The open-ZB open-source toolset has been made publicly available at http://www.open-zb-net (released 24/NOV/2006) and already witnessed over 20000 visits and 800 downloads in 13 months. This toolset is being developed within the IPP-HURRAY Research Group (http://www.hurray.isep.ipp.pt), in the context of the The ART-WiSE (Architecture for Real-Time communications in Wireless Sensor networks, http://www.hurray.isep.ipp.pt/art-wise) research framework.

We have been focusing on new communication architectures and mechanisms to support large-scale embedded computing applications with critical requirements, particularly concerning timing behaviour, which will be increasingly important in emerging and future cyber-physical systems.

The ART-WiSE research framework aims at the specification of a scalable multiple-tiered communication architecture for improving the timing and reliability behaviour of WSNs. One of our major goals is relying as far as possible on existing standard communication protocols and commercial-off-the-shell (COTS) technologies – IEEE 802.15.4/ZigBee for Tier 1 and IEEE 802.11 for Tier 2.

Concerning the IEEE 802.15.4/ZigBee protocols, which show up interesting potentialities for WSNs [1, 2], we already have significant scientific and technological contributions.

We have provided methodologies to analyse and dimension star and cluster-tree 802.15.4/ZigBee networks, namely being able to compute throughput and message delay bounds for the Guaranteed Time Slot (GTS) mechanism and ZigBee Router’s buffer requirements in cluster-tree networks (e.g. [3, 4]).

We have also proposed several important add-ons to these protocols that are backward compatible, namely the following that were already implemented and validated using the open-ZB toolset:
- an implicit GTS allocation mechanism (i-GAME, [5]); this enables to improve the bandwidth utilization by several nodes sharing a GTS;
- a solution for the problem of beacon/superframe scheduling in ZigBee cluster-tree networks (the Time Division Beacon Scheduling mechanism, [6]); this enables to engineer a synchronized cluster-tree WSN, where each cluster may operate with different and low duty-cycles, thus prolonging network lifetime;
- a hidden-node avoidance mechanism (H-NAMe; under submission); this enables to eliminate hidden-node collisions in synchronized multiple cluster WSNs, leading to improved network throughput, energy-efficiency and message transfer delays.

We have developed an open-source toolset for the IEEE 802.15.4/ZigBee protocols – Open-ZB (http://www.open-zb.net) [7, 8, 2], including:
- the IEEE 802.15.4 protocol developed in TinyOS, for the MICAz and TelosB motes;
- ZigBee Network Layer functionalities for supporting synchronized multiple cluster topologies (the Cluster-Tree topology) developed in TinyOS, for the TelosB motes;
- a simulation model of the IEEE 802.15.4 protocol developed in OPNET;
- tools for timing analysis and network dimensioning.

There are some ongoing collaborations around the open-ZB protocol stack, namely with ATMEL (http://www.atmel.com), SICS (http://www.sics.se) and SSSUP (http://erika.sssup.it) to port it to their platforms. We are also the only non-USA partner in the TinyOS Network Protocol Working Group (http://tinyos.stanford.edu:8000/Net2WG), to implement a ZigBee compliant stack for TinyOS 2.0.

REFERENCES (complete list at http://www.hurray.isep.ipp.pt/art-wise):
[1] A. Koubaa, M. Alves, E. Tovar, “IEEE 802.15.4: a Federating Communication Protocol for Time-Sensitive Wireless Sensor Networks”, chapter of the book “Sensor Networks and Configurations: Fundamentals, Techniques, Platforms, and Experiments”, Springer-Verlag, Germany, pp. 19-49, January 2007.
[2] A. Cunha, “On the use of IEEE 802.15.4/ZigBee as federating communication protocols for Wireless Sensor Networks”, HURRAY-TR-070902, MSc Thesis, September 2007.
[3] A. Koubaa, M. Alves, E. Tovar, “GTS Allocation Analysis in IEEE 802.15.4 for Real-Time Wireless Sensor Networks”, 14th International Workshop on Parallel and Distributed Real-Time Systems (WPDRTS 2006), special track on Wireless Sensor Networks, Rhodes Island, Greece, April 2006.
[4] A. Koubaa, M. Alves, E. Tovar, “Modeling and Worst-Case Dimensioning of Cluster-Tree Wireless Sensor Networks”, 27th IEEE Real-time Systems Symposium (RTSS’06), Rio de Janeiro, Brazil, December 2006, pp. 412-421, IEEE Computer Society.
[5] A. Koubaa, M. Alves, E. Tovar, A. Cunha “An Implicit GTS Allocation Mechanism in IEEE 802.15.4 for Time-Sensitive Wireless Sensor Networks: theory and practice”, to be published in the Real-Time Systems Journal, Springer. This is an extended version resulting from an invitation from the TPC of the 18th Euromicro Conference on Real-Time Systems (ECRTS’06), upon selection of the best papers of the conference.
[6] A. Koubaa, A. Cunha, M. Alves, “A Time Division Beacon Scheduling Mechanism for IEEE 802.15.4/Zigbee Cluster-Tree Wireless Sensor Networks”, 19th Euromicro Conference on Real-Time Systems (ECRTS 2007), Pisa (Italy), July 2007.
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[7] P. Jurcik, A. Koubaa, M. Alves, E. Tovar, Z. Hanzalek, “A Simulation Model for the IEEE 802.15.4 protocol: Delay/Throughput Evaluation of the GTS Mechanism”, 15th IEEE International Symposium on Modeling, Analysis, and Simulation of Computer and Telecommunication Systems (MASCOTS´07), Istanbul, Turkey, October 2007.
[8] A. Cunha, A. Koubaa, R. Severino, M. Alves, “Open-ZB: an open-source implementation of the IEEE 802.15.4/ZigBee protocol stack on TinyOS”, 4th IEEE International Conference on Mobile Ad-hoc and Sensor Systems (MASS´07), Pisa, Italy, October 2007.
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