## 2 Background
### Related Work
The communication approach proposed in this paper considers a static topology (no mobile nodes), where the information is gathered through message exchanges. [PERSON] and [PERSON] [13] implemented a multihop GTS mechanism for IEEE 802.15.4 beacon-enabled networks. The aim of this work is to allocate GTS slots from the requesting nodes to the sink node along the cluster-to-cluster path. In turn, [PERSON] et al. [19] proposed the GLHOVE approach for cluster-tree networks to maximise the number of received messages in the base station, prioritizing specific clusters of the network. GLHOVE comprises a distributed algorithm that guarantees a balanced message delivery per cluster and, at the same time, avoiding congestion. However, none of these approaches consider any message prioritisation mechanism. [PERSON] et al. [54] and [PERSON] et al. [55] proposed two different approaches to decrease the number of hops to reach the sink node using the neighbouring table defined in the ZigBee standard. The purpose is to reduce the routing costs, improving the energy consumption, network lifetime and end-to-end delays. In [54], each node keeps a neighbouring table with relevant information about the neighbouring nodes, such as: depth, link quality and device type. Thus, the algorithm defines the shortest path using three criteria: minimum hop count, minimum congestion and maximum link quality. Each criterion has its weight and the next hop (neighbour node) is selected based on a minimum cost function. In [55], the algorithm inspects the neighbouring table in order to find a node with a shortest tree path until the destination node and selects it as next hop. Although these algorithms define shortest paths until the sink node, they do not consider any information about network scheduling (top-down or bottom-up), nor about how data communication is performed. [PERSON] [56] presented an approach using border nodes (slave-slave bridge) to interconnect neighbouring clusters. The slave-slave bridge nodes listen to the beacons generated by source and sink clusters. During the source cluster active period, the bridge node can receive data from the coordinator, and during the sink cluster active period, the bridge node delivers its data to the sink coordinator. Considering this approach, the cluster-tree path is not used. Although using bridge nodes to avoid the cluster-tree path, data is transmitted during the clusters' active period, and therefore, end-to-end delays are still dependent of the cluster scheduling. Also, [PERSON] et al. [57] proposed an adoptive-parent-based framework for ZigBee networks, to increase the bandwidth between the source and sink nodes. For this, a specific cluster-head can request bandwidth from neighbour cluster-heads (adoptive parents) during a given time period. However, even using additional paths to transmit messages between source and sink nodes, these additional paths are cluster-tree paths and therefore the problem of a higher energy consumption of the cluster-heads still remains. The communication scheme proposed in this paper defines new alternative paths for message streams, instead of cluster-tree paths, which can improve end-to-end communication delays and reduce network congestion. Regarding beacon frame scheduling, some relevant works have also been presented in [7; 14; 15; 39; 58]. The key idea of these works is to schedule the clusters' active period, considering the main constraints imposed by the message streams supported by the cluster-tree network. [PERSON] and [PERSON] [15] and [PERSON] et al. [58] follow a multichannel approach to avoid overlapping cluster collisions, while maintaining the cluster connectivity. In [15], the proposed approach schedules adjacent clusters in alternate timeslices, using different channels to avoid cross-channel interferences. Thus, while a coordinator schedules its own superframe, its adjacent clusters can not schedule their owns. Therefore, collisions are avoided between neighbour clusters.