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Emergency vehicles have been employed in rescue operations and supportive services, attending victims and managing critical situations in smart cities. Such vehicles, notably ambulances, fire trucks, police cars and transit agents vehicles, may be tracked and monitored in some applications for different functions. When such emergency vehicles are not equipped with GPS receivers, cameras can be used to view emergency signs printed on them, allowing indirect identification of emergency vehicles, although many complexities have to be considered when performing visual sensors-based tracking and monitoring. In this context, this paper proposes a mathematical model focused on the evaluation of the coverage efficiency of a group of visual sensors over moving vehicles, aimed at visual coverage of emergency signs. For that, vehicles, emergency signs and visual sensors are mathematically modelled in this paper, with coverage interactions among these elements being computed based on proposed geometry equations and algorithms. Doing so, the effectiveness of the positioning and configurations of visual sensors can be evaluated without requiring actual deployment, potentially reducing costs when assessing visual monitoring systems in this scenario.

Wireless sensor networks comprising nodes equipped with cameras have become common in many scenarios, providing valuable visual data for some relevant services such as localization, tracking, patterns identification and emergencies detection. In this context, algorithms and optimization approaches have been designed to perform different types of quality assessment or performance enhancement tasks, addressing challenging issues such as networking, compression, availability, reliability, security, energy efficiency and virtually any subject related to the operational challenges of those networks. However, the dynamics of coverage failures have not been properly modelled in visual sensor networks, resulting in unrealistic perceptions when optimizing or assessing quality in most visual sensing scenarios. Particularly, the Field of View of visual sensors will be affected by occlusion caused by obstacles in the monitored field, which may turn such sensors inadequate for the expected monitoring services of the considered network. Therefore, this article proposes a mathematical model to assess occlusion caused by mobile obstacles such as vehicles on a road or forklifts in an industrial plant, aiming at the selection of the visual sensor nodes that will not have their coverage significantly restricted by those obstacles. Doing so, the proposed model can be exploited by any optimization or quality assessment approach in wireless visual sensor networks, providing a preprocessing method when selecting visual nodes.

The adoption of sensors-based monitoring systems supported by Internet of Things technologies has opened new possibilities for data retrieving and processing in urban areas. Among such possibilities, emergencies management is expected to play an important role in how modern cities will evolve, reducing the negative impacts of critical events and improving the quality of life perceived by their inhabitants. Actually, when an emergency is detected and alerted, emergency vehicles, notably ambulances, fire trucks, police cars and transit agents vehicles, should be quickly assigned to respond to that situation, as soon as possible. In this context, we propose a dynamic algorithm to automatically assign emergency vehicles in smart city scenarios, exploiting for that a sensors-based emergency detection system to provide emergency alerts. The proposed algorithm can then be used to quickly assign a number of emergency vehicles in the first moments of an emergency, which can potentially save lives and improve existing crisis management applications in smart cities.

In this work we address a problem of practical relevance -the delivery of X.509 v3 digital certificates to end users on personal computers and smartphones- and we seek to describe the design, construction and evaluation of an artifact with which it is possible to solve it. Seven meta-requirements were defined, which have been instantiated into an artifact, which in turn has been empirically evaluated. The evaluation of the artifact has demonstrated its effectiveness based on actual use and perceived ease of use. The results of this work contribute to positioning personal computers and smartphones as alternative cryptographic modules to smart cards for some usage scenarios.

Wireless visual sensor networks (WVSN) bring a more comprehensive perception of monitored environments, leading to an increase adoption of such networks as a promising solution for a wide range of applications. Among many examples, highlight industrial applications related to the industry 4.0 paradigm, which increasingly require more data from manufacturing systems. Those sensor-based applications are in many cases safety-critical, requiring dependability guarantees mainly related with reliability and availability, that should be maintained during the whole network operation. Although several approaches have provided network deployment with dependability guarantees, sometimes the monitored environment or the application configurations can change during the network operation, which can violate the dependability requirements and demand network redeployment in order to keep those guarantees. In this paper we propose a novel algorithm to redeploy WVSN guided by the optimization of the application dependability, considering changes on cameras’ orientations. A methodology is defined to support dependability analysis. We compare the results of the proposed algorithm with previous algorithms found in literature. The achieved results show that the proposed algorithm is useful and efficient to provide network redeployment, keeping or improving the application dependability.

Nowadays one of the web browsers constraints is their limited capacity for interaction with end-user physical cryptographic devices, such as smart cards and cryptographic tokens. For this reason, in this article, we present an artifact that provides to web applications the ability to perform asymmetric cryptographic operations on the client-side using these devices. The artifact, designed as a service, presents two functions: (1) creation of electronic signatures of electronic documents and (2) electronic authentication of users into websites. It was designed with the purpose of allowing web developers to perform integrations in a simple and fast way. Through an empirical evaluation, in a controlled experiment, short-time integration was evidenced, confirming its potential usefulness for web developers.

Area coverage is an inherent and important topic when dealing with wireless visual sensor networks, since it may be desired when addressing availability and fault tolerance in critical applications. This problem arises because more than one visual sensor may cover the same area, generating overlapped regions that can be exploited for different kinds of optimization and quality enhancement approaches. Actually, some methods to compute the resulted covered area by a set of sensors have been proposed, and they are initial steps to compute availability metrics that are necessary for many monitoring scenarios. Particularly, approximate approaches are promising when computing area coverage, potentially achieving good results, although such methods lack proper evaluation and analysis about complexity, performance and precision. In this context, we perform an evaluation of a recent algorithm based on approximation for area coverage computing, comparing it with a precise algorithm developed in this work for this purpose. Doing so, it is desired to assess performance and accuracy of both algorithms, indicating the most appropriate approach when addressing availability in visual sensor networks.

Modern cities are subject to a lot of periodic or unexpected critical events, which may have different monitoring and control requirements according to the expected impacts on people safety and urban mobility. When multiple monitoring and automation systems are deployed, adaptive wireless sensor networks may adjust sensing and transmission configurations according to the detected events, optimizing the network overall operation. In this context, mobile sinks come as an effective way to enhance monitoring performance in smart city environments. However, practical issues related to the available roads and traffic load should be considered, allowing the computation of the best final positions and movement paths for each sink. Therefore, this paper proposes algorithms to compute dynamic sinks movement in reactive wireless sensor networks, supporting efficient adaptation to event-based monitoring in smart cities.

The use of Wireless Sensor Network (WSN) based technologies is an attractive solution for large-scale sensing applications (wide area deployment), such as environmental monitoring, precision agriculture and industrial automation. IEEE 802.15.4/ZigBee standards are the most used communication protocols for WSN technologies, where the clustertree topology is pointed out as a suitable topology to support the implementation of large-scale WSNs. These networks are usually scheduled to prioritise convergecast (upstream) traffic generated from sensor nodes toward the sink node. However, this scheduling pattern results in higher delays for control messages (downstream traffic). Within this context, this paper proposes a Hybrid Beacon Scheduling (Fast-HyBeS) scheme to enable the periodic reconfiguration of cluster-tree WSNs. The underlying idea is to periodically schedule a downstream opportunity window, to allow a faster dissemination of control messages. This opportunity window follows a top-down scheduling approach that prioritises the downstream traffic. Simulation results show that the use of Fast-HyBeS can significantly decrease the end-to-end communication delay for control messages, when compared to the use of static convergecast scheduling schemes. Moreover, the simulation results also highlight that the Fast-HyBeS has a negligible impact upon end-to-end communication delays of the monitoring traffic.

The IEEE 802.15.4/ZigBee standards support the implementation of cluster-tree networks, which are a suitable topology to deploy wide-scale networks. In this type of topology, an important issue is the configuration of the superframe duration that is allocated to each of the network clusters. In this paper, we propose an allocation scheme for setting-up these superframe duration values. The proposed scheme allocates adequate duration values for each cluster coordinator, based on the message load originated from its child nodes (including child coordinators and its descendants). The target of the proposed allocation scheme is to improve the network throughput and to reduce network congestion and dropped messages. Its main advantages are: 1) to have a balanced allocation of network resources to the different clusters; 2) the message generation at the application layer does not need to be synchronised with the beacon arrival; and 3) the message periods do not need to be multiple of the beacon interval.

Wireless sensor networks are being widely used to support time-critical applications, even knowing that previous versions of the IEEE 802.15.4 standard have a limited capacity to ensure a timely communication service. Its main limitation is the reduced number of available GTS slots. More recently, the IEEE 802.15.4e LLDN mode has been released to address this issue. It provides static TDMA-based communication, with a larger number of available slots to support time-critical communication. However, it does not provide any guidelines on how to allocate slots, in order to guarantee the message real-time requirements. In this paper, we propose a methodology to allocate LLDN slots based on the traffic load imposed by the supported message streams. The proposed allocation scheme is able to handle message streams with periodicities that are not multiple of the beacon interval, and also to support static and dynamic prioritizing of message streams.

The LLDN (Low Latency Deterministic Network) mode is a IEEE 802.15.4e amendment specifically designed for industrial applications requiring low latency and low loss rate. It is based on a static TDMA scheme composed of fixed-size slots. One of its limitations regards the support of messages with different sizes and different periodicities. In this paper, a slot allocation scheme is proposed, enabling the support of heterogeneous message streams. The rationale is to compute a suitable timeslot size to communication devices, enabling adaptive control of the superframe without changing the LLDN standard. This paper shows that it is possible to accommodate heterogeneous message streams while maintaining low cycle times when transmitting messages with variable payload.

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.

The IEEE 802.11 standard has been evolving over the past decade, introducing a set of new mechanisms at the MAC layer to improve the Quality of Service (QoS) provided to the communication. Among such improvements, we highlight the evolution from earlier DCF and PCF, until more recent EDCA and HCCA MAC layer mechanisms. In this paper we perform a simulation assessment of these four MAC mechanisms, evaluating their ability to support real-time (RT) communication. More specifically, we assess their ability to handle RT traffic in open communication environments composed of RT and non-RT stations operating in the same frequency channel and coverage area. The target of this paper is to highlight and understand the limitations of each mechanism when supporting RT communication. We show that for most of the situations, including less demanding scenarios, these mechanisms are not adequate to support RT traffic.

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.

Fault Tolerance for Flexible Time-Triggered Ethernet-based systems (FT4FTT) is a project to devise an architecture for distributed embedded systems that provides both flexibility to changing real-time requirements and high reliability through fault tolerance. One of the key parts of such an architecture is the communication subsystem. When designing such a subsystem many decisions have to be made. To understand how such decisions impact the reliability of the final design, in this paper we present a framework to evaluate the reliability of a large number of potential designs. The approach is based on storing a finite subset of the design space for the communication subsystem of FT4FTT in an undirected graph and then generating a continuous-time Markov chain from the graph to evaluate the reliability of each design belonging to the subset.

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.

The IEEE 802.15.4 standard proposes a flexible communication solution for Low-Rate Wireless Personal Area Networks, including Wireless Sensor Networks. In these networks, the transmission of messages or the Guaranteed Time Slot allocation are coordinated by the traditional CSMA/CA scheme, which does not provide any additional traffic differentiation mechanism. In a previous work, we proposed an approach that assigns priorities to the messages based on the (m, k)-firm task model. This paper proposes, in a complementary way, an experimental assessment in order to define the best parameters for messages prioritization. The experimental assessment considers a realistic scenario in which a set of sensor nodes, implementing the proposed scheme works in the same coverage area of another set of nodes that does not belong to the same Wireless Sensor Networks infrastructure.

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.

Wireless sensor networks may be deployed to retrieve visual information from the monitored field, enriching monitoring and control applications. Whenever a set of camera-enabled sensor nodes are deployed for time-critical monitoring, visual information as still images and video streams may need to reach the sink as soon as possible, requiring a differentiated treating of the network when compared with non-critical visual data. In such way, considering that source nodes may have different sensing relevancies for the application, according to the desired monitoring tasks and the current sensors' poses and fields of view, we propose a delay-aware multihop routing mechanism where higher relevant visual data packets are routed through paths with lower end-to-end delay. As sensor nodes are expected to be energy-constrained, transmitting only high-relevant packets through shorter/faster paths may prolong their lifetime and assure longer time-critical delivering, with low impact to the overall monitoring quality.

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.

In this paper is proposed a communication approach for implementation of parallel data fusion techniques in IEEE 802.15.4 star networks. In the approach, the network configures itself in order to adjust the number of messages sent periodically by sensor nodes, establishing a trade-off between the number of sent messages and the dependability of data fusion in base station. Conventional techniques for data fusion can be employed using the proposed approach, such as Dempster-Shafer or Kalman filter. A prototype was implemented and experiments were carried out to evaluate the approach. Results showed the effectiveness of the proposal.

In this paper we propose the use of a coordination layer to handle real-time communication in infrastructured WiFi networks. This layer combines a TDMA scheme with a traffic separation mechanism (FCR MAC), which enables the prioritization of real-time (RT) traffic over uncontrolled (external) traffic sources. The target of this paper is to assess the behavior of this coordination layer when supporting RT communication and to compare these results with those obtained with IEEE 802.11e EDCA. The simulation assessment considers an open communication environment, where a set of RT and non-RT stations share the same coverage area and frequency band. A realistic error-prone channel was used to measure the impact of interferences against an error-free channel. We show that the proposed solution offers a significant improvement when compared with EDCA, in what concerns average deadline losses and delay.

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.

In this paper, we propose a TDMA-based communication approach to be used upon the EDCA communication mechanism defined in the IEEE 802.11e standard. This approach allows the coexistence of real-time (RT) traffic together with uncontrolled (external) traffic sources. In the context of this paper, RT traffic means small sized packets generated in periodic intervals, which must be delivered before the end of the message stream period. The target of this paper is to highlight some limitations of the EDCA mechanism when supporting RT communication and to compare these results with those obtained with the proposed TDMA approach. We have assessed these two mechanisms considering an open communication environment, where there are RT and non-RT stations operating in the same frequency band. Furthermore, a realistic error-prone model channel was used to measure the impact of interferences against an error-free channel. We show that the proposed TDMA approach offers an almost constant access delay and it also improves the EDCA behavior in what concerns the average functional throughput and the average deadline losses.

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.

In this paper, we analyze the timing behavior of the EDCA communication mechanism defined in the IEEE 802.11e standard, when it is used to support real-time (RT) traffic. In the context of this paper RT traffic means small sized packets generated in periodic intervals, which must be delivered before the end of the message stream period. Otherwise. the message is considered to be delayed and a deadline loss occurs. The tat-get of this paper is to understand the limitations of the highest priority level of the EDCA mechanism (voice category) when supporting RT communication. We have assessed this mechanism considering an open communication environment, where there are RT and non-RT stations operating in the same frequency band. Furthermore, a realistic error-prone model channel was used to measure the impact of interferences against an error-free channel. we show that in the most cases evaluated, both the number of packet losses and the average packet delays forecast an unacceptable number of deadline losses for the RT Message streams, even for intermediate load cases. As a conclusion of this paper, we present some potential future directions toward improved QoS in wireless networks.

The main objective of this thesis is to address the following question: ldquohow to guarantee the timing requirements of real-time (RT) control data, when the communication medium is shared with timing unconstrained traffic?rdquo. That is, to propose mechanisms that enable the support of RT communication services in CSMA-based communication infrastructures, specifically when such infrastructures are also being used to support multipurpose data transfer applications. The target is to address the timing requirements of typical industrial applications. Thus, a new RT-communication approach (VTP-CSMA) has been proposed, which is based on the use of traffic separation mechanisms. Such mechanisms are able to prioritize RT-traffic over multipurpose traffic, without directly controlling the latter. That is, instead of controlling all the traffic generated by all the stations, this approach controls only the traffic generated by the RT stations. The VTP-CSMA approach forces the collision resolution in favor of the RT stations, enabling the fulfillment of the RT communication requirements.

Keeping up with the timing constraints of real-time traffic in wireless environments is a hard task. One of the reasons is that the real-time stations have to share the same communication medium with stations that are out of the sphere-of control of the real-time architecture. That is, with stations that generate timing unconstrained traffic. The VTP-CSMA architecture targets this problem in IEEE 802.11 wireless networks. It is based on a Virtual Token Passing procedure (VTP) that circulates a virtual token among real-time stations, enabling the coexistence of real-time and non realtime stations in a shared communication environment. The worst-case timing analysis of the VTP-CSMA mechanism shows that the token rotation time is upper-bounded, even when the communication medium is shared with timing unconstrained stations. Additionally, the simulation analysis shows that the token rotation mechanism behaves adequately, even in the presence of error-prone communication channels. Therefore, the VTP-CSMA architecture enables the support of real-time communication in shared communication environments, without the need to control the timing behavior of every communicating device. A ring management procedure for the VTP-CSMA architecture is also proposed, allowing real-time stations to adequately join/leave the virtual ring. This ring management procedure is mandatory for dynamic operating scenarios, such as those found in VoIP applications.

Currently there is a trend for the implementation of industrial communication systems on top of wireless communications. However, keeping up with the timing constraints of real-time traffic in open medium environments is a hard task. The main source for complexity is that, if the set of communicating devices is not previously agreed, the timing constraints imposed by such devices cannot be predicted at the system setup time, nor can be effectively controlled at the system run-time. In this paper, we propose the VTP-CSMA architecture to deal with this problem. This architecture allows the coexistence of default IEEE 802.11e devices with real-time devices sharing the same communication medium, enabling the prioritization of real-time traffic. The proposed solution is based on the control of the medium access rights by means of a Virtual Token Passing procedure (VTP), complemented by an underlying traffic separation mechanism that prioritizes real-time traffic over the traffic from default IEEE 802.11 stations. The simulation analysis shows that the VTP-CSMA architecture guarantees values for both the throughput and the average access delay that significantly improves the results obtained for default IEEE 802.11e stations operating under EDCA mode.

The paper proposes a simulation model which enables the evaluation of IEEE 802.11e EDCA networks, focusing in the behavior of the Enhanced Distributed Channel Access (EDCA) function associated to Quality of Service (QoS) stations. When compared with currently used simulation models, the proposed model provides a more accurate implementation of some aspects (timeouts, EIFS), a flexible implementation, and an easiness of use. Besides, a large number of different types of performance measures can be obtained. The model is based on a high level Stochastic Petri Net formalism referred as Stochastic Activity Networks (SAN), able to produce very compact and efficient models, and which is supported by the Mobius tool.

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.