Wireless Sensor Networks (WSNs) based on the Long-Range radio modulation (LoRa) can use the LoRaWAN protocol as the medium access layer. However, this protocol only supports a single-hop star topology. As a consequence, devices can not use retransmissions along the network to extend their coverage area or to circumvent signal attenuation with distance, obstacles or interference from other radio sources. This paper proposes a multi-hop LoRaWAN wide-scale WSN based on a scheduled cluster-tree topology. This methodology can expand the spatial coverage of the network, decrease collisions, and improve overall network performance. A multi-hop cluster-tree topology eliminates the need for adjustments of LoRa radio parameters as an attempt to expand the single-hop coverage limitation. Simulation results show that the scheduled cluster-tree topology can scale the network coverage and significantly improve communication and energy consumption performances.

The use of adequate relay selection techniques is crucial to improve the behavior of cooperation based approaches in Wireless Sensor Networks (WSN). The Optimized Relay Selection Technique (ORST) is a relay selection technique that may be reduced to the application on classic set-covering problem (SCP) to WSN. The SCP seeks to find a minimum number of sets that contain all elements of all data sets. The SCP can be solved with different types of algorithms. This paper assesses the performance and quality of three different algorithms to solve the SCP generated by the previously proposed ORST technique, considering performance metrics relevant within WSNs context. The analysis was performed by simulation using the OMNeT++ tool and the WSN framework Castalia. The simulation results show that the branch and bound algorithm excels when compared to other state-of-the-art approaches.

Cluster-tree topologies are commonly employed in large-scale Wireless Sensor Networks applications where nodes are randomly deployed. This topology presents several challenges related to connectivity and message schedulability. As a consequence, effective methods for network formation and operation are required. This work proposes a combination of methods of neighborhood discovery, network formation and message scheduling to deal with this problem. This proposal aims to build a flexible network, with better performance and is also able to adapt for different scenarios. Simulation results highlight significant improvements of the network performance, when compared to the use of LEACH protocol.

Topology formation in wireless sensor networks is usually done assuming just either the geographical proximity between nodes or the signal strength of communication. In this paper, a heuristic called DbCTF is proposed to guide the formation of cluster-tree networks, which also considers data clustering techniques. The use of DbCTF allows the setup of a data-based topology in the cluster-tree, and also the prioritisation of monitored regions in which relevant events may be occurring. The performance of DbCTF has been compared with a state-of-the-art algorithm, for the specific case of a classical WSN laboratory experiment. The simulation assessment revealed that the cluster-tree formed by DbCTF was able to reduce by more than 20% the average communication delay of message streams conveying critical data, and was also able to increase by more than 35% the average lifetime of the network.

Cloud Computing, Big Data Analytics and NoSQL databases are technologies that have been rapidly evolving in recent years. These technologies have demonstrated the importance of horizontal scalability, a key feature of the infrastructure elasticity of large internet companies. However, elasticity is often priced out of the reach of small and medium sized businesses (SMBs), which have a slower adoption rate for these technologies. In this paper, we present the Rapid Elasticity to Prevent Overload (REPO) system, a microservices elastic management system for cost reduction in the cloud. It was designed to enhance a virtual private server (VPS) without dynamic load balancing capabilities, a kind of server heavily used by budget-restricted companies such as SMBs and web startups.

Wireless Sensor Networks (WSNs) arc susceptible to faults both in sensors and in communication. Information fusion techniques allow to extract precise information from a large amount of data. Detection, identification and treatment of outlier, in these techniques, is a key point. Outlier detection in WSNs is a challenge due to the low capacity of the nodes and low bandwidth of the network. This paper proposes a methodology that applies the clustering and lightweight statistics techniques for detection of outliers in WSNs. The assessment of the methodology involves a case study with temperature sensors in WSN nodes. The results show that this methodology is able to provide precise information, even in the presence of outliers.

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.

Till nowadays, the Controller Area Network (CAN) has been a de facto standard for communication in automotive applications. To meet the requirements of new high-end vehicular systems, new communication protocols such as the FlexRay Communication System and the CAN with Flexible Data-rate (CAN-FD) has been developed. In the near future, it is expected the coexistence of those protocols in the same vehicle, with electronic control units (ECUs) connected to different network buses exchanging information through gateways. In this paper we investigate the following problem: “how to schedule the communication in a vehicular system, considering that a message is transmitted through a network that is composed of CAN-FD, FlexRay and CAN segments interconnected by gateways.” We propose a method for the schedulability analysis of such systems, focusing on the case where a message is generated in a ECU connected to a CAN-FD segment is used by an ECU connected to a CAN bus.

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.

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.

In wireless visual sensor networks comprised of multiple camera-enabled sensors, source prioritization can be exploited to soften the impact of congestion, packet loss and energy depletion when higher relevant packets are processed. However, for such optimizations, source nodes have to be properly prioritized according to some effective metric. When performing visual sensing over moving targets, sensors may view different parts of the targets, which may have particular relevance for monitoring applications. In this context, this paper proposes a low-cost mathematical approach that associates a priority level to each visual source node according to the viewed segments of the targets' perimeter, and such priority may then be exploited for a large set of optimizations. A complete mathematical formulation and numerical results are presented to base the proposed approach.

IEC 61850 is a communication standard for electrical Substation Automation Systems (SAS). It defines both the information model and services used for communication between Intelligent Electronic Devices (IEDs) in a substation. The adoption of this standard brings several advantages for the design and operation of substations. The abstract data models defined in IEC 61850 can be mapped upon application protocols, such as MMS, GOOSE or SMV. These protocols can run upon TCP/IP networks or upon specific high speed Ethernet LANs, in order to match the timing requirements associated to protective relaying mechanisms. For the specific case of GOOSE messages, the standard specifies the use of VLANs (Virtual LANs) with priority tagging (IEEE 802.1q) to implement separate virtual networks with the appropriate message priority levels, in order to ensure the specified response times. The lack of adequate simulation models that enable the response time assessment of both SMV and GOOSE messages is one of the shortcomings of available simulation tools. In this paper, we propose simulation models for the IEC 61850 communication standard, targeting application that use GOOSE and SMV messages. This simulation models has been built upon OMNeT++/INET. The simulation results obtained from a typical IEC 61850 communication scenario show the effectiveness of the developed models. Some of these results have been experimentally validated.

Energy consumption is a major problem in Wireless Sensor Networks (WSNs) since in most scenarios nodes operate on batteries with reduced size and low capacity. Battery lifetime estimation is therefore crucial to evaluate the network behavior over time. In addition, battery lifetime estimation is a major challenge since it depends on many factors, such as the duty cycle of supported applications and the intrinsic chemical reactions within the batteries. Several battery models have been proposed to deal with battery lifetime estimation. However, most of the available models address only high-capacity batteries. This paper discusses a method for extracting the parameters for the Kinetic Battery Model (KiBaM) used within WSN context. The target is to define a battery model that helps to estimate the lifetime of nodes using low-capacity batteries and which operate in a duty cycle scheme. Results show that KiBaM is an adequate model for WSNs since it presents low error when compared with experimental results using real batteries.

The use of wireless sensor networks (WSN) to support critical monitoring applications is becoming a relevant topic of interest. These networks allow a highly flexible approach to data monitoring and, consequently, a major breakthrough for several application domains, from industrial control applications to large building domotics and health care applications. One of the major impairments of using wireless networks to support critical monitoring applications is the electromagnetic noise, which may increase the packet loss ratio to unacceptable values. In this paper, we assess different techniques of cooperative communication and network coding that can be useful to mitigate the aforementioned problem. These techniques may be implemented in WSN nodes in conformance with the IEEE 802.15.4 standard, to reduce the impact of electromagnetic interferences upon the packet loss ratio. In this paper, we report an experimental assessment of the network coding and cooperative diversity techniques, where the network is subjected to a controlled electromagnetic interference inside of an anechoic chamber. The experimental results show that, by using these techniques, it is possible to increase the success rate of communication in typical electromagnetic noisy environments.

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.

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.

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 use of cooperative diversity techniques and network coding concepts are promising solutions to improve the communication reliability in industrial Wireless Sensor Networks (WSNs). In this paper, we propose the NetCoDer scheme to address this problem, whose design is based in these concepts. The effectiveness of the NetCoDer scheme is evaluated through both an experimental setup with real WSN nodes and a simulation assessment, comparing its performance against state-of-the-art TDMA-based retransmission techniques.

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.

In this paper we propose a novel approach, named Real-Time Alternative-Route Definition (RT-ARounD), to support real-time message streams over large-scale wireless sensor networks. Starting from a standard cluster-tree topology, where each cluster has active and inactive periods, the proposal selects a set of nodes to build a mesh topology which is used during the inactive periods of the respective clusters to transmit messages with real-time requirements. This new topology is built considering shortest path routes between source and sink nodes. In that way, the number of hops that a real-time message needs to transverse can be drastically reduced, thereby improving the real-time responsiveness of the overall network. Moreover, the use of the inactive period guarantees that the normal cluster-tree operation remains unaffected.

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.

This paper assesses the interference caused by uncontrolled traffic sources upon real-time traffic in IEEE 802.11s mesh networks. It was considered a mesh network transmitting real-time traffic and an external interference was injected in order to evaluate its impact over the real-time communication. A set of experiments were performed in ns-3 and different scenarios were evaluated, where the end-to-end delay, message loss and deadline miss ratios were chosen as performance metrics. It can be inferred that the real-time traffic in 802.11s is highly affected by external interferences, and therefore that must be taken into account when setting-up such type of 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).

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.

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.

In some Wireless Sensor Network (WSN) applications, it may be necessary to keep a large number of nodes sensing and transmitting data to a base station in order to have unbiased measurement values. Therefore, in addition to the traditional energy consumption issues, the spatial diversity of the monitored area is another relevant metric to evaluate the performance of a WSN. Nevertheless there is a clear trade-off between these two parameters, as keeping the sensors active all the time will deplete the nodes batteries and therefore will shorten the network lifetime. Fortunately, for the case of some applications it is possible to specify as a Quality of Service (QoS) parameter the number of periodically expected messages in the base station. Therefore, it will be possible to balance QoS against energy consumption. This paper proposes an approach called (m,k)-Gur Game that aims a trade-off between the expected QoS and the spatial coverage diversity. Simulation results show the effectiveness of the proposed approach.

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.

The IEEE 802.15.4 standard provides a flexible communication approach for LR-WPAN networks. When active, the beacon-enabled mode allows real-time communication by adopting a GTS (Guaranteed Time Slot) mechanism. However, this mechanism supports only up to seven real-time communicating devices. Motivated by this problem, this paper proposes the use of a dynamic GTS scheduling approach based on the (m, k)-firm task model, to deal with the GTS starvation problem. The proposed approach ensures that, for each admitted message stream Siat least m messages will be transmitted within each window of k consecutive deadlines. The effectiveness of the proposed scheduling approach is demonstrated by a set of simulation and experimental results. The proposed approach is fully compatible with the IEEE 802.15.4 standard.

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.

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.

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 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.

Low-power wireless communication protocols have been shown to experience a significant timeliness and reliability degradation under certain interference profiles. Provided that this impairment has hindered their wide adoption in demanding real-time scenarios, the Wireless Flexible Time Triggered (WFTT) protocol has been recently proposed targeting applications with stringent timeliness requirements. This protocol builds on the medium access determinism granted by the bandjacking technique in open environments, where different technologies may contend for the medium. Besides providing a deeper definition of the WFTT architecture and protocol operation, this paper presents a thorough analysis addressing its implementation feasibility.

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.

Wireless Mesh Networks (WMNs) are a promising communication technology that may offer greater flexibility and reliability, when compared to traditional wireless networks. WMNs open up new applications domains, but still need to find efficient mechanisms to deal with scalability and timeliness requirements. This paper proposes a scheme for Path Selection and Message Forwarding in IEEE 802.11s networks, that is suitable to be used in industrial environments. We present the DHT-based Cluster Routing Protocol (DCRP), a routing protocol based on DHTs, clustering of nodes and use of proxies. DCRP allows to improve the overall network performance by reducing the time required for path selection and the number of communication hops in large sized networks.

We present a family of reliable broadcast protocols designed to take advantage of the dual scheduling TDMA (DuST) scheme provided by current state-of-the-art automotive control networks such as FlexRay. These protocols are a complement to FlexRay's native communication services, which do not provide sufficient fault tolerance for safety-critical applications. A reliability evaluation of the proposed protocols carried out with the help of the probabilistic model checker PRISM shows that the proposed protocols can achieve reliability levels suitable for safety-critical applications.

IEEE 802.15.4/Zigbee is one of the most widespread Wireless Sensor Network technologies. It may work either in beacon-enabled mode or in beaconless mode. When operating in beacon-enabled mode, it divides the superframe in sixteen equally-sized time slots, where the request for slot allocation is achieved through a CSMA/CA approach. This allocation method does not allow the provision of any service differentiation scheme, which is required for the transfer of time constrained messages. This paper proposes a QoS provision approach with minimal changes to existing protocols. Our approach improves the CSMA/CA algorithm by assigning decentralized priorities based on the (m,k)-firm task model. In this paper, a set of experimental results is presented and compared with the traditional approach currently offered by the standard. This set of experiments highlight the promising behavior of the proposed approach, when dealing with high network load scenarios.

This paper studies the adoption of the ZigBee technology to replace broadcast-based wired communications in automation systems for Health Smart Homes. An analytical study is presented as well as the experimental results validating the derived expressions. Results show that the timeliness of ZigBee broadcast communications, in interference-free environments, can cope with the requirements of home automation applications and those found in building automation.

Wireless sensor networks (WSN) are being targeted for use in applications like security, resources monitoring and factory automation. However, the reduced available resources raise a lot of technical challenges. Self-organization in WSN is a desirable characteristic that can be achieved by means of data fusion techniques when delivering reliable data to users. In this paper it is proposed a genetic machine learning algorithm (GMLA) approach that makes a trade-off between quality of information and communication efficiency. GMLA is based on genetic algorithms and it can adapt itself dynamically to environment modifications. The main target of the proposed approach is to achieve self-organization in a WSN application with data fusion. Simulations demonstrate that the proposed approach can optimize communication efficiency in a dense WSN.

Wireless sensor network (WSN) is an emerging technology that targets multiple applications in the different environments. Its infrastructure is composed of a large number of sensor nodes with a limited physical capacity and low cost. The energy consumption must be as optimized as possible in order to extend its lifetime. The use of data compression techniques can be an advantage in the WSN context, once these techniques eliminate transmission of redundant information and consequently can be adopted to minimize the consumption of energy in the sensor nodes. WSN for monitoring applications can benefit from this technique as it may maximize the lifetime of batteries. The main motivation of this paper is to investigate the performance of a data compression algorithm for WSN in the context of monitoring applications. To validate the proposal, simulation experiments have been performed using the Network Simulator (NS-2) tool.

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.

The three papers in Part II of this special issue focus on communication in automation.

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 towards the implementation of industrial communication systems using wireless networks. However, keeping up with the timing constraints of real-time traffic in wireless environments is a hard task. The main reason is that real-time devices must share the same communication medium with timing unconstrained devices. The VTP-CSMA architecture has been proposed to deal with this problem. It considers an unified wireless system in one frequency band, where the communication bandwidth is shared by real-time and non-real-time communicating devices. The proposed architecture is based on a virtual token passing (VTP) procedure that circulates a virtual token among real-time devices. This virtual token is complemented by an underlying traffic separation mechanism that prioritizes the real-time traffic over the non-real-time traffic. This is one of the most innovative aspects of the proposed architecture, as most part of real-time communication approaches are not able to handle timing unconstrained traffic sharing the same communication medium. A ring management procedure for the VTP-CSMA architecture is also proposed, allowing real-time stations to adequately join/leave the virtual ring.

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 IEEE 802.11e standard was published as an amendment to the original IEEE 802.11 standard. This amendment is intended to provide differentiated levels of QoS to the supported applications. The 802.11e amendment incorporates an additional coordination function called hybrid coordination function (HCF) that uses both a contention-based channel access method, called the enhanced distributed channel access (EDCA) and a controlled channel access, referred to as the HCF controlled channel access (HCCA). Under the EDCA mechanism, it is a common assumption to consider the highest access category (voice) adequate to support real-time communication. In this paper, we analyze the timing behavior of the EDCA function, when it is used to support real-time traffic. Basically, we assess the behavior of the voice category in open communication environments (i.e., a communication environment subject to external disturbances) when this access category is used to transfer small sized packets, generated in periodic intervals. We show that the transmission opportunity (TXOP) mechanism included in the IEEE 802.11e amendment improves the system throughput, for the case of message streams with small packet sizes. However, the impact of external disturbances upon the transfer of real-time messages is highly relevant. For instance, the average access delay for the real-time messages is more than one order of magnitude larger when the external disturbance increases the network load from just 10% to 30%. Furthermore, both the number of packet losses and the average size of the MAC queues forecast an unacceptable number of deadline losses for the real-time message streams, even for intermediate load cases.

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 realtime 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

Traditional shared Ethernet networks have a non-deterministic behaviour, which impairs the support of real-time communication. In this paper, it is proposed a shared Ethernet deterministic architecture, able to interconnect sensors, controllers and actuators at the field level, allowing the coexistence of standard devices with enhanced (real-time) devices. Such solution is based on the control of the medium access right, by means of a virtual token passing procedure among enhanced stations, complemented by an underlying prioritization mechanism. Such underlying mechanism guarantees that, whenever an enhanced (real-time) station is contending for the bus access, it will be able to access the bus prior to any other station. Thus, it enables the traffic separation between standard and enhanced (real-time) stations, being able to guarantee real-time communication in unconstrained traffic environments

In this paper, the enhanced collision resolution algorithm, referred as high priority binary exponential backoff (h-BEB) is analyzed. Such algorithm provides high priority traffic separation, enabling the support of real-time communication in shared Ethernet networks. It allows multiple Ethernet standard devices to coexist in the same network segment with one h-BEB enhanced station, imposing higher priority for the transfer of h-BEB related traffic. The simulation analysis shows that the h-BEB algorithm guarantees values for both the average access delay and the related standard deviation, that are significantly smaller for the h-BEB station than for the BEB stations.

Controller area network (CAN) is a fieldbus network suitable for small-scale distributed computer controlled systems (DCCS), being appropriate for sending and receiving short real-time messages at speeds up to 1 Mbit/sec. Several studies are available on how to guarantee the real-time requirements of CAN messages, providing preruntime schedulability conditions to guarantee the real-time communication requirements of DCCS traffic. Usually, it is considered that CAN guarantees atomic multicast properties by means of its extensive error detection/signaling mechanisms. However, there are some error situations where messages can be delivered in duplicate or delivered only by a subset of the receivers, leading to inconsistencies in the supported applications. In order to prevent such inconsistencies, a middleware for reliable communication in CAN is proposed, taking advantage of CAN synchronous properties to minimize the runtime overhead. Such middleware comprises a set of atomic multicast and consolidation protocols, upon which the reliable communication properties are guaranteed. The related timing analysis demonstrates that, in spite of the extra stack of protocols, the real-time properties of CAN are preserved since the predictability of message transfer is guaranteed.

PROFIBUS is an international standard (IEC 61158) for factory-floor communications, with some hundreds of thousands of world-wide installations. However, it does not include any wireless capabilities. In this paper we propose a hybrid wired/wireless PROFIBUS solution where most of the design options are made in order to guarantee the proper real-time behaviour of the overall network. We address the timing unpredictability problems placed by the co-existence of heterogeneous transmission media in the same network. Moreover, we propose a novel solution to provide inter-cell mobility to PROFIBUS wireless nodes.

The Controller Area Network (CAN) is a fieldbus network with real-time capabilities. It is generally considered that CAN guarantees atomic multicast properties, through its extensive error detection/signalling mechanisms. However, there are error situations where messages can be delivered in duplicate by some receivers or delivered only by a subset of the receivers, leading to inconsistencies in the supported applications. In order to prevent such inconsistencies, a set of atomic multicast protocols is proposed, taking advantage of CAN synchronous properties to minimise its run-time overhead. This paper presents such set of protocols, focusing on the timing analysis of the supported reliable real-time communication. It demonstrates that, in spite of the extra stack of protocols, the real-time capabilities of CAN are preserved, since the predictability of message transfer is guaranteed.

It is generally considered that the controller area network (CAN) guarantees atomic broadcast properties through its extensive error detection and signalling mechanisms. However, it is known that these mechanisms may fail, and messages can be delivered in duplicate by some receivers or delivered only by a subset of the receivers. This misbehaviour may be disastrous if the CAN network is used to support replicated applications. In order to prevent such inconsistencies, a set of atomic broadcast protocols is proposed, taking advantage of CAN synchronous properties to minimise its run-time overhead. The paper presents such set of protocols, and demonstrates how they can be used for the development of distributed real-time applications.

Fieldbus communication networks aim to interconnect sensors, actuators and controllers within distributed computer-controlled systems. Therefore, they constitute the foundation upon which real-time applications are to be implemented. A specific class of fieldbus communication networks is based on a simplified version of token-passing protocols, where each station may transfer, at most, a single message per token visit (SMTV). In this paper, we establish an analogy between non-preemptive task scheduling in single processors and the scheduling of messages on SMTV token-passing networks. Moreover, we clearly show that concepts such as blocking and interference in non-preemptive task scheduling have their counterparts in the scheduling of messages on SMTV token-passing networks. Based on this task/message scheduling analogy, we provide pre-run-time schedulability conditions for supporting real-time messages with SMTV token-passing networks. We provide both utilisation-based and response time tests to perform the pre-run-time schedulability analysis of real-time messages on SMTV token-passing networks, considering RM/DM (rate monotonic/deadline monotonic) and EDF (earliest deadline first) priority assignment schemes.

In this paper we address the real-time capabilities of P-NET, which is a multi-master fieldbus standard based on a virtual token passing scheme. We show how P-NET's medium access control (MAC) protocol is able to guarantee a bounded access time to message requests. We then propose a model for implementing fixed priority-based dispatching mechanisms at each master's application level. In this way, we diminish the impact of the first-come-first-served (FCFS) policy that P-NET uses at the data link layer. The proposed model raises several issues well known within the real-time systems community. Message release jitter; pre-run-time schedulability analysis in non pre-emptive contexts; non-independence of tasks at the application level. We identify these issues in the proposed model and show how results available for priority-based task dispatching can be adapted to encompass priority-based message dispatching in P-NET networks.

The paper provides a comprehensive study on how to use Profibus networks to support real time communications, that is, ensuring the transmission of the real time messages before their deadlines. Profibus is based on a simplified Timed Token (TT) protocol, which is a well proved solution for real time communication systems. However, Profibus differences from the TT protocol prevent the application of the usual TT analysis. The main reason is that, conversely to the TT protocol, in the worst case, only one high priority message is processed per token visit. The major contribution of the paper is to prove that, despite this shortcoming, it is possible to guarantee communication real time behaviour with the Profibus protocol.

Addresses the problem of transmitting real-time periodic traffic in a DQDB network. In a DQDB network, connection-oriented isochronous services use the pre-arbitrated (PA) access. The standard specifies that isochronous connections must have guaranteed periodic bandwidth but mechanisms to provide it, such as the slot allocation scheme, are not described. We propose a real-time service provider (RTSP) based on the use of the PA access. The RTSP consists of an off-line centralized scheduling algorithm and an on-line mode change algorithm which allows to take into account load changes. Means are also provided to guarantee a minimum fairness level for the asynchronous traffic.>