# HARE: Supporting Efficient Uplink Multi-Hop Communications in Self-Organizing LPWANs
## 4 HARE Operation  
The HARE protocol stack conceives end devices as elements controlled by the GW by means of beacons. This centralized approach allows STAs to remain asleep the majority of the time, so that their single concern is to be awake enough in advance to listen to the next beacon. Network synchronization is thus achieved and allows the GW to ask for specific data and/or distribute configuration changes easily.  
The GW is considered to be appropriately placed close to a power source or an energy harvesting solution. Therefore, it may always stay in an active state and is provided with the ability to directly communicate (i.e., via single-hop communications) with any node of the network through unicast and/or broadcast messages as well as to redirect gathered data to other networks or the Internet.  
Conversely, STAs can take advantage of their neighbors to create multi-hop paths over which data is transmitted to the GW by means of lower transmission power levels. During the association process, and depending on their position within these paths, STAs are ideally organized into _rings_, resulting in network topologies as the ones shown in Figure 2. The number of hops to reach the GW determines the ring number (i.e., STAs from ring 2 require two hops to reach the GW).  
Each uplink data transmission phase (consisting of one or more _transmission windows_) begins with a beacon signal from the GW. Transmission windows are in turn virtually split into as many TDMA slots as network rings, so that STAs are only active during their own slot (for transmitting data) and the previous one belonging to their children (for receiving data). _Children_ refers here to all STAs of an adjacent higher ring from which an STA receives packets. Similarly, _parent_ refers to that STA from an adjacent lower ring to which an STA transmits its own packets (after aggregating the ones from its children) in its way to the GW.  
The first slot is allocated to the highest ring and the rest are scheduled consecutively. Data received by STAs is aggregated to that generated by themselves, and finally sent to the corresponding parent at the minimum power level that ensures reliable communications. This process is repeated as many times as rings that the network has.  
The correct reception of data transmissions at the GW is acknowledged at the end of every transmission window with a broadcast message, so that STAs are not only aware of their own end-to-end reliability, but also of those STAs in the same path to the GW. These acknowledgment  
Figure 2: Network topology of typical multi-hop LPWANs. (**a**) multi-hop LPWAN consisting of a GW and 30 STAs (\(N_{1}\)–\(N_{30}\)) deployed in four rings (\(R_{1}\)–\(R_{4}\)); (**b**) multi-hop LPWAN affected by an obstacle, with a GW and 10 STAs (\(N_{1}\)–\(N_{10}\)) deployed in four rings (\(R_{1}\)–\(R_{4}\)).  
beacons, together with the information obtained from their adjacent nodes, allow STAs to decide whether they should remain awake to perform retransmissions of lost network packets in successive transmission windows.  
Network association (also started by a beacon) remains stable until a change in the topology is detected or the mechanism is reset by the GW. Nevertheless, the agreed transmission power between adjacent nodes in the association phase is constantly monitored and adjusted in a decentralized way in order to reduce the energy consumption.