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Availability is a major design issue that should play an important role when deploying and operating wireless sensor networks, specially for critical monitoring applications. Actually, sensing redundancy can be exploited to enhance the attainable availability level of sensor networks, since redundant nodes can replace faulty nodes. When employing camera-enabled sensors, the perception of sensing redundancy is considerably changed when compared to scalar sensors, with direct impact on network availability. In such way, some characteristics as deployment density, viewing angle and sensing range should be properly evaluated in wireless visual sensor networks, in order to better estimate the network availability. Nevertheless, when deploying visual sensors on occluded environments, viewed areas and resulted overlapping may be significantly altered, redefining sensing redundancy. We then propose an algorithm to automatically select redundant nodes in wireless visual sensor networks deployed on areas with occlusion, according to network configurations and application availability requirements. Additionally, an algorithm to adjust cameras’ orientations in occluded environments is also proposed. Mathematical assessment of the proposed algorithms are performed, allowing the discussion of how parameters of deployed networks can influence applications monitoring availability.

The development of wireless sensor networks for control and monitoring functions has created a vibrant investigation scenario, where many critical topics, such as communication efficiency and energy consumption, have been investigated in the past few years. However, when sensors are endowed with low-power cameras for visual monitoring, a new scope of challenges is raised, demanding new research efforts. In this context, the resource-constrained nature of sensor nodes has demanded the use of prioritization approaches as a practical mechanism to lower the transmission burden of visual data over wireless sensor networks. Many works in recent years have considered local-level prioritization parameters to enhance the overall performance of those networks, but global-level policies can potentially achieve better results in terms of visual monitoring efficiency. In this paper, we make a broad review of some recent works on priority-based optimizations in wireless visual sensor networks. Moreover, we envisage some research trends when exploiting prioritization, potentially fostering the development of promising optimizations for wireless sensor networks composed of visual sensors.

The use of Wireless Sensor Network (WSN) technologies is an attractive option to support wide-scale monitoring applications, such as the ones that can be found in precision agriculture, environmental monitoring and industrial automation. The IEEE 802.15.4/ZigBee cluster-tree topology is a suitable topology to build wide-scale WSNs. Despite some of its known advantages, including timing synchronisation and duty-cycle operation, cluster-tree networks may suffer from severe network congestion problems due to the convergecast pattern of its communication traffic. Therefore, the careful adjustment of transmission opportunities (superframe durations) allocated to the cluster-heads is an important research issue. This paper proposes a set of proportional Superframe Duration Allocation (SDA) schemes, based on well-defined protocol and timing models, and on the message load imposed by child nodes (Load-SDA scheme), or by number of descendant nodes (Nodes-SDA scheme) of each cluster-head. The underlying reasoning is to adequately allocate transmission opportunities (superframe durations) and parametrize buffer sizes, in order to improve the network throughput and avoid typical problems, such as: network congestion, high end-to-end communication delays and discarded messages due to buffer overflows. Simulation assessments show how proposed allocation schemes may clearly improve the operation of wide-scale cluster-tree networks.

This paper proposes a real-time communication scheme for wireless local area network (WLAN) infrastructure. This strategy was called Group Sequential Communication (GSC). The GSC has the main goal to reduce the network overhead due to the Polling, Ack and QoS Null Frames of Hybrid Coordination Function Controlled Channel Access (HCCA) in IEEE 802.11e, because it uses a Virtual Token Passing procedure to priority the real-time communication over WLAN. In order to improve the confiability of the new scheme is presented a fault-tolerant mechanism based on Block-Ack strategy to avoid missing real-time messages. The GSC was implemented in NS2 and simulation results have proved the superior performance of GSC in comparison with HCCA for real-time industrial communications.

The IEEE 802.15.4 standard offers a flexible communication solution for low-rate wireless small area networks. When the beacon mode is enabled, real-time communication is possible by using the Guaranteed Time Slots (GTS) mechanism. However, the beacon-enabled mode only supports a maximum of seven GTS allocated in each superframe, preventing other nodes to benefit from the guaranteed service. This paper introduces the SIGHT (Skips In GTS scHeduling for IEEE 802.15.4 neTworks) model, which incorporates a dynamic GTS allocation policy for IEEE 802.15.4 networks based on the (m,k)-firm task model. Our approach ensures that at least m messages are allocated in a guaranteed slot, for any k consecutive superframes. It adopts a deterministic schedulability test and an on-line prioritization heuristic algorithm called Slotted DBP (Distance Based Priority) to attend requests according to this schedulability model. A simulation model has been developed, in order to compare the behaviour in overload conditions of the proposed Slotted DBP with other approaches (DWCS and FIFO).

Wireless Sensor Networks (WSNs) have been widely considered as a promising solution to support different types of applications on industrial environments. Many of these applications impose strict dependability requirements, since a system failure may result in economic losses, or damage for human life or to the environment. The absence of an effective approach enabling the dependability evaluation of WSNs prevents system designers to forecast these type of scenarios or to optimize decisions regarding the criticality of the devices, network topology, levels of redundancy and network robustness that minimize the occurrence of faults. To bridge the gap between research achievements and industrial development, we present in this paper a framework to support the dependability evaluation of WSNs based on the automated generation of analytical dependability models from high level AADL (Architecture Analysis and Description Language) architecture models. The main objective of this framework is to relieve the end user from a deep knowledge of dependability modeling techniques and evaluation methods, focusing on their knowledge of the behavior and structure of the system.

In some Wireless Sensor Network applications the sensor nodes share the same sensing activity, which means that for a considerable number of applications, not all nodes are required to perform sensing tasks during the network lifetime. Sleep-scheduling approaches can be applied in this scenario, enabling that some nodes turn off their radios, saving energy and bandwidth, as long as there are enough nodes to ensure the required Quality of Service (QoS) of the network. This paper presents a new adaptive approach for QoS and energy management in IEEE 802.15.4 networks, entitled Skip Game. This approach targets a trade-off between increasing the network lifetime and maintaining the QoS of the network, aiming a greater number of nodes to participate in the monitoring application. In order to evaluate the proposed approach, we performed some experiments using the OMNeT++ simulator tool under the MiXiM framework. The results show that the Skip Game outperforms both the traditional Gur Game and Gureen Game approaches in terms of QoS provision and network lifetime.

Wireless sensor networks can provide visual information from the monitored field when sensor nodes are equipped with low-power cameras. In general, visual monitoring applications supported by sensing technology will have to address many challenging issues when visual information has to be transmitted over resource-constrained sensors. When addressing energy efficiency, sensing redundancy can be exploited to enlarge the network lifetime, whenever inactive sensors are used to replace faulty nodes. The monitoring of multiple targets may be optimized reducing the number of active visual sensors, but the required perspectives of the targets must be considered. In this paper we propose an algorithm to compute the minimum number of visual sensors that should be activated to cover all desired targets, especially addressing the particular problem when single nodes can view multiple targets at the same time. As different concurrent perspectives of the targets may be required, the proposed algorithm can bring significant results to wireless visual sensor network applications.

Many Wireless Sensor Network (WSN) applications operate autonomously in unreliable or inaccessible environments, precluding maintenance or human intervention. Redundant deployment schemes are usually considered in this scenario, making the network resilient to failure and environmental changes. Furthermore, sleep-scheduling techniques can also be applied, enabling redundant nodes to turn off their radios, while active nodes perform monitoring services. This paper investigates the behavior of the (m, k)-Gur Game approach. The main goal of the (m, k)-Gur Game is to provide an uniform network coverage for monitoring applications, with autonomic nodes performing a self-regulated choice between sending message to a base station or sleep until the next period. The proposal was evaluated using the OMNeT++ simulator tool under the MiXiM framework. Preliminary results shows that the (m, k)-Gur Game outperforms the traditional GurGame approach in terms of QoS provision and network coverage.

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

Visual monitoring in wireless sensor networks can provide valuable information of the monitored field, enriching surveillance and control applications. For those networks, however, some active visual sources may fail or run out of energy, potentially degrading the application monitoring quality. Visual sensors may be deployed to monitor a set of targets that are critical for the monitoring tasks of the application, demanding some level of redundancy to compensate sensor failures. In this context, it may be desired to know the probability of a specific target to be covered by at least one visual sensor along the network operation. We propose an approach for the availability assessment in wireless visual sensor networks for the specific case of target coverage, relating sensing redundancy to energy discharging and sensors disconnection. The proposed approach can then be used to predict coverage holes, directly benefiting critical monitoring applications.

Wireless visual sensor networks can provide significant information for a large set of monitoring and surveillance applications. In these networks, mobile sinks are often used to reduce energy consumption over the network, where many algorithms have been proposed for higher energy efficiency. Frequently, visual sensors may have different relevancies for the monitoring functions of the applications, according to their potential to provide significant data. Additionally, the relevancies of visual sensors may be quickly adjusted according to the occurrence of some critical event. In such cases, higher relevant source nodes may be concurrently transmitting visual information with higher quality or frequency, potentially increasing energy consumption in intermediate nodes from those sources toward the sink. We propose an autonomous balanced positioning algorithm for mobile sinks in order to shorten the transmission paths from higher relevant sources, directly benefiting multi-hop sensor networks with multiple active visual source nodes.

As Wireless Sensor Networks have been employed to support critical monitoring applications, network availability has become a major design concern. In these networks, redundancy can be exploited to enhance the attainable availability level, where redundant sensors can replace faulty nodes. When camera-enabled sensors are deployed to retrieve visual information, the perception of redundancy changes considerably, since the redundancy of visual sensors depends on the monitoring requirements of the applications. In such context, characteristics as deployment density, viewing angle and sensing range are relevant when planning wireless sensor network applications, directly impacting in the number of redundant nodes. We propose an algorithm to select redundant nodes in Wireless Visual Sensor Networks, according to the application requirements. Moreover, we discuss how parameters of the deployed network can influence on the number of redundant nodes.

Wireless sensor networks can be deployed for a large set of monitoring functions, providing information as humidity, pressure, temperature, luminosity, among many others. When monitored data is transmitted over wireless links, packets can be corrupted requiring some error recovery strategy. Hop-by-hop retransmission can provide an acceptable level of reliability, but can potentially increase the energy consumption of the network. In fact, wireless communications are error prone, and interferences may be concentrated in specific parts of the network. We propose a semi-reliable retransmission mechanism where only packets carrying critical information will be always retransmitted if corrupted. The remaining corrupted packets will not be retransmitted, saving energy of the network. We designed an energy consumption model to evaluate the proposed approach.

The evolution of industrial networks can be summarized as a constant battle to define the universal technology that integrates field devices and applications. Since the Fieldbus wars in the 1980s, diverse wired solutions have been proposed. However, this scenario has been changing due to the introduction of industrial wireless sensor networks. In the last 10 years, the development of deterministic scheduling techniques, redundant routing algorithms, and energy saving issues has brought wireless sensor networks into the industrial domain. This new communication paradigm is governed by a de facto standard, the IEEE 802.15.4, and more recently also by the IEEE 802.15.5. However, there are signs of a new battle on the horizon with the new publicly available specifications of WirelessHART, ISA100.11a, and IEC 62601. In this chapter, to the authors analyze the advantages and drawbacks of these emerging technologies for industrial wireless sensor networks.

In several monitoring applications, such as those that can be found in industrial factory floor, it may be necessary to obtain an uniform sensing coverage, providing as much as possible the same coverage degree for the entire network area. The IEEE 802.15.4 has become an important standard in wireless sensor networks. However, the use of cluster-tree topology in these networks hampers a fair and uniform coverage in the sensing area. Clusters more distant from the base station are eventually adversely affected, with messages from its sensors delayed and discarded. In this paper, we propose a framework, entitled GLHOVE, whose goal is to make a trade-off between a minimum and uniform coverage area and the energy consumption of the network. The simulation results show an increase up to 20% of the fairness metric (w.r.t. messages received by base station). (c) 2013 IEEE.

There is a growing need to develop new tools to help end users in tasks related to the design, monitoring, maintenance and commissioning of industrial processes. The complexity of the industrial environment requires that these tools have flexible features in order to provide valuable data at the early planning and design phases. Furthermore, it is known that industrial processes have stringent requirements for dependability (reliability and availability), since failures can cause economic losses, environmental damages and danger to people. The lack of tools that enable the evaluation of faults in industrial processes could mitigate these problems. In this work we present a framework which is able to estimate the reliability and availability of industrial processes that can be modeled as a graph. The framework transforms the graph structure into a Fault Tree model in order to analyze their dependability. In order to validate the proposal we conduct a dependability study of an industrial process. © 2013 IEEE.

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.

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.

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.

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.

Characterization of the behaviour of link-layer retransmissions is helpful when designing MAC protocols and scheduling algorithms for Factory Automation WLANs. In this paper, we measure retransmission rates and consecutive retransmission occurrences of IEEE 802.11g communication links in an industrial indoor environment. The employed wireless channel at 2.4 GHz is characterized. Experimental results show that the distribution of consecutive retransmission occurrences tends to concentrate around just one retry when devices operate within their transmission range. It is also shown that smaller frames result in fewer retransmissions.

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.

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.

We present a reliability evaluation of a group membership protocol (GMP), by computing the probability of violating the fault assumptions made in its proof. The evaluation of the reliability of a GMP is of paramount importance because group membership services are often used as building blocks in the design of fault-tolerant applications. The GMP that we consider here has been proposed for dual scheduled TDMA networks such as FlexRay, a protocol that is likely to become the de-facto standard for next generation automotive networks. Our study is carried out by modeling the GMP with discrete-time Markov chains. The models consider different fault scenarios, including permanent, transient and common-mode faults, affecting both channels and nodes. Furthermore we perform a sensitivity analysis to assess the influence of different parameters on the protocol's reliability. The results show that the GMP can achieve reliability levels in the range required for safety critical applications.

The paper proposes a dependability model to evaluate the behavior of a CAN network in scenarios of transient faults which affect data communications. Fault occurrence is modeled by a Markov Modulated Poisson Process (MMPP) which is capable to describe the typical behavior of electromagnetic interferences (EMI) that occur in industrial environments. An accurate and efficient representation of the network behavior is achieved by adopting a set of assumptions that reduce the pessimism level and which are closer to the real operating conditions. The model is based on Stochastic Petri Nets, which are a high-level modeling formalism able to produce very compact and efficient models, supporting both analytical and simulation solutions. Dependability measures are established from the fulfillment of the real-time constraints (deadlines) defined on messages exchanged between network nodes. Analytical and simulation solutions are both investigated. A case study is proposed to asses both model Performance and network dependability. Copyright © 2005 IFAC.

The paper presents a framework to evaluate dependability of PROFIBUS-DP networks. An hierarchical modeling approach is proposed, which considers both the effects of permanent and transient faults. Stochastic Petri Nets are used as the modelling technique to obtain dependability models with emphasis in the use of analytical/numerical methods to provide model's solutions. In scenarios where its difficult to apply this approach, compact simulation models are derived.

Distributed control systems operation strongly depends on the communication system performance. Therefore, their analysis becomes an important aspect to be considered, particularly in situations where communications have real-time constrains and the network operates on faulty environments. In this paper the behavior of PROFIBUS network in the presence of faults is analyzed. This analysis is performed in an experimental base supported by a real network. The network operation is disturbed through a set of fault injection experiments from which performance metrics related with PROFIBUS outage times are evaluated.

The paper proposes a dependability model which enables the evaluation of Profibus-DP networks in scenarios of transient faults which affect data communications. The full behavior of Profibus-DP communication stack is modeled, including: cyclic process data exchange between master and slave stations; configuration, parameterization and diagnostics of slave stations. The model is based on a high level stochastic Petri net formalism referred as stochastic activity networks (SAN), supported by the Mobius tool. High modeling power, state-of-the-art analytical and simulation solutions, and a flexible and integrated environment are their main features. Dependability measures are established from the fulfillment of the real-time constraints (deadlines) defined on process data messages exchanged between master and slave stations. The reward concept is used to define the measures, which are obtained by means of a simulation approach. A case study is proposed to assess the model performance

Fieldbus networks have been assuming a high acceptance in the industrial environment, replacing the old centralized control architectures. Due to time critical nature of the tasks involved in these environments, the fulfillment of dependability attributes is usually required. Therefore, the dependability is an important parameter on system design, which should be evaluated. Several factors can affect system dependability. The environmental ones are the most common and due to the particularity of the industrial environment this susceptibility is increased. In this paper, we propose a framework based on fault injection techniques, supported by a hardware platform, which emulates a fault set, representative of industrial environment scenarios, intending to disturb data communications on a PROFIBUS network. From these fault injection experiments, relevant data is gathered and a further analysis is carried out in order to evaluate dependability attributes.

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.

The objective of this work was to evaluate the NPK influence on yield and oil, proteins and phytochemical contents in sunflower grain and to explore the potential of mathematical model of marginal contributions of fertilization effects. The field experiment was conducted in 3 3 incomplete factorial model, in two randomized blocks, with 15 parcels for block. The treatments were zero, 30 and 60 kg ha -1 of N, P and K. The achene yield increased with fertilizer doses until 55 kg of N ha -1 , 41 kg ha -1 of K 2 O and 46 kg ha -1 of P 2 O 5 . The achene oil content answered positively to increased K 2 O and P 2 O 5 . Proteins content decreases as K 2 O rate increases. No influence of the fertilization was observed in concentrations of chlorogenic acid, total sterols and total phosphorus on achene.

This paper presents a framework to evaluate fieldbus; dependability in presence of transient faults. Several fault models, which copes most situations found in an industrial environment, are proposed. Fault impact on fieldbus communications is evaluated taking into account different communication protocols and specific operating modes. Bus redundancy and common-cause faults are also considered. Analytical models are proposed and closed-form expressions derived, enabling the identification of the most relevant parameters from a dependability viewpoint. Copyright (C) 2001 IFAC.

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.