# Design and Analysis of Adaptive Hierarchical Low-Power Long-Range Networks
## 2 A Hierarchical Communication Scheme  
The main idea of the proposed architectural organization is that devices are equipped with two different networking interfaces. One network interface provides short-range high-rate communication capabilities by exploiting IEEE 802.15.4 technologies. The second network interface provides long-range low-rate communication based on recently introduced LPWAN technologies. Each networking interface operates independently and it is controlled by the software firmware that can choose for each interface if it will be active or not. When a networking interface is inactive, the corresponding power consumption is reduced to very low levels (at the order of a few \(\upmu\)A). A self-organizing distributed algorithm is used to assign devices to groups based on the short-range 802.15.4 network. In the sequel, each group selects in a distributed manner one device that assumes the role of the controller of the group. The controller activates the LPWAN network and becomes the gateway of the group to the internet, while all other devices of the group keep their LPWAN network interfaces deactivated, thus reducing power consumption and at the same time limiting congestion of the LPWAN. In this way, a hierarchy is created within the network where (i) the vast majority of the devices are positioned at the lowest level of the hierarchy, (ii) the controllers form the intermediate level of the hiercarchy, and finally (iii) the LPWAN gateways form the top layer of the hierarchy that connect the IoT deployments with the Internet. Figure 1 provides an example of such a hierarchical topology. A central idea of the communication scheme is the inherent redundancy of IoT deployments in terms of devices, where a specific physical location is usually monitored by more than one device. Based on this deployment design property, each device independently uses a duty-cycling scheme that periodically activates and deactivates the IEEE 802.15.4 network interface in order to conserve energy. Depending on the chosen level of redundancy selected by the network administrator, the duty-cycling scheme is suitably adjusted so that, on average, a minimum, yet adequate number of devices to achieve proper monitoring is always active. The self-organization protocol continuously monitors the IEEE 802.15.4 network topology and reassigns devices to groups when nearby devices switch between active and inactive states. A second central idea of the communication scheme is that devices that assume the role of the controller are spending more energy than the other devices within the same group since they are forwarding upstream and downstream messages through the LPWAN. In order to limit the overspending of energy, the controller role is periodically reassigned to another device within each group. When a device stops acting as the controller, the self-organization protocol identifies another device to become the new controller. The protocol guarantees that each group will always maintain connection with the LPWAN by properly and timely selecting a controller device. Network organization has been vastly presented in literature in the past. In the relevant bibliography, several surveys (e.g., [21; 22; 23; 24]) categorize and classify the various protocols based on the methodologies used to organize the network. For example, in [21], they classify the protocols based on their main objectives in the following categories: load balancing, fault tolerance, increased connectivity and reduced delay, minimal cluster count, and maximal network longevity. Ref. [23] organizes the literature based on Cluster Count, Cluster Overlap, Cluster-head Selection, Node Mobility and Time Complexity but also distinguishes the algorithms in two main categories: Probabilistic and Non-probabilistic, depending on the criteria and parameters used for controller selection. Ref.