# Wireless Sensor Network-Based Rockfall and Landslide Monitoring Systems: A Review
## 2 State-of-the-Art Overview of Wireless Sensor Networks for Monitoring Applications  
In recent years, WSNs have received increasing interest from industrial and research perspectives [24; 25; 26]. In a WSN, a group of nodes forms a network where the elements sense and may control the environment. The data is exchanged through a gateway to a sink via single or multiple hops. Figure 1 shows the general architecture of a WSN monitoring network.  
The sink unit can be used to monitor the information locally or remotely, if it is connected to another network. In this second case, the internet is the commonly used structure, so to form an Internet of Things [27; 28] sensor network. The nodes can be stationary or moving and can be heterogeneous in sensor type and communication technology. Figure 2 shows the general architecture of an IoT-enabled WSN in the example case of rockfall monitoring.  
The application fields of WSNs are numerous and, ideally, unlimited. Every application demands specific requirements, and it is the designer who selects the appropriate technological solution for the case. We found several application fields in the literature and industrial fields, such as environmental monitoring [29], structural monitoring [30], and health-related applications [31], including wearable systems [32], early warning tools [33; 34], and industrial control [35]. These deployments benefit from WSN applications, especially concerning installation accessibility and wiring constraint reduction [36], along with the possibility of an increase in overall cost effectiveness. New applications are enabled by WSNs, but on the other hand, the design is affected by several constraints that must be taken into consideration [37; 38]. Sensing, processing, and communication usually take place using a limited amount of energy. For this reason, the design approach requires particular attention to data processing and communication protocols [39]. Power constraints are crucial for sensor nodes. In some cases, the nodes are deployed in areas difficult to reach, so it is not possible to recharge or replace the battery of the system. In this case, it is necessary to employ techniques to reduce the power consumption of the nodes to improve the lifetime of the WSN [40]. To this extent, energy harvesting techniques can be applied to WSN nodes to obtain recharge mechanisms [41]. Wireless transmission technology is strictly related to the power performances of the nodes. The current trend is to employ low power wide area network (LPWAN) [42; 43] solutions which allow, in many cases, good coverage and energy performance to be achieved [44; 45]. This aspect is particularly crucial in harsh environments, both in terms of radio propagation and power supply [37]. LPWANs are often used in mixed-type systems, where several different solutions are used in combination [46; 47]. Security in a WSN, for topology, is strictly application-dependent, and the sensor nodes should be robust to physical damage and to communication issues such as data leakage and attacks. Detailed aspects of the security of WSNs were thoroughly investigated in [48; 49; 50]. The fundamental performance  
Figure 1: General architecture of a WSN.  
Figure 2: General architecture of an IoT WSN.  
limits of WSNs were analyzed in depth in [51], where the authors studied the constraints that affect these structures.  
In Table 1, we find a summary of the pros and cons of the aspects previously discussed about WSNs.