A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> B. ANALYSIS <s> The rising tide of the Internet of Things has brought to the surface numerous low-power, long-range and low-bitrate wireless network technologies. One of them, LoRaWAN, is being intensely popularized as a solution for sensor networks, however, its potential and limitations are unclear, because there is still neither accurate study nor massive LoRaWAN deployment. This paper surveys and analyzes LoRaWAN operation, focusing on performance evaluation of its channel access as the most crucial component for massive machine type communication. We reveal and point out weaknesses of the LoRaWAN specification and propose solutions to improve LoRaWAN performance. <s> BIB001 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> B. ANALYSIS <s> Recently, the role of Low-Power Wide Area Networks (LPWANs) as enablers of the Internet of Things (IoT) is emerging. However, many concerns arise on the performance of LPWANs if higher Quality of Service (QoS) requirements are needed by certain classes of IoT services. In this work, we will focus on one of the most prominent of these technologies, i.e., Long-RangeTM (LoRa). Our aim is to study to what extent a LoRa network is able to support uplink (UL) traffic that requires to be confirmed by the Network Server (NS). The simulation results show that the network performance is severely impacted by the downlink (DL) traffic generated by feedback packets. <s> BIB002 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> B. ANALYSIS <s> The need for low power, long range and low cost connectivity to meet the requirements of IoT applications has led to the emergence of Low Power Wide Area (LPWA) networking technologies. The promise of these technologies to wirelessly connect massive numbers of geographically dispersed devices at a low cost continues to attract a great deal of attention in the academic and commercial communities. Several rollouts are already underway even though the performance of these technologies is yet to be fully understood. In light of these developments, tools to carry out `what-if analyses' and pre-deployment studies are needed to understand the implications of choices that are made at design time. While there are several promising technologies in the LPWA space, this paper specifically focuses on the LoRa/LoRaWAN technology. In particular, we present LoRaWANSim, a simulator which extends the LoRaSim tool to add support for the LoRaWAN MAC protocol, which employs bidirectional communication. This is a salient feature not available in any other LoRa simulator. Subsequently, we provide vital insights into the performance of LoRaWAN based networks through extensive simulations. In particular, we show that the achievable network capacity reported in earlier studies is quite optimistic. The introduction of downlink traffic can have a significant impact on the uplink throughput. The number of transmit attempts recommended in the LoRaWAN specification may not always be the best choice. We also highlight the energy consumption versus reliability trade-offs associated with the choice of number of retransmission attempts. <s> BIB003 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> B. ANALYSIS <s> Today the LoRaWAN is among the most widely adopted Low Power Wide Area Network (LPWAN) technologies. In contrast to the other alternatives, LoRaWAN enables deploying both continent-wide public networks, and private networks composing one or several gateways. In this paper, we empirically investigate how does the presence of the downlink traffic affects the performance of uplink for LoRaWAN operating in 868 MHz EU bands. Our results show that in real-life the downlink transmissions can compromise performance of uplink and must be accounted for when planning a network. Also we demonstrate the effects that the selection of secondary frequency channels and data rates have on the performance of a LoRaWAN network. The reported results reveal new dependences and provide the ground truth reference for the future analytical works. For these reasons, the paper can be of interest to both practitioners, planning and deploying LoRaWAN networks, and the researchers performing analytical work on LoRaWAN. <s> BIB004 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> B. ANALYSIS <s> LoRaWAN is gaining momentum in the arena of IoT connectivity technologies thanks to the low cost, ease of deployment, and support for adaptable transmission rates and bidirectional communications. The research community has then been working to develop suitable analytical models that can be instrumental in the study of this technology. In this work we propose a mathematical model that makes it possible to accurately estimate the packet success probability of a LoRaWannetwork in presence of bi-directional traffic, i.e., both uplink (UL) and downlink (DL) transmissions, and that accounts for the most critical features of the LoRa chipset and the LoRaWanstandard. The proposed model, furthermore, makes it possible to study the effect of different parameters configurations, thus offering a valid tool to investigate possible improvements to the system configuration. The proposed model is first validated by comparison with some accurate simulation results and, then, its potential is exemplified by analyzing the system settings that yield the best network performance. <s> BIB005 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> B. ANALYSIS <s> Although LoRaWAN offers many advantages, one of its main limitation is the insufficient downlink capacity in large-scale scenarios. This makes reliable communication impractical as, in LoRaWAN, reliability is achieved through the acknowledgement of confirmed messages. The contribution of this paper is to provide a comprehensive description of the problems associated with the presence of downlink traffic and to propose solutions to enable the use of acknowledgments for low to medium downlink load. The proposed mechanisms build on a more adequate and flexible choice of the gateway for each downlink transmission by the network server. Thus, they are entirely compatible with existing LoRaWAN technologies and deployments. Our simulation results show that the proposed solutions enable the successful realization of a higher percentage of LoRaWAN confirmed messages. Thus, the common practice of strongly discouraging the use of confirmed messages could be reconsidered to allow applications that require reliable communication. <s> BIB006 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> B. ANALYSIS <s> LoRaWAN is a Low Power Wide Area Network technology featuring long transmission range and a simple MAC layer that have favored its adoption and usage in large-scale deployments. While LoRaWAN's main use case consists in unconfirmed sensor data collection, the standard also considers confirmed traffic, enabling control applications and reliable services. Although this feature does have important implications in terms of system settings and performance, it has mostly been overlooked in the literature. To contribute filling this gap, in this paper we provide a mathematical model to estimate the performance of a LoRaWAN gateway serving a set of devices that may employ confirmed traffic. The model also features a set of parameters that can be adjusted to investigate different behaviors of the gateway, making it possible to carry out a systematic analysis of various trade-offs. The accuracy of the proposed model is validated against ns-3 LoRaWAN simulations. <s> BIB007 </s> A Survey on the Viability of Confirmed Traffic in a LoRaWAN <s> B. ANALYSIS <s> LoRaWAN is an emerging low-power wide-area network (LPWAN) technology, which is gaining momentum thanks to its flexibility and ease of deployment. Conversely to other LPWAN solutions, LoRaWAN indeed permits the configuration of several network parameters that affect different network performance indexes, such as energy efficiency, fairness, and capacity, in principle making it possible to adapt the network behavior to the specific requirements of the application scenario. Unfortunately, the complex and sometimes elusive interactions among the different network components make it rather difficult to predict the actual effect of a certain parameters setting, so that flexibility can turn into a stumbling block if not deeply understood. In this article, we shed light on such complex interactions for a single-gateway (GW) system by analyzing the effect of some built-in features and configurations, including the GW’s limitations in terms of duty cycle and the number of parallel reception paths, the number of allowed retransmissions for confirmed traffic, and the preconfigured data rate used in downlink transmissions. The simulation-based analysis reveals various tradeoffs and highlights some inefficiencies in the design of the LoRaWAN standard. Furthermore, we show how significant performance gains can be obtained by wisely setting the system parameters, possibly in combination with some novel network management policies (e.g., enabling selective prioritization of downlink transmissions at the GW). <s> BIB008
The currently published work examines downlink viability by almost exclusively focusing on the impact of confirmed traffic, i.e. sending ACKs to end devices. However, downlink traffic consists of more than just ACK transmissions, there are MAC command transmissions and application layer data to also consider. Even if confirmed traffic is not allowed by a network operator, downlink traffic would still occur BIB006 . Improvements in those areas will reduce the total downlink traffic, easing DC restrictions and thus also improving the performance of confirmed traffic. A potential improvement for downlink traffic is offered in  , BIB008 , who notes that when end devices open their receive windows, there is the potential that they accidentally lock onto the uplink transmission of another node instead of the gateway's response. This is caused by the preamble of downlink and uplink transmissions being identical, so the receiver is not immediately aware that it is locking onto an uplink transmission. As the issue of viability only comes into play in large networks, analysis of confirmed traffic has predominately been done using LoRaWAN simulators. The currently available simulators have differences in how the LoRaWAN protocol is implemented. As an example, BIB002 shows in their Table 1 that ACK_TIMEOUT has a value of 1 s, the LoRaWAN protocol specifies that this should be a random delay between 1 and 3 seconds [31]. In general, simulators are used to investigate a specific topic and thus assumptions/decisions are made on other areas deemed not relevant. Direct comparisons between results are also difficult as different payload lengths, sending intervals, number of nodes, number of channels and so forth are used. The LoRaWAN protocol could potentially be optimised by modifying the current receive window behaviour. The published work has shown that gateways struggle with the current implementation. Currently, the second window is opened in a downlink only 10 % DC restriction channel. If this channel is rather used for the first receive window, more devices would be able to receive an ACK as not only is due DC restrictions reduced in this channel but there would be no collisions with uplink traffic  , BIB004 . Matching the downlink's SF with the original SF of the uplink packet would further improve how many responses can be sent before the 10 % DC restriction is reached. Whilst the issues raised above must be considered, the existing literature can be summarised to capture what is currently known about the state of confirmed traffic in LoRaWANs and what parameters influence its viability. Researchers found that sending two ACKs instead of one decreases network performance, receive window 2's use of SF12 is not ideal and that the impact of prioritising reception over transmission at gateways depends on NbTrans and traffic volume BIB005 , BIB008 , BIB007 . Adjusting the NbTrans parameter can be effective in increasing the PDR but this improvement is limited to small networks  , BIB003 . Consensus on the exact threshold after which it becomes negative and what the optimum number of transmissions was not achieved amongst researchers. In large networks, the main cause in the reduction in performance was the DC restrictions imposed on gateways BIB003 . This is problematic, as nodes are creating a retransmission snowball effect for data that was successfully received. Adding additional gateways can be an effective solution to easing the DC restrictions on gateways, but load balancing between the gateways must be considered BIB006 . The ACK_TIMEOUT random backoff interval is too short for packets sent using high SFs, they will simply collide again due to their long transmission times  , BIB001 . It is good that version 1.1.x of the protocol no longer increase the SF between retransmission attempts, as this only made congestion worst in 1.0.x networks  , BIB001 .