## 1 Introduction
### Application Domains  
Operating mechanisms in Wireless Sensor Networks are usually application-dependent, in what concerns: node deployment, network topology, communication traffic patterns and communication protocols [20; 21; 22; 23; 24; 25]. Regarding WSN traffic patterns, convergecast traffic is the most common traffic pattern in WSNs [26; 27; 28]. Convergecast traffic corresponds to typical sensing traffic (upstream) being periodically generated by sensor nodes and forward towards a sink node (usually at the PAN coordinator). Figure 1 illustrates a typical monitoring cluster-tree WSN, where static sensor nodes continuously transmit convergecast traffic.  
Other applications may require multiple destination nodes for message streams, i.e., WSNs can be designed with one or more sink nodes, depending on the application requirements [21]. This behaviour is especially observed whenever sensor nodes are deployed with actuating capabilities. Remarkable examples of multi-sink scenarios are local irrigation management systems in precision agriculture or smart control for shading systems in building automation [1; 29; 30; 31], where actuating commands are generated according to locally sensed variables. Other applications are related to collaboration purposes, where sensor nodes can exchange messages with neighbour nodes to take local decisions, for instance detection of specific events in a given monitored environment [28; 32]. In this type of sensing/actuating (or collaborative) applications, the network can be required to implement local control loops, based on local variables, without the intervention of a global controller (located at the sink node). Therefore, it would be of major relevance to have WSNs where any pair of nodes can be source/destination of message streams [33]. Figure 2 illustrates this type of source-to-destination communication pattern.  
A major limitation when using traditional cluster-tree networks to support source-to-destination traffic is that data communication must follow the tree routing paths (dashed arrows in Figure 2). Thus, if source and destination nodes are not located in the same cluster, the message stream will be required to traverse multiple clusters towards the PAN coordinator (or, at least, a common parent cluster-head) and then down until reaching the destination node. This type of  
Figure 1: Typical convergecast communication traffic in a Wireless Sensor Network.  
Figure 2: Source-to-destination traffic in a Wireless Sensor Network.  
higher end-to-end communication delays and potentially higher network congestion of the cluster-tree backbone nodes.  
Within this context, the communication scheme proposed in this paper uses available communication resources to set alternative communication paths, enabling the support of direct source-to-destination message streams (double arrows in Figure 2). Although being widely studied in MANET (Mobile Ad hoc Networks) literature [33; 34; 35], there are two fundamental differences in what concerns the application domains for these alternative routing paths: ARounD considers WSN clusters where sensor nodes are most part of the time in sleeping mode, in order to save energy; and there are no mobile nodes in the communication environment. Instead, there is a set of static WSN nodes that may be used during their idle periods to support the proposed alternative routing paths.