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In this paper we make a comparative assessment between the RT-WiFi and HCCA approaches to handle realtime traffic in IEEE 802.11 networks operating in open communication environments. The main focus will be on the assessment of the admission control mechanism proposed for the RT-WiFi upper layer, that is based on a non-preemptive EDF scheduling algorithm combined with a mechanism that measures the blockage times on medium access of real-time stations due to retransmissions and delays. The goal is to achieve a good compromise between medium utilization and timeliness. This proposal is compared with HCCA, and the results show that RT-WiFi offers a significantly better behavior in what concerns the maximum number of admitted streams.

WirelessHART currently appears as a promising solution for the last mile connection in process control applications. Most of these applications have stringent dependability requirements, where a system failure may result in economic losses, or damage for human life or the environment. Among the different type of faults that can lead to a system failure, permanent faults on network devices have a major impact. They can hamper communication over long periods of time and consequently they may disturb, or even disable, control algorithms with all resulting problems. In this work we perform a dependability evaluation of the best practices indicated by the HART Communication Foundation (HCF), when network devices are subject to permanent faults.

The usage of Wireless Sensor Networks (WSNs) in industrial environments has been steadily increasing, due to the reduced deployment and maintenance costs, when compared to the use of wired networks for connecting single I/O points in industrial applications. The most promising WSN standard is the one defined by the IEEE 802.15.4 standard. However, it does not support the adequate mechanisms to deal with the unreliable nature of an industrial communication environment. In this paper, we propose an extension of the use classes available for this standard, together with the proposal of adequate message retransmission schemes, in order to increase the reliability of message exchanges in industrial environments. To demonstrate the effectiveness of the proposed schemes, we present a simulation assessment of both the proposed message retransmission schemes and the Guaranteed Time Slots (GTS) mechanism, as defined by the standard. © 2012 IEEE.

Wireless sensor networks composed of camera-enabled source nodes can provide visual information of an area of interest, potentially enriching monitoring applications. While few bytes can represent scalar data, even low-resolution still images may require thousand of bytes, turning data fragmentation into a relevant design issue. Different optimization approaches have been proposed in recent years to achieve energy saving in wireless image sensor networks. However, the impact of image fragmentation upon the adopted MAC technology has been neglected in most cases. In this work we investigate the effect of frame size on image transmissions over wireless sensor networks, linking the maximum frame size, the useful payload and the frame error rate effects. Additionally, we discuss different approaches for transmissions of DWT-based encoded images and the impact of inserting application-specific information into the frame header. We believe that our discussions can contribute to the advance of the design of wireless image sensor networks. Â(c) 2012 IEEE.

One of the motivations for use of wireless sensor networks (WSNs) in industrial environments is related to the reduced deployment and maintenance costs, when compared to the use of wired networks for connecting single I/O points. Nevertheless, a well-known problem for the use of wireless communication in industrial environments is related to the unreliable nature of the communication medium. Therefore, the search for communication reliability in WSNs is a relevant topic of research for the scientific community. In this paper we propose an innovative message retransmission scheme, based on network coding techniques, to increase the reliability of message exchanges in industrial environments. The proposed technique: NetCod, that improve the probability of message deliver by allowing the message retransmission whenever a loss occurs is experimentally compared with state of the art techniques: BlockACK, Redundant TDMA and Master/Slave. A comparative experimental assessment of these four techniques was performed using MicaZ motes. Metrics such as: message loss, message transmission delay and power consumption were used to comparatively assess the behavior of these retransmission techniques. Â(c) 2012 IEEE.

Recent trends in distributed embedded systems, such as those found in avionics and trains, have shown an increase in the amount and heterogeneity of the information that needs to be exchanged, together with a growing importance of supporting dynamic reconfiguration and adaptive behaviors. In this paper we focus on Ethernet technologies with real-time reconfiguration support and we address the case of middle-size networking infrastructures with a few switches. We use the FTT-SE protocol with the needed adaptations to support dynamic heterogeneous real-time transactions in multi-hop networks. The paper presents a worst-case response-time analysis that provides timeliness guarantees, improving the results obtained with another previous analysis, decreasing the needed network capacity for guaranteed schedulability by 25% on average. Practical experiments and simulation results validate the proposed approach and analysis. © 2012 IEEE

In recent years, the FlexRay communication protocol been promoted as a de facto standard for in-vehicular communication. In the FlexRay protocol, the communication timeline is organized as a sequence of four segments, whereas the static segment defines a set of slots specifically designed for the transmission of synchronous messages. In this paper, we investigate the following problem: "how to allocate a minimum number of static slots to each FlexRay node, while guaranteeing that all synchronous messages will be transmitted before their deadlines." Unlike previous studies that use linear programming based techniques, we evaluate the number of allocated slots using traditional response time analysis (RTA) techniques. The use of RTA techniques allow us to consider the timing requirements imposed by the set of synchronous message streams. Unlike other approaches, the RTA-based technique proposed in this paper is able to deal with a) message stream sets where periods are not multiple of the FlexRay cycle duration, b) generation of messages at the application layer that are not synchronized with the FlexRay cycle, and c) on-line scheduling of the traffic transferred during the static segment, according to its deadline requirements. The proposed technique is integrated as a RT-Middleware in each node, to take full advantage of the proposed allocation scheme. © 2012 IEEE.

Whenever reliable transmissions are assured by retransmission mechanisms, higher packet error incur in additional energy consumption due to packet retransmissions, even though many monitoring applications can tolerate some loss in the quality of the received images. In fact, visual information retrieved from source nodes may have different relevancies for the applications, according to the desired monitoring tasks and the current sensors' poses and fields of view. We propose a semi-reliable energy-efficient hop-by-hop retransmission mechanism, where the reliability level of each packet is function of the relevance of the source nodes associated with DWT subbands. Doing so, some of the corrupted packets will not be retransmitted, saving energy of intermediate nodes with low impact to the visual monitoring quality. © 2012 IEEE.

In this paper we present an admission control mechanism to handle real-time traffic in IEEE 802.11 networks operating in open communication environments. The proposal is based on a non-preemptive EDF scheduling algorithm, combined with a measurement-based mechanism that assesses blockage times on medium access of real-time stations due to retransmissions and delays. This proposal will be part of the upper layer of the RT-WiFi architecture that was proposed by us in previous works. The main goal is to achieve a better compromise between medium utilization and timeliness. Â(c) 2012 IEEE.

When packets are transmitted over braided-paths in visual sensor networks some intermediate nodes may relay most part of the traffic, turning part or the entire network inactive when they run out of energy. Due to the nature of visual monitoring, source nodes may have different relevancies for the application, according to the desired monitoring tasks and the current sensors' poses and fields of view. Based on such relevancies, packets transmitted from different source nodes will have different relaying priorities, which are associated with energy thresholds. We propose an energy-efficient relay scheme where packets are forwarded to the next hop toward the sink only if the associated energy threshold is below the current energy level of the relaying node. Doing so, energy saving is achieved potentially prolonging the network lifetime, but with reduced impact to the overall monitoring quality of the application. © 2012 IEEE.

In this paper is proposed a novel communication approach for implementation of parallel data fusion techniques in Wireless Sensor Networks. The approach is carried out by a distributed algorithm implemented in each network sensor node. In this approach, the network configures itself in order to adjust the number of messages sent periodically, establishing a tradeoff between the number of sent messages and the quality of data fusion (QoF). A prototype was implemented and experimental results show the effectiveness of this proposal.

Camera-enabled sensor nodes deployed for visual monitoring can considerably enlarge the applicability of wireless sensor networks. Due to the stringent requirements of visual data transmission and processing, when compared with scalar wireless sensor networks, quality assessment becomes a relevant issue. Although academic investigation has been focused on QoS parameters such as end-to-end delay, throughput and packet error rate, what is being seen by source nodes may be more important for the application than the quality of received data. In such way, we propose the novel concept of Quality of Viewing (QoV) to be employed as an important QoS parameter when assessing the monitoring quality in visual sensor networks. Some issues for the establishment of the QoV of monitoring applications will be presented, as well as practical exploitation of this parameter for dynamic verification, control and management of wireless sensor networks composed of camera-enabled source nodes. Â(c) 2012 IEEE.

WirelessHART currently appears as a leading solution for interconnection of wireless devices in industrial process control applications. However, the lack of knowledge about the influence of transient faults in WirelessHART networks can lead to the choice of less reliable topologies. In this work, we propose a simulation model to evaluate WirelessHART networks in the presence of transient faults. We assume that these faults result from noisy environments that disturb communications between devices. The model was developed using the Stochastic Petri Net (SPN) formalism, supported by the Mbbius tool. For further developments, we target the development of an application that automates the assessment of a WirelessHART network in transient fault scenarios.

Recent trends in distributed embedded systems have shown an increase in the amount and heterogeneity of the information that needs to be exchanged, together with a growing importance of supporting dynamic reconfiguration and adaptive behaviors. In this paper we focus on Ethernet technology and we address the case of middle-size networking infrastructure with a few switches. We use the FTT-SE protocol to support dynamic heterogeneous real-time transactions with temporal isolation and we propose the needed scheduling adaptations to support multi-hop network configurations. The paper also includes a companion worst-case response-time analysis that allows verifying the timeliness of the system. Recent trends in distributed embedded systems have shown an increase in the amount and heterogeneity of the information that needs to be exchanged, together with a growing importance of supporting dynamic reconfiguration and adaptive behaviors. In this paper we focus on Ethernet technology and we address the case of middle-size networking infrastructure with a few switches. We use the FTT-SE protocol to support dynamic heterogeneous real-time transactions with temporal isolation and we propose the needed scheduling adaptations to support multi-hop network configurations. The paper also includes a companion worst-case response-time analysis that allows verifying the timeliness of the system. Copyright 2011 ACM.

The purpose of this paper is to survey the state-of-the-art on real-time (RT) communication in CSMA-based networks and to identify the most suitable approaches to deal with the requirements imposed by next generation communication environments. The paper focuses in the two of the most relevant solutions that operate in shared broadcast environments, according to the CSMA medium access protocol (IEEE 802.3/802.11). RT communication solutions are classified according to two classification axes. The first axis is related to how collisions are dealt with. A second classification axis is related to the compatibility degree achieved by each of the proposed approaches. Finally, a RT communication framework that is adequate to support RT communication in shared environments is briefly discussed.

This work has the objective to present the first development results of a WirelessHART module for the ns-3. Our focus is the implementation of the Physical layer in order to provide the basis for the development of the superior layers such as MAC and Application. Thus, we presente an energy consumption model, a Gilbert/Elliot error model and an analysis for the currently avaliable ns-3 propagation loss models. For further development we mainly aim for the implementation of the time slot scheduler ( Network Manager) and the development of an inter protocol simulation with mutual interference.

In this paper, we propose the VOA algorithm (Variable Offset Algorithm) to deal with the optimization of communication efficiency in dense WSN s with star topologies. The use of the VOA algorithm has been assessed by means of an experimental setup. The results highlight that the use of the VOA algorithm, implemented as a light middleware at the application layer, clearly enhances the communication efficiency in a WSN with star topology. Â(c) 2010 IEEE.

O padrão IEEE 802.15.4 oferece uma solução flexível de comunicação para redes LR-WPAN. Quando ativo, o modo com beacon habilita comunicação de tempo real através da adoção de GTS (Guaranteed Time Slot). No entanto, este modo de transmissão suporta apenas que sete dispositivos da rede possam se beneficiar deste recurso em cada superquadro. Este trabalho, introduz uma nova técnica de escalonamento dinâmico de GTS's baseada no modelo de tarefas (m,k)-firm. A abordagem proposta garante, através de um teste dinâmico de escalonabilidade, que ao menos m mensagens sejam transmitidas em compartimentos garantidos em uma janela de k superquadros consecutivos. A avaliação da abordagem foi conduzida em duas vias: analiticamente e experimentalmente através de um protótipo em nodos Micaz. Os resultados analíticos garantem a exequibilidade da solução apresentada, enquando que os experimentais apontam para a simplicidade de implementação da proposta em um cenário real.

An important aspect when dealing with computer control systems is the impact of control delay upon the performance of the controlled item. Such control delay may be a consequence of the computer controlled implementation or, for the case of a network based control system, can be the consequence of the use of a communication network to interconnect sensors and actuators with the controller. Basically, control applications require a predictable delay between measurement and actuation, with small variability, i.e., small jitter. In this paper we present some simulation results concerning the impact of a varying control delay upon the performance of a DC-Motor, both for a centralized computer-controlled setup and for a network based control system. The results for the centralized setup were also experimentally confirmed. For the network-based setup it was used the h-BEB real-time communication protocol. The evaluated scenarios were simulated using the True Time simulator that is based in MATLAB/Simulink. The experimental tests were developed in LabVIEW, where the controller task was executed as a software routine. The tests consisted in the variation of the sampling period and control delay to determinate the steady region of the system.

In this paper, we propose a real-time publish-subscribe communication scheme for IEEE 802.11e networks, named Group Sequential Communication (GSC). The GSC intends to improve the HCCA mechanism by adopting a simple approach based on the Virtual Token Passing (VTP) protocol which uses a real-time group concept, where the real-time members of the group are granted a high-priority and sequential access to the communication medium. Their main achievements are the reduction of the Polling, individual ACKs and NULL frames overheads between the controller and the polled stations. In order to improve the reliability of the GSC scheme was developed a fault-tolerant mechanism based on a block acknowledge strategy to avoid missing real-time message deadlines.

The integration between industrial applications and industrial networks technologies is a required condition to share information at the factory level. Industrial networks are known for having different communication protocols. Therefore wired gateway technologies play an important role in the integration of different protocol stacks, wherever that integration is necessary. However, if there are physical constraints that make hard setting up a wired gateway, such as, difficult access for the required cabling, the use of a wireless gateway may present both a technical and an economical advantage when compared with wired gateway. This paper illustrates two industrial automation case studies, where both solutions use ELPRO wireless gateways proposed by TecWise Company®. The assessment results confirm that the use of wireless gateways may be an interesting solution for the industrial communication interconnection being analyzed. In this paper, we make a technical and economical assessment of the use of wireless gateways in industrial networks as a contributive solution for the integration of different industrial communication protocols.

Wireless Sensor Networks (WSN) are usually composed for a huge number of tiny nodes deployed in an environment that has to be monitored. Due to high probability of faults in communication level and in the nodes itself, network topology becomes very dynamic and unpredictable. Several challenges are faced in these networks like real-time and quality of service. Moreover, it is very difficult to fix the network due to environmental issues or the huge number of nodes. This way self-organizing and self-organizing approaches are required to maintain the network services. In this paper a selforganizing approach for real-time data fusion applications is presented. Our approach showed that is possible to achieve the required levels of communication efficiency or quality of service required by system administrators.

The paper proposes a dependability model to evaluate the behavior of a CANnetwork in scenarios of transient faults which affect data communications. Fault occurrenceis modeled by a Markov Modulated Poisson Process (MMPP) which is capable todescribe the typical behavior of electromagnetic interferences (EMI) that occur in industrialenvironments. An accurate and efficient representation of the network behavior isachieved by adopting a set of assumptions that reduce the pessimism level and which arecloser to the real operating conditions. The model is based on Stochastic Petri Nets, whichare a high-level modeling formalism able to produce very compact and efficient models,supporting both analytical and simulation solutions. Dependability measures are establishedfrom the fulfillment of the real-time constraints (deadlines) defined on messagesexchanged between network nodes. Analytical and simulation solutions are both investigated.A case study is proposed to asses both model performance and network dependability.

Within industrial environments a multitude of different working practices, manufacturing technologies and information systems are continually appearing. Significant benefits can be achieved through integration of the information allowing key decisions to be made based on timely and accurate data. This communication presents the work carried out at an industrial site consisting on the integration of several areas and based on the use of different communication standards. The implementation satisfies several types of requirements, which include not only the functional requirements, but also the preservation of previous investments. The use of OSI (Open Systems Interconnection) based technologies, such as those developed in the aim of CCE-CNMA (CIME Computer Environment by integrating a Communication Network for Manufacturing Applications), contributed to the success of this work.

The operation of networked control systems strongly depends on the performance of its communication infrastructure. Therefore, their evaluation becomes an important aspect to be considered, particularly in situations where network traffic has real-time constraints and the network operates in faulty environments. In this paper a set of fault injection experiments were performed on a Profibus-DP network in order to assess the impact of faults in the network operation. The experiments reproduce typical conditions found in industrial applications, considering different traffic loads and fault scenarios. Several performance metrics were evaluated, such as the bus cycle time, messages response times and worst case response times. Copyright © 2007 IFAC.