A framework for reliable and dynamic wireless sensor-actuator networks
The number of persons requiring medical assistance in industrial nations grows with the demographic change. Unfortunately, the growth is unproportional to the availability of well-trained care personal. Wireless sensor-actuator networks have the potential to support care personal. Nodes worn by patients may supervise individually critical vital parameters and trigger an alarm if a critical value is reached. Medical help can be provided earlier lowering the risk of permanent health issues. Nevertheless, wireless sensor-actuator networks are error-prone. They have to be designed fault-tolerant to function reliably. However, an approach for provably reliable wireless networks is still non-existing. This thesis proposes a solution for this problem by providing a framework for the generation, supervision and maintenance of fault-tolerant wireless sensor actuator networks. A network is defined as fault-tolerant and reliable, if it is biconnected. At least two node-disjoint paths between every pair of nodes have to exist within the network. The main contributions of this thesis are threefold. First, an algorithm for the generation of fault-tolerant networks for given floor plans is provided. The generated topologies are biconnected and require only a reasonable number of nodes. Furthermore, they provide the infrastructure for a localization of nodes by covering each position within the floor plan with three signals. Second, a heuristic and distributed algorithm for the detection of bottlenecks in dynamic networks is introduced. Bottlenecks are possible breakpoints of the network and have to be discovered early to avoid a disconnection of nodes. The core is the assumption that every network is representable in the form of a graph. Graph theoretic measures are applied to detect topological changes. The two characteristics used for the heuristic are the algebraic connectivity and the Fiedler vector, both reflect the connectivity of the network. Time variations indicate critical topological changes in dynamic networks. A decentralized continuous algorithm is proposed, which estimates both characteristics utilizing the properties of a propagating discrete wave. The algorithm requires only local information avoiding a single-point of failure in the form of a central node. Third, an algorithm for the analysis and correction of faulty topologies is introduced. The topology of a network is analyzed using the well-known Ford-Fulkerson algorithm. The Ford-Fulkerson algorithm determines the maximal flow within a flow network. The maximal flow equals the number of edge-disjoint paths within a network with an edge capacity of one. Through a slight modification of the network, node-disjoint paths are found. If the number of node-disjoint paths is less than two, a bottleneck exists within the network. The bottleneck node is then identified and the network corrected through the placement of correction nodes. Finally, the placements are examined to avoid redundant correction nodes. The correct function of the algorithms is validated using a case study of a retirement home. All algorithms work without human interaction. Their application facilitates and accelerates the design, supervision and maintenance of wireless networks. The described framework provides a basis for the reliable application of sensor-actuator network in health care facilities.
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