Mário Alves

Improving Quality-of-Service in Wireless Sensor Networks by Mitigating “Hidden-Node Collisions”

Wireless sensor networks (WSNs) emerge as underlying infrastructures for new classes of large-scale networked embedded systems. However, WSNs system designers must fulfill the quality-of-service (QoS) requirements imposed by the applications (and users). Very harsh and dynamic physical environments and extremely limited energy/computing/memory/communication node resources are major obstacles for satisfying QoS metrics such as reliability, timeliness, and system lifetime. The limited communication range of WSN nodes, link asymmetry, and the characteristics of the physical environment lead to a major source of QoS degradation in WSNs-the ldquohidden node problem.rdquo In wireless contention-based medium access control (MAC) protocols, when two nodes that are not visible to each other transmit to a third node that is visible to the former, there will be a collision-called hidden-node or blind collision. This problem greatly impacts network throughput, energy-efficiency and message transfer delays, and the problem dramatically increases with the number of nodes. This paper proposes H-NAMe, a very simple yet extremely efficient hidden-node avoidance mechanism for WSNs. H-NAMe relies on a grouping strategy that splits each cluster of a WSN into disjoint groups of non-hidden nodes that scales to multiple clusters via a cluster grouping strategy that guarantees no interference between overlapping clusters. Importantly, H-NAMe is instantiated in IEEE 802.15.4/ZigBee, which currently are the most widespread communication technologies for WSNs, with only minor add-ons and ensuring backward compatibility with their protocols standards. H-NAMe was implemented and exhaustively tested using an experimental test-bed based on ldquooff-the-shelfrdquo technology, showing that it increases network throughput and transmission success probability up to twice the values obtained without H-NAMe. H-NAMe effectiveness was also demonstrated in a target tracking application with mobile robots over a WSN deployment.

Real-Time Communications Over Cluster-Tree Sensor Networks with Mobile Sink Behaviour

Modelling the fundamental performance limits of wireless sensor networks (WSNs) is of paramount importance to understand the behaviour of WSN under worst case conditions and to make the appropriate design choices. In that direction, this paper contributes with a methodology for modelling cluster tree WSNs with a mobile sink. We propose closed form recurrent expressions for computing the worst case end to end delays, buffering and bandwidth requirements across any source-destination path in the cluster tree assuming error free channel. We show how to apply our theoretical results to the specific case of IEEE 802.15.4/ZigBee WSNs. Finally, we demonstrate the validity and analyze the accuracy of our methodology through a comprehensive experimental study, therefore validating the theoretical results through experimentation.

Hybrid wired/wireless PROFIBUS networks supported by bridges/routers

Fieldbus networks are becoming increasingly popular in industrial computer-controlled systems. More recently, there has been the desire to extend the capabilities of fieldbuses to cover functionalities not previously considered in such networks, with particular emphasis on industrial wireless communications. Thinking about wireless means considering hybrid wired/wireless solutions capable of interoperating with legacy (wired) systems. One possible solution is to use intermediate systems (IS) acting as repeaters to interconnect the wired and wireless parts. In contrast, we analyze a solution where intermediate systems are implemented as bridges/routers. We detail the main advantages in terms of dependability and timeliness, and propose mechanisms to manage message transactions and intercell mobility.

A Traffic Differentiation Add-On to the IEEE 802.15.4 Protocol: Implementation and Experimental Validation over a Real-Time Operating system

The IEEE 802.15.4 is the most widespread used protocol for Wireless Sensor Networks (WSNs) and it is being used as a baseline for several higher layer protocols such as ZigBee, 6LoWPAN or Wireless HART. Its MAC (Medium Access Control) supports both contention-free (CFP, based on the reservation of guaranteed time-slots GTS) and contention based (CAP, ruled by CSMA/CA) access, when operating in beacon-enabled mode. Thus, it enables the differentiation between real-time and best-effort traffic. However, some WSN applications and higher layer protocols may strongly benefit from the possibility of supporting more traffic classes. this happens, for instance, for dense WSNs used in time-sensitive industrial applications. In this context, we propose to differentiate traffic classes within the CAP, enabling lower transmission delays and higher success probability to time critical messages, such as for event detection, GTS reservation and network management. Building upon a previously proposed methodology (TRADIF), in this paper we outline its implementation and experimental validation over a real-time operating system. Importantly, TRADIF is fully backward compatible with the IEEE 802.15.4 standard, enabling to create different traffic classes just by tuning some MAC parameters.Effective cooling is of paramount importance for low speed permanent magnet (PM) machines due to their high power density and low speed. In this paper, a forced air cooling system for multibrid permanent magnet wind generator is presented, and the three dimensional temperature fields are calculated for this type of machine to verify the feasibility of the system. The steady state heat transfer mathematic models are established, and hypothetical conditions and loads are calculated. This thermal analysis technique is then validated after experimentation carried out on a 10kW prototype equipped with thermal sensors, and some valuable conclusions are obtained.

Web Site Access Analysis for a National Statistical Agency

Web access log analysis is gaining popularity, especially with the growing number of commercial web sites selling their products. The driver for this increase in interest is the promise of gaining some insights into the behaviour of users/customers when browsing through their Web site, fuelled by the desire to improve the user experience. In this chapter we describe the approach taken in analysing web access logs of a non-commercial Web site disseminating Portuguese statistical data. In developing the approach, we follow the common steps for data mining applications (the CRISP-DM phases), and give details about several phases involved in developing the data mining solution. Through intensive communication with the web site owner, we identified three data mining problems which were successfully addressed using different tools and methods. The solution methodology is briefly described here accompanied with some of the results for illustrative purposes. We conclude with an attempt to generalize our experience and provide a number of lessons learned.

Evaluating the duration of message transactions in broadcast wired/wireless fieldbus networks

Determining the response time of message transactions is one of the major concerns in the design of any distributed computer-controlled system. Such response time is mainly dependent on the medium access delay, the message length and the transmission delay. While the medium access delay in fieldbus networks has been thoroughly studied in the last few years, the transmission delay has been almost ignored as it is considered that it can be neglected when compared to the length of the message itself. Nevertheless, this assumption is no longer valid when considering the case of hybrid wired/wireless fieldbus networks, where the transmission delay through a series of different mediums can be several orders of magnitude longer than the length of the message itself. In this paper, we show how to compute the duration of message transactions in hybrid wired/wireless fieldbus networks. This duration is mainly dependent on the duration of the request and response frames and on the number and type of physical mediums that the frames must cross between initiator and responder. A case study of a hybrid wired/wireless fieldbus network is also presented, where it becomes clear the interest of the proposed approach.

A Theory of Persistent Containers and Its Application to Ada

I argue for a standard container library for Ada featuring indefinite elements and persistence. I present a theory of persistent containers as a systematization of the motivations and technical issues involved in creating the library. I hope to show that having such a library represents a major improvement of the language.

The use of ada, GNAT.spitbol, and XML in the sol-eu-net project

We report the use of Ada in the European research project Sol-Eu-Net. Ada was used in a web mining subproject, mainly for data preparation, and also for web system development. Open source Ada resources e.g. GNAT.Spitbol were used. Some such resources were modified, some created anew. XML and SQL were also used in association with Ada.

Internet Support to Collaboration

The knowledge gathered by an organization throughout its activity is too valuable an asset to be kept volatile, always dependent on those who produced it. Organizational knowledge also tends to be tacit, and distributed, so only a small part of it is likely to be acquired and retained. This chapter describes a number of techniques and tools for capturing this kind of knowledge, applied to a particular research project organization. These techniques cover the design of versatile information collection templates and ways of collecting information from members of the organization. Another important aim is to make such information available internally to the organization, as well as externally to the world. The collection and dissemination of organizational knowledge is realized through Web systems. The Web is also used to link the (distributed) set of tools into an integrated system, allowing some of these tools to communicate automatically. Important issues such as security and ease of maintenance are also addressed.

Integration of manufacturing applications: Overcoming heterogeneity to preserve investment

The work presented in this paper consisted on the integration of several areas within a factory by using different communication standards. The described implementation satisfies several types of requirements, which include not only the functional requirements, but also the preservation of previous investments. The fulfilment of the latter imposed the use of heterogeneous technologies, both in terms of vendors and generations. This resulted in additional difficulties for the integration that were overcome. The use of OSI based technologies, such as those developed in the scope of CCE-CNMA, contributed to the success of this work.