# Adaptive Data Synchronization Algorithm for IoT-Oriented Low-Power Wide-Area Networks
## 1 Introduction  
The Internet of Things has now become reality and is expected to have by the 2020s over a billion devices connected to the Internet, everywhere, at any time [1]. Smart homes, wearable devices, smart cities, health care, transportation, and farming represent just few reference scenarios where the application of IoT-based models would be successful. The goal of IoT is to realize an environment within which _things_ are uniquely identified and able to interact with one another through the exchange of information. Moreover, the development of this new paradigm has been sped up thanks to incoming 5G technology that will provide ultimate performance in terms of data rates, latency, and network coverage. Therefore, the concept of connectivity among devices will soon be completely revolutionized. Providing good performance is paramount when dealing with mobile communications and, in general, with real-time services that request high data rates and low latency. However, there are many other application scenarios where the sporadic interaction among devices makes energy saving the main aspect to take care of. In this regard, Bluetooth Low Energy, ZigBee, and Low-Power Wi-Fi will be used in a large part of the so-called _consumer_ IoT (cloT) market [2] that refers to all those applications aiming to improve citizen life quality. Specifically, the technologies cited above are tailored to Personal Area Networks (PANs) as they essentially provide single-user coverage area. The Internet Engineering Task Force (IETF) has recently standardized several IoT-oriented protocols, such as the one considering the use of IPv6 over Low-Power Wireless Personal Area Networks (identified as 6 LoWPAN) [3; 4]. Several devices especially employed for health and fitness activity monitoring have been already implemented with the 6 LoWPAN protocol suite and launched into the market. In parallel to the cloT, the world of _industrial_ IoT (iIoT) has been recently rising. The iIoT collects those scenarios where information technologies are integrated into industrial and public facilities contexts, to make activity management and monitoring more efficient [5]. As with cloT, many iIoT applications are characterized by limited device mobility and sporadic data transmission, but on the other hand a wider coverage area than in PAN-based services is requested. The satisfaction of these requirements has been achieved through the development of specific radio access technologies, referred to as Low-Power Wide-Area Networks (LPWANs), that best match the features of iIoT. Several standards and vendors in the field of LPWANs have emerged during the last decade [6]. The first one was Sigfox [7] that in 2009 presented on the market its ultra-narrow band (UNB) patented technology. A few years later, LoRa Alliance standardized the LoRaWAN networking protocol [8] for LPWANs exploiting a chirp spread spectrum (CSS)-based technology developed and provided by the Semtech Corporation [9]. The third main potential solution offered on the market is Narrowband-IoT (NB-IoT), representing the result of a 3 GPP (3 rd Generation Partnership Project) standardization process begun in 2014 [10] and currently approaching its Release 15 [11]. The NB-IoT features originate from the LTE framework, with the employed frequency bands that are part of the LTE and GSM spectrum [12]. This is one of the marked differences with Sigfox and LoRa which, on the other hand, share the unlicensed ISM spectrum. However, each technology provides different performances [13]. Thanks to their characteristics, LPWANs have been recognized as particularly fitting for the IoT context, especially with respect to emerging scenarios such as remote health and industrial monitoring, smart cities and living. As shown in Table 1, different kinds of activities and measurements can be considered for each specific environment, therefore the use of technology that is as flexible and scalable as possible is fundamental to providing good network performance.