose realization by wireless multi-hop networks becomes increasingly popular, are a particular challenge. For such systems, it is of crucial importance that communication protocols are deterministic and come with the required amount of efficiency and predictability, while additionally considering scarce hardware resources that are a major limiting factor of wireless sensor nodes. This, in turn, does not only place demands on the behavior of a protocol but also on its implementation, which has to comply with timing and resource constraints. The first part of this thesis presents a deterministic protocol for wireless multi-hop networks with time-critical behavior. The protocol is referred to as Arbitrating and Cooperative Transfer Protocol (ACTP), and is an instance of a binary countdown protocol. It enables the reliable transfer of bit sequences of adjustable length and deterministically resolves contest among nodes based on a flexible priority assignment, with constant delays, and within configurable arbitration radii. The protocol's key requirement is the collision-resistant encoding of bits, which is achieved by the incorporation of black bursts. Besides revisiting black bursts and proposing measures to optimize their detection, robustness, and implementation on wireless sensor nodes, the first part of this thesis presents the mode of operation and time behavior of ACTP. In addition, possible applications of ACTP are illustrated, presenting solutions to well-known problems of distributed systems like leader election and data dissemination. Furthermore, results of experimental evaluations with customary wireless transceivers are outlined to provide evidence of the protocol's implementability and benefits. In the second part of this thesis, the focus is shifted from concrete deterministic protocols to their model-driven development with the Specification and Description Language (SDL). Though SDL is well-established in the domain of telecommunication and distributed systems, the predictability of its implementations is often insufficient as previous projects have shown. To increase this predictability and to improve SDL's applicability to time-critical systems, real-time tasks, an approved concept in the design of real-time systems, are transferred to SDL and extended to cover node-spanning system tasks. In this regard, a priority-based execution and suspension model is introduced in SDL, which enables task-specific priority assignments in the SDL specification that are orthogonal to the static structure of SDL systems and control transition execution orders on design as well as on implementation level. Both the formal incorporation of real-time tasks into SDL and their implementation in a novel scheduling strategy are discussed in this context. By means of evaluations on wireless sensor nodes, evidence is provided that these extensions reduce worst-case execution times substantially, and improve the predictability of SDL implementations and the language's applicability to real-time systems