## 2 State of the Art  
Various works have investigated the reliability of LoRa networks. In [6], the authors investigate the problem of collision and propose two distinct mechanisms for collision free transmission, namely TDMA (Time-Division Multiple Access)-based and FDMA (Frequency Division Multiple Access)-based with an ultimate aim of increasing the reliability of the service. The first mechanism allows all clusters to transmit in sequence where up to six EDs belonging to the same cluster can transmit using different SFs in parallel whereas the latter allows all clusters to transmit in parallel, each cluster on its own frequency. However, within each cluster, all EDs transmit in sequence. The simulation results provide better performances than standard LoRaWAN in terms of Packet Delivery Rate (PDR) even if the number of EDs is high. Similarly in [7], a two-step lightweight scheduling is proposed to divide nodes into groups where similar transmission powers are used in each group to reduce the capture effect. The nodes are guided by the GW coarse-grained scheduling to use different SFs to enable simultaneous transmissions through the use of beacon signals at every pre-defined interval, thus reducing packet collisions. The validity of the proposed scheme is assessed using NS-2 simulations showing better performance than legacy LoRaWAN in terms of packet error ratio, throughput, and energy efficiency. However, inter-SF transmission is still a problem due to the loss of the orthogonality between the two signals [8; 9]. Since the ALOHA mechanism of the LoRaWAN drastically decreases the performance because of the non-negligible on-air collision probabilities, some authors in literature have proposed the synchronization of LoRa networks by assigning slots to each node using fine-grained scheduling [10]. In order to overcome the problems of classical ALOHA in LoRaWAN, [PERSON] et al. [11] propose a time-slotted mechanism where data are buffered locally and transmitted whenever a GW is available by avoiding bursts of collisions. Similarly, in [12] a Time Slotted (TS)-LoRa that allows the nodes to organize autonomously and determine their slot positions in a frame is proposed. This is achieved by sharing an easy-to-compute hash algorithm between the network server and the nodes able to map the nodes' addresses that are assigned during the join phase into unique slot numbers. Moreover, this mechanism ensures backward compatibility with legacy LoRaWAN nodes and liberates TS-LoRa from the huge schedule dissemination overhead. The last slot in each frame is used for sending synchronization ACK responsible for handling time synchronization and ACKs. The considered TS-LoRa achieves a very high packet delivery ratio for all the tested SFs. The availability of CONF messages is one of the important features in the LoRaWAN networks that is not available in some of its competing low power technologies like Sigfox, Bluetooth Low Energy (BLE) etc. This feature can be used in those scenarios where data reliability is concerned. However, very few works in literature have investigated the CONF traffic, its effects and, more in general, the DL viability. [PERSON] et al. [13] provides an analysis of use cases requiring CONF traffic and concludes that CONF traffic is viable in small networks, especially when data transfer is infrequent. Additionally, aspects likes duty-cycle regulations, SF12 for RX window 2, maximum re-transmission numbers and ACK_TIMEOUT transmission back-off interval negatively impact the viability of the CONF traffic. Similarly, [PERSON] et al. [14] conclude that the performance of a single LoRaWAN cell can significantly degrade when the fraction of nodes that require CONF traffic grows excessively. Moreover, they also suggest that it is necessary to carefully choose the maximum number of transmission attempts for CONF packets, based on the node density and traffic load to get the best performances.