## 2 Materials and Methods
### System Description  
The meshed network, whose architecture is represented in Figure 1, is composed of multiple nodes that retrieve acceleration data from multiple on-board accelerometers and communicate with a master data logger. Communication is performed through a redundant architecture. Each node post-processes acquired data from the accelerometers producing a spectrogram and giving the freedom of both retrieving raw data as well as processed data.  
As seen in Figure 1, data sending and visualization are implemented through a redundant structure that uses both a wired and a wireless protocol. Communication redundancy allows a robust and wide-range SHM detection system; in particular, a serial standard is used with a wireless protocol. In the first case, RS485 is used; this robust and fast choice allows the positioning of nodes at distances up to several hundreds of meters, guaranteeing good reliability of received packets. However, a limitation of this standard is its wired structure, which requires the nodes to be physically connected. Toovercome this constraint, LoRaWAN (Long Range World Area Network) protocol is used. LoRaWAN is a protocol that uses spread spectrum communication to reach very long tx-rx distances, up to several kilometers, depending on terrain and environmental factors, with low power consumption. However, rx-tx times of packets interchanged with this protocol are larger with respect to the serial wired protocol, making it less reliable for real-time measurements with stringent constraints on time. In this system, the user can preset each node to communicate with both protocols when the node distance is under almost a kilometer, which is an estimation of the maximum RS485 range, so this protocol is used as the main communication protocol, and LoRa is considered a backup solution, while if the distance exceeds the aforementioned estimation, LoRaWAN can be selected as the only protocol for communication. For a better understanding of the two protocols used, the main differences that characterize RS485 and LoRaWAN are reported in Table 1.  
Thanks to a custom application-level tx-rx protocol (described in Section 2.4) data integrity can be ensured both when using RS485 as well as LoRaWAN thanks to a 16-bit CRC embedded in the custom data packet. Measurement of modal frequencies is finally performed through a spectrogram, chosen because of the possibility to track the changes in modal frequencies through time.  
Figure 1: Architectural scheme of the system.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Protocol**} & \multirow{2}{*}{\begin{tabular}{c} **Communication** \\ **System** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Total Driver and** \\ **Receiver on** \\ **One Line** \\ \end{tabular} } & \multicolumn{2}{c}{\begin{tabular}{c} **Maximum** \\ **Operational** \\ **Distance** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Maximum Data** \\ **Transmission Rate** \\ \end{tabular} } & \multirow{2}{*}{**Duplex Mode**} \\ \hline RS485 & \begin{tabular}{c} Wired, \\ differential \\ \end{tabular} & \begin{tabular}{c} 32 drivers, \\ 32 receivers \\ (one driver \\ active at a time) \\ \end{tabular} & 0.36–0.91 km & 10 Mbit/s & Half duplex or \\  & \begin{tabular}{c} Full Duplex \\ \end{tabular} \\  & \begin{tabular}{c} Full Duplex \\ \end{tabular} \\ \multirow{4}{*}{\begin{tabular}{c} LoRaWAN \\ \end{tabular} } & Wireless & \
\begin{tabular}{c} 120 receivers \\ (20 bytes each \\ 16.7 min) \\ \end{tabular} & 2–8 km &
\begin{tabular}{c} 0.98–12.5 kBit/s \\ (Downlink) \\ \end{tabular} & Half Duplex \\ \hline \hline \end{tabular}
\end{table}
Table 1: Main differences between RS485 and LoRaWAN protocols.