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Issue Date: 3-May-2011
Authors: Collotta, Mario
Title: Novel Network Architectures and Algorithms for Real-Time Industrial Wireless Networks
Abstract: The use of wireless technologies in real-time automation systems offers attractive benefits, but introduces a number of new technological challenges. Behind the success of wireless networks in automation there are several advantages such as cost reduction, easy placement and installation, easy extension and mobile device connectivity. Wireless systems are in great demand also thanks to their flexibility. Devices can be rapidly installed without the added cost and time required by the installation of cables. Wireless technology is even more advantageous if installations are temporary. Wireless networks provide noticeable advantages in terms of mobility and costs reduction and they are in great demand due to their flexibility. The proper functioning of wireless infrastructures is of vi-tal importance for the acceptance and the success of the whole automation system. In addition to its main function of supporting data exchange, the network has to provide suit-able traffic management for efficiently dealing with the typi-cal requirements of automation systems. One of the main challenges of wireless networks in auto-mation is to provide a flexible architecture for the Quality of Service (QoS) management and for the support of real- time traffic. The QoS ability refers to the capability for providing different applications, users, or data flows, with different pri-orities, or to guarantee a certain level of performance to a data flow. This is done by traffic differentiation, i.e., assign-ing different priority levels to traffic flows and adopting suit-able mechanisms to deal with different traffic priorities in a different way. In real-time applications, for instance, the main issue is to provide QoS by achieving predictability on the packet delivery times. Predictability is usually achieved by reserving resources and employing admission control under a priori assumed workloads, real-time constraints and failure conditions. The QoS mechanism can be influenced by many factors, such as node movement, node removal or addi-tion, system updates or reconfiguration (due to network to-pology changes), routing, etc. Consequently, QoS manage-ment involves dynamic resource reallocation to cope with real-time constraints, in order to maximize the system ben-efit. To provide control and monitoring of automation systems, hybrid networks can be employed, where the wireless infra-structure is connected to existing traditional wired back-bones so that the information flow from the field device to the control room (and vice versa) is ensured. Hence, a co-operating group of both wireless and wired subsystems will be fulfilling their control tasks beyond physical layer bor-ders. All of that will be carried out without compromising the real-time behaviour of the whole system. Therefore, based on the targeted application class, upper boundaries of the exe-cution times will be determined and then real-time guaran-tees will be provided. Many manufacturers in the discrete automation industry are investigating the feasibility of deploying wireless technology in order to improve process performance and/or optimize their asset utilization or combined with traditional wired connections. Over the past few years, intensive wireless networks developments have been done for industrial cases. Wireless technology is even more advantageous if installations are temporary. This is the case, for example, when machinery is being measured and tested for certification purposes priori to start-up or for maintenance, or again when it is necessary to modify wiring between control systems and the various devices operating in a plant. However, wireless networks will not totally replace wired networks, but will seamlessly integrate with them. In large factories, multiple wireless cells are deployed to efficiently implement automation cells to be interconnected by a real-time wired backbone. A large number of process control network hardware manufacturers offer proprietary or standard wireless systems, instead of, or in combination with, wired systems. If we neglect proprietary solutions, which can be used in a limited number of contexts, and recent standards such as WirelessHART, which are slowly entering the market, due to the limited number of available chipsets and manufacturers, currently the most interesting solutions, thanks to their wide availability and acceptance, are those based on COTS standard communication protocols such as Bluetooth, IEEE 802.11, IEEE 802.15.4. Among all the problems that should be faced to effectively use wireless in automation environment, this PhD thesis focuses on the integration of IEEE 802.11, Bluetooth and IEEE 802.15.4 on the factory floor. This integration seems to be very promising to cover a large spectrum of applications and to boost the use of wireless technologies in automation. However, each of these technologies has some limitations. Some of them will be discussed in next sections and innovative mechanisms to overcome these limitations and improve performance will be proposed. The experience gained during the PhD work and reflected in this thesis contributed to the flexWARE project, an EU project funded under the 7th FP (Seventh Framework Programme). The aim of the flexWARE project is the integration of different technologies on the factory floor, to implement a flexible real-time wireless industrial network based on a two-tiered hybrid wired/wireless infrastructure. The rest of the thesis is organized as follows. In Chapter 2 the requirements of a typical industrial network are discussed along with a two-tiered architecture that integrates a wired backbone with multiple wireless cells. The wireless tier connects the nodes that operate in each industrial automation cells. In Chapter 3 the IEEE 802.11 protocol is presented. The CSMA/CA protocol adopted by the IEEE 802.11 standard cannot guarantee known delay times when several nodes compete for the channel. An innovative approach is therefore discussed to solve the problem of the increased number of collisions with increasing workloads that, in the standard protocol, cause bandwidth waste and abrupt throughput degradation. In Chapter 4 a two-tiered IEEE 802.15.4 wireless architec-ture is presented. The advantages of the proposed architec-ture are discussed. The proposed architecture makes it pos-sible to use the wired real-time network for highly-critical tasks that could not be supported over an unreliable me-dium, while the wireless network is used to cut the costs and increase the flexibility of the industrial network. Chapter 5 shows a case study based on Bluetooth. One as-pect investigated is how to enhance the support provided by BT to discrete manufacturing traffic through a novel trans-mission scheduling algorithm. Moreover, a novel frequency hopping management mechanism for Bluetooth networks used in industrial environments is proposed, which reduces the interference among co-located piconets, thus improving the network performance in terms of transmission delay and throughput. Finally, Conclusions in Chapter 6 summarize the thesis.
Appears in Collections:Area 09 - Ingegneria industriale e dell'informazione

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