Tree-based topology design for multi-sink wireless sensor networks

This paper aims at designing tree-based topologies for Multi-Sink Wireless Sensor Networks (MS-WSNs). Tree height and the average number of children per parent, are the degrees of freedom of such a topology. The objective is to maximise the amount of information gathered at the sinks, that is, network reachability, having fixed node and sink densities and the air interface capacity. Once the number of levels for trees is fixed, network reachability is maximised by a proper choice of the average number of nodes at each level (i.e. the average number of children per parent). The study is performed by means of a statistical model and simulations. A mathematical framework taking into account connectivity aspects, radio channel fluctuations, random node and sink deployment, and the capacity of the air interface is considered; the mathematical analysis is based on a deterministic multiple access control (MAC) scheme. The framework has some limits: it does not consider a contention-based MAC protocol and assumes that nodes are distributed over an infinite plane. To overcome these limits we have developed a multi-sink simulator, which implements an IEEE 802.15.4 standard compliant network, based on a three-level tree based topology. Simulation results show trends very similar to the ones obtained through the mathematical model; the causes of the differences between the two are discussed in detail. Finally, the difference between a mono-sink scenario, as usually considered in the literature, and the case with multiple sinks, is discussed by means of numerical comparisons.