Wireless Sensor Networks (WSN) have been attracting growing interests for developing a new generation of large-scale embedded computing systems with a great potential for wide range of applications such as surveillance, environment monitoring, emergency medical response or building automation. However, the communication paradigms in wireless sensor networks differ from the ones associated to traditional wireless networks, triggering the need for new communication protocols. The large-scale (number of nodes and geographical region) and the node’s scarce resources (e.g. energy, memory, communication bandwidth, radio coverage) are key aspects that turn WSNs into a challenging research field. 

Within this context, we have been focusing on new communication architectures and mechanisms to support large-scale embedded computing applications with critical requirements. We are particularly concerned with systems requiring real-time behaviour, as time will play an increasingly important role in emerging and future cyber-physical systems, where computers will be tightly integrated and interacting with their physical environment.

The ART-WiSE (Architecture for Real-Time communications in Wireless Sensor networks, (http://www.hurray.isep.ipp.pt) 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 (Fig. 1).

Figure 1: Example of the ART-WiSe two-tiered network architecture

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 ZigBee networks, which enable time-bounded and energy-efficient WSNs, namely we can compute message end-to-end delay bounds for the Guaranteed Time Slot (GTS) mechanism and ZigBee Router’s buffer requirements in ZigBee Cluster-Tree networks (e.g. [3, 4]). 

We have also proposed several important add-ons to these protocols that are backward compatible with the standard specifications, 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]); engineer a synchronized Cluster-Tree WSN, where each cluster may operate with dynamically adaptable 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 (available at http://www.open-zb.net) [ 2, 7, 8], that includes:

• 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.

The open-ZB web site already witnessed over 40000 visits and 2000 downloads in 18 months. There are some ongoing collaborations to port our protocol stack to other platforms, namely to the Contiki (http://www.sics.se) and ERIKA operating systems (http://erika.sssup.it). We are also participating (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 available here)

[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, University of Porto, 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”, Real-Time Systems Journal, Volume 39, Numbers 1- 3, pp. 169-204, Springer, August 2008. Published on-line: 21/NOV/2007.

[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. Best paper award.

[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. < 5% (12/265) long paper acceptance ratio