Paulo Portugal

Reliability and Availability Evaluation of Wireless Sensor Networks for Industrial Applications

Wireless Sensor Networks (WSN) currently represent the best candidate to be adopted as the communication solution for the last mile connection in process control and monitoring applications in industrial environments. Most of these applications have stringent dependability (reliability and availability) requirements, as a system failure may result in economic losses, put people in danger or lead to environmental damages. 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 communications over long periods of time and consequently disturb, or even disable, control algorithms. The lack of a structured approach enabling the evaluation of permanent faults, prevents system designers to optimize decisions that minimize these occurrences. In this work we propose a methodology based on an automatic generation of a fault tree to evaluate the reliability and availability of Wireless Sensor Networks, when permanent faults occur on network devices. The proposal supports any topology, different levels of redundancy, network reconfigurations, criticality of devices and arbitrary failure conditions. The proposed methodology is particularly suitable for the design and validation of Wireless Sensor Networks when trying to optimize its reliability and availability requirements.

VTP-CSMA: A Virtual Token Passing Approach for Real-Time Communication in IEEE 802.11 Wireless Networks

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.

Simulation Analysis of the IEEE 802.11e EDCA Protocol for an Industrially-Relevant Real-Time Communication Scenario

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.

An approach to implement data fusion techniques in wireless sensor networks using genetic machine learning algorithms

Wireless Sensor Networks (WSNs) can be used to monitor hazardous and inaccessible areas. In these situations, the power supply (e.g. battery) of each node cannot be easily replaced. One solution to deal with the limited capacity of current power supplies is to deploy a large number of sensor nodes, since the lifetime and dependability of the network will increase through cooperation among nodes. Applications on WSN may also have other concerns, such as meeting temporal deadlines on message transmissions and maximizing the quality of information. Data fusion is a well-known technique that can be useful for the enhancement of data quality and for the maximization of WSN lifetime. In this paper, we propose an approach that allows the implementation of parallel data fusion techniques in IEEE 802.15.4 networks. One of the main advantages of the proposed approach is that it enables a trade-off between different user-defined metrics through the use of a genetic machine learning algorithm. Simulations and field experiments performed in different communication scenarios highlight significant improvements when compared with, for instance, the Gur Game approach or the implementation of conventional periodic communication techniques over IEEE 802.15.4 networks.

Adaptive Monitoring Relevance in Camera Networks for Critical Surveillance Applications

Camera networks have been considered for a large set of visual monitoring applications. For some of them, cameras may be continuously monitoring scenes or groups of targets, but some events may trigger a critical level of visual monitoring, as in public security, industrial automation, and response to natural disasters. In such way, some critical events as a bomb explosion, a volcanic eruption, forest wildfire, or a car accident must be captured with high relevance, potentially helping when identifying responsibilities and during rescue operations. New relevance levels, which can be reflected in higher quality of transmitted images or video streams or even higher priority during transmission over the network must be quickly assigned to cameras that can view the critical events. In this work we propose a methodology to dynamically assign relevancies to cameras that view the area of critical events, employing scalar sensors and a decentralized decision mechanism. The resulting multimodal camera network can considerably enhance the critical surveillance in different monitoring applications. We propose some optimizations that exploit the monitoring relevance in such scenarios.

A Stochastic Petri Net Model for the Simulation Analysis of the IEEE 802.11e EDCA Communication Protocol

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.

A new MAC scheme specifically suited for real-time industrial communication based on IEEE 802.11e

This paper proposes a new real-time communication scheme for 802.11e wireless networks. This scheme is called Group Sequential Communication (GSC). The GSC improves the efficiency of the Hybrid Coordination Function Controlled Channel Access (HCCA) mechanism by reducing the protocol overheads of the 802.11e amendment. The GSC approach eliminates the polling scheme used in traditional scheduling algorithm, by means of a virtual token passing procedure among members of the real-time group to whom is granted a high-priority and sequential access to communication medium. In order to improve the reliability of the proposed scheme, it is also proposed an error recovery mechanism based on block acknowledgment. The GSC was implemented in network simulator software and the performance results were compared to HCCA scheme, showing the efficient of the proposed approach when dealing with traditional industrial communication scenarios.

A TDMA-based mechanism for real-time communication in IEEE 802.11e networks

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.

Limitations of the IEEE 802.11e EDCA protocol when supporting real-time communication

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 target 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.

Assessment of PROFIBUS networks using a fault injection framework

Industrial control systems architectures have been evolving to the decentralization of control tasks. This evolution associated with the time-critical nature of these tasks, increases the dependability requirements for one of their most critical components: the communication system. Therefore, this is an important aspect on the control system design which must be properly evaluated. In this paper the dependability of a PROFIBUS network is assessed. By using of a fault injection framework the network operation is disturbed with fault scenarios which are representative of industrial environments. From these experiments, the stability of the PROFIBUS logical ring is analyzed, the main outages causes are identified and their probabilities are obtained