# Design and Analysis of Adaptive Hierarchical Low-Power Long-Range Networks
## 2 A Hierarchical Communication Scheme
### Network Model and Device Capabilities  
The communication scheme presented in this work is designed for heterogeneous wireless sensor network installations that span over large areas (i.e., a public building, a neighborhood or a whole city). It is designed to benefit from the merits of the different networking interfaces and achieve the best mixture in terms of power consumption and data exchange rates. For example, Figure 1 provides a graphical representation of an installation that spans over two physical locations (LPW1 and LPW2). Each location contains an LPWAN gateway, and a set of devices, capable of communicating with the LPWAN gateway. In both locations, the devices form a set of sub-networks (depicted with the different colors). The controllers of each sub-network have activated their LPWAN interfaces and act as the sub-network gateway to the LPWAN (depicted with a thicker border). Inside the sub-networks, the devices communicate using the short-range IEEE 802.15.4 network interfaces that operate over completely different frequency bands and do not interfere with LPWAN. They can also use these radio interfaces to communicate with nearby sub-networks if needed.  
Based on the requirements of the installation, once the sub-networks are formed, the applications running on the devices can benefit from techniques like duty cycling to increase the life of the network. We assume that this functionality is implemented on the lower level of the network stack. The low-level mechanism divides time into slots of fixed duration during which the network interface of the device can either be active or inactive. The state of each single device is completely independent from the others and there is no actual synchronization resulting in a network operation like the one presented in Figure 2 where active periods are presented with gray and inactive with white.  
The network monitoring mechanism used by the self-organization protocol can detect possible changes in the network topology also when devices change their location due to passive (e.g., when attached to a mobile object) or active mobility (e.g., in the case of robots). Each group will suitably  
Figure 1: An example of a topology with two Long-power Wide-area Networks (LPWAN) installations with sub-networks formed in the 802.15.4 network.  
Figure 2: Two devices alternating between active and inactive states: Device 1 broadcasts while Device 2 is sleeping (white arrows).  
adjust the network structure in order to remove a device that has moved away, or include a new device that has appeared.  
Given existing hardware platforms, the network interfaces provided to each device implement a naming scheme that offers a unique identifier to each device. It is assumed that each device generates a unique identity based on the unique identifiers provided by the network interfaces.  
Finally, each device has access to an internal clock. The clocks of the devices are not required to be synchronized in any way. It is assumed that the hardware of the clocks of each device are similar in terms of capabilities, thus each device is measuring the same amount of time for a given period of time (i.e., clocks have similar drift rates).