# HARE: Supporting Efficient Uplink Multi-Hop Communications in Self-Organizing LPWANs
## 2 State-of-the-Art
### Multi-Hop Approaches in LPWANs  
Most LPWAN technologies operate in the sub-1 GHz band to achieve large coverage ranges, and are built following a star topology, where end devices are directly connected to the base station. Though this single-hop approach simplifies the network design, it makes communications rely heavily on transceivers' capabilities (i.e., transmission power, antenna gain, data rate, etc.). In contrast, in low- or medium-range networks, usually operating at 2.4 GHz, IoT-aimed multi-hop environments are common:  
* IETF RPL (IPv6 routing protocol for low-power and lossy networks) [23] is a distance vector routing protocol, which provides a mechanism whereby multipoint-to-point traffic from devices inside a low power and lossy networks (LLN) towards a central control point, and vice versa.
* Time-slotted channel hopping (TSCH) is one of the MAC modes defined in the IEEE 802.15.4e standard [24]. It combines time-slotted access and channel hopping, thus providing predictable latency, energy efficiency, communication reliability, and high network capacity. TSCH is topology independent, as it can be used to form any network topology (e.g., star, tree, partial mesh, or full mesh) [25].
* A new Working Group called 6 TiSCH was created at the IETF to enable IPv6 over the TSCH mode of the IEEE 802.15.4e standard. In addition, 6 TiSCH manages the schedule created by the time-slotted access of TSCH and matches it against the traffic needs in the network. Its distributed scheduling mode allows nodes to agree on a schedule by using distributed multi-hop protocols and neighbor-to-neighbor negotiation [26].  
From combining both conceptions (long range links and heterogeneous network topologies) in the aforementioned unlicensed spectrum LPWANs, novel multi-hop approaches have been proposed in recent times:  
* LoRaBlink [27] is a protocol on top of LoRa's physical layer designed to support reliable and energy efficient multi-hop communications. Time synchronization is used to define slotted channelaccess, with a reliability of 80% according to tests. While downlink messages are distributed through flooding, nodes use a directed flooding approach for uplink communications.
* One of the main singularities of DASH7 is enabling both star and tree topologies to facilitate the management of large networks [28]. In the latter case, stations (STAs) not directly reaching the gateway (GW) may transmit to sub-controllers or other STAs, which forward the messages to the GW. Regarding the MAC layer, STAs check the channel periodically for possible pull requests or downlink transmissions.
* To improve data transmission efficiency and extend range coverage, IEEE 802.11 ah allows for using a dual-hop relay system in between the AP and end stations [29]. According to simulations from [30], these benefits are especially important for uplink communications. In addition, since an STA is closer to the relay than to the AP, it can transmit frames (i) at lower power and (ii) at higher data rate making the transmission shorter, consequently reducing its energy consumption.
* To reduce the time an STA is competing for the channel as well as to increase its sleep time, IEEE 802.11 ah uses a scheme called _TIM and Page Segmentation_[31], which has inspired the hybrid MAC sublayer of HARE. Thus, distribution of STAs into groups is used not only for organizational purposes but also for effectively allocating available channel resources.  
Unlike the aforementioned multi-hop approaches, HARE considers energy consumption as a key element in the decision making processes of the different network self-organization mechanisms (for instance, routing or power management). The typical single-hop approach of LPWAN technologies is therefore called into question and even replaced by multi-hop topologies when proving energetically more efficient. Furthermore, HARE's high flexibility when allocating channel resources ensures communications reliability, network scalability, and facilitates future adoption of QoS schemes.