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On the use of IEEE 802.15.4/ZigBee as federating communication protocols for Wireless Sensor Networks
Ref: HURRAY-TR-070902       Publication Date: 20, Sep, 2007

Abstract:
We are witnessing the dawn of a new paradigm in Information and Communication Technologies (ICT), where large-scale embedded computing systems will interact with the physical environment in a ubiquitous and pervasive fashion. The landscape of potential new applications is tremendous; homeland security, environmental monitoring, health care, domotics, or factory automation are just a few elucidative examples of how these emerging embedded technologies will impact our daily life and society at large. In order for these applications to become a reality, it is mandatory to find adequate Wireless Sensor Network (WSN) infrastructures that comply with complex requirements. In this context, IEEE 802.15.4/ZigBee seems potentially interesting to act as federating communication protocols for WSNs. Nevertheless, several issues in the standard specifications are still open. The main research objectives of this Thesis are to evaluate the adequateness of the IEEE 802.15.4/ZigBee protocols for supporting WSN applications, the identification of open issues in the standard specifications and the proposal of solutions to effectively solve some of these problems and to enhance some native mechanisms. For this purpose, an implementation of the IEEE 802.15.4/ZigBee protocol stack under the TinyOS operating system was conducted. This protocol stack has been used to leverage our research work, in the sense that it has been enabling the test, validation and demonstration of our scientific findings through experimentation. This work has also been driven by the need for an open-source implementation of the IEEE 802.15.4/ZigBee protocols, filling a gap between some newly released complex C implementations and black-box implementations from different manufacturers. The problem of building synchronized cluster-tree networks, which are quite suitable for ensuring real-time and energy-efficient communications, is also addressed by this Thesis, demonstrating the feasibility of the Time Division Beacon Scheduling approach. Reliability is also an important aspect in WSN applications, particularly those with critical requirements. ZigBee Cluster-Tree networks are prone to the single point of failure problem in the ZigBee Routers (cluster-heads). This Thesis presents a mechanism that improves on the ZigBee default behaviour by reducing or even eliminating network inaccessibility times in case of ZigBee Router failure. Additionally, timeliness is an important feature of the IEEE 802.15.4 protocol, turning it quite appealing for applications under timing constraints. The native (explicit) Guaranteed Time Slot (GTS) allocation mechanism may be inefficient concerning bandwidth utilization. Thus, an implicit allocation mechanism (i-GAME) was implemented and proved to overcome that problem, by allowing several nodes to share the same GTS. Finally, this Thesis presents a performance evaluation of the slotted CSMA/CA mechanism, comparing experimental and simulation results.

André Cunha

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