## 2 Background
### Related Work  
In recent years, due to the high demand of applications supported by wireless sensor networks, there have been a large number of works addressing cluster-tree WSNs. Multiple issues are being addressed, such as: network formation schemes, routing protocols, cross-layer approaches, real-time requirements, reliability and availability issues, energy-efficiency, scalability, congestion control and beacon frames scheduling. Each of these issues have their own challenges and special considerations. Additionally, with the increased demand for wide-scale applications that need to operate with time-sensitive data and must be energy-efficient, the design of wide-scale WSNs is becoming more challenging due to the large number of constraints that need to be simultaneously addressed. The state-of-the-art indicates that the use of hierarchical clustering protocols has some advantages over flat protocols for wide-scale applications with respect to scalability [36; 37; 38], energy-efficiency [7; 14; 36; 38; 39] and time-sensitive data operation [7; 14; 38; 39; 40; 15]. Therefore, several works have been published in the literature, encompassing clustering for WSNs, with different concerns. As an example, there are several surveys that summarise popular clustering protocols and analyse the strengths and weaknesses of available routing protocols based on different metrics [41; 42; 43; 44; 45; 46]. For example, LEACH (Low Energy Adaptive Clustering Hierarchy) protocol [47] is one of the most cited hierarchical protocols in the literature, which has motivated the design of several other protocols. In LEACH, sensor nodes are grouped in clusters for sending data to their cluster-heads, which aggregate data and send it to the base station. In order to balance the energy consumption of sensor nodes, this protocol randomly selects a set of different cluster-heads during each round. Also, [PERSON] et al. [36] proposed an Energy Efficient Heterogeneous Clustered Scheme (EEHC) based on the LEACH protocol, considering the heterogeneity of network resources. However, as it uses single-hop routing, the cluster-heads send directly data to the base station, which leads to an increase of energy consumption. Thus, this type of communication may be impracticable for wide-scale environments. Additionally, several cross-layer approaches have been proposed to deal with energy-efficient communication in Wireless Sensor Networks [48; 49; 50; 51; 52]. Typically, cross-layer approaches blend common protocol layer functionalities into a cross-layer module, addressing simultaneously congestion control, routing and medium access control issues to enhance the efficiency of the traditional layered protocol architectures. As the communication scheme proposed in this paper addresses setting-up new communication paths in wide-scale cluster-tree networks, this state-of-the-art study does not consider the use of cross-layer approaches, as the use of this type of architectures would not be compatible with the use of standard IEEE 802.15.4/ ZigBee cluster-tree networks. Instead, it is focused on relevant approaches that can be implemented upon cluster-tree networks, regarding cluster-tree formation, its synchronisation, congestion control and data communication support in wide-scale cluster-tree networks. Within this context, [PERSON] et al. [53] proposed a Tree-Cluster-Based Data-Gathering Algorithm (TCBDGA) using a mobile sink to improve the non-uniform energy consumption of WSNs. The basic idea of this algorithm is to build a cluster-tree network considering specific characteristics of the nodes, such as their residual energy, distance to base station and number of neighbours. After this, the network is decomposed in several sub-trees and each sub-tree has its own set of collecting nodes. The mobile sink is responsible to collect data from nodes. This algorithm is a location-aware based algorithm, which implies more energy consumption and complexity.