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Real-Time Communications over Hybrid Wired/Wireless PROFIBUS-Based Networks
Ref: HURRAY-TR-0302       Publication Date: 1, Feb, 2003

Abstract:
The communication infrastructure of current Distributed Computer-Controlled Systems (DCCS) is usually based on fieldbus networks, since they provide adequate levels of performance, dependability, timeliness, maintainability and cost. Nevertheless, cabling starts to be an obstacle for an increasing number of industrial automation applications, which impose or benefit from the use of mobile devices such as handheld computers or transportation equipment. Within this context, there is a trend to extend fieldbus systems with wireless capabilities, leading to hybrid wired/wireless communication networks, which must support wireless/mobile communications while still fulfilling stringent DCCS requirements. Wireless communications must cope with real-time and dependability features at least similar to the ones encountered in traditional (wired) fieldbus networks. The support of inter-cell mobility turns this task even more difficult, since mobile nodes must handoff between radio cells in a transparent way. The main research objectives of this thesis are the specification of a hybrid wired/wireless communication architecture based on a standard fieldbus protocol (PROFIBUS) and the proposal of the appropriate mechanisms and approaches to support and guarantee real-time communications with such an architecture. For this purpose, several design approaches for the architecture of the hybrid fieldbus network are analysed. Particular focus is given to how real-time communications can be guaranteed over such a network and to how mobility can be supported without affecting the realtime performance of the hybrid network. The interconnection of heterogeneous physical media in a broadcast network leads to traffic congestion (increasing queuing delays) in the Intermediate Systems (ISs). Therefore, an innovative mechanism for eliminating traffic congestion in the ISs is proposed, which permits to reduce and bound system turnaround times through the insertion of additional idle time before a master End System (ES) issues request frames. This is implemented by setting appropriate values for the PROFIBUS Idle Time parameters, for every master ES. Additionally, a methodology to compute the worst-case system turnaround time and duration of message transactions is also proposed. This permits to set the PROFIBUS Slot Time parameter, for all master ESs in the communication network and enables the evaluation of the worst-case response time of message transactions. Finally, the impact of inter-cell mobility in the timing behaviour of the network is addressed and a timing analysis of the adopted mobility management mechanism is carried out. This analysis permits to compute values for the mobility-related network parameters. It should also be stressed that neither the system architecture nor the methodologies proposed in this thesis impose any changes to the PROFIBUS protocol.

Mário Alves

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