# The Current Development of Structural Health Monitoring for Bridges: A Review
## 2 Monitoring Technology
### Sensor Monitoring Technology in BHM
#### 2.1.2 Wireless Sensor Technology  
The monitoring system of long-span bridge will produce a large number of monitoring data sets every day. If all the sensors used to collect this data were traditional wired sensors, the cost would undoubtedly be huge, which provides a demand for the research and application of wireless sensors. A wireless sensor network (WSN) is a network form that combines a large number of sensor nodes in the monitoring area into a network system through wireless communication technology, as shown in Figure 2.  
The WSNs deployed on bridges are characterized by a layered network communication protocol consisting of five layers, namely the physical layer, data link layer, network layer, transport layer, and application layer. The physical layer facilitates signal monitor \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Classification**} & \multirow{2}{*}{**Test Method \({}^{*}\)**} & \multicolumn{2}{c}{**Measurement**} & \multirow{2}{*}{**Disadvantage**} & \multirow{2}{*}{**Application**} \\  & & **Distance and/or** & & \\  & & **Spatial Resolution** & & \\ \hline \multirow{3}{*}{Rayleigh scattering} & \multirow{3}{*}{OTDR} & Tens to hundreds & Low spatial & \multirow{3}{*}{Vibration sensing} \\  & & of kilometers & resolution & \\ \cline{2-5}  & & & Short measuring & & \\ \cline{2-5}  & & OFDR & Resolution at mm level & & \\ \hline \multirow{3}{*}{Raman scattering} & ROTDR & \multirow{3}{*}{Ten kilometers} & Low spatial & \multirow{3}{*}{Distributed temperature sensor} \\  & & & & \\ \cline{2-5}  & & & & \\ \hline \multirow{3}{*}{Brillouin scattering} & ROTDR & Dozens of kilometers, & \multirow{3}{*}{A complex system, \multirow{3}{*}{and strain sensor}} \\  & & & 0.4–0.5 m resolution & \\ \cline{1-1}  & & & & \\ \cline{1-1}  & & & & \\ \hline \hline \end{tabular} \({}^{*}\)OTDR: optical time domain reflection; OFDR: optical frequency domain reflection; OTDA: optical time domain analysis; OFDA: optical frequency domain analysis.  
\end{table} 
Table 1: Classification and comparison of DFOS. ing, transmission, and reception, with the design goal of minimizing energy loss while maximizing link capacity [27]. The data link layer primarily enhances the functionality of transmitting raw bits from the physical layer [28]. The network layer is responsible for packet routing and network interconnection [29], forming the foundation of data transmission. The transport layer provides reliable and efficient means for data transmission. Finally, the application layer transforms data into usable information for the physical world. [PERSON] and [PERSON] [30] proposed an improved WSN transport layer protocol based on the pump slowly, fetch quickly (PSFQ) protocol. They analyze the proposed protocol in terms of average delay and average fault tolerance, and the results are better than the ordinary PSFQ protocol. In SHM, a WSN is more convenient to deploy and maintain, and has a lower cost. However, the wireless characteristics of the network also bring a series of new challenges to researchers, including communication delay and security, effective routing, and network scalability. Among numerous wireless protocols, WiFi (IEEE 802.11), Bluetooth (IEEE 802.15.1), and ZigBee (IEEE 802.15.4) are suitable examples for use in WSNs. Among them, WiFi has a longer transmission distance and an extremely fast transmission rate, but also consumes a considerable amount of energy. Therefore, without special modification methods, WiFi technology is not suitable for WSNs that require long-term monitoring. Like WiFi, Bluetooth also faces the dilemma of high power consumption, even though its performance is good. In the existing research [32], ZigBee has been used to design low-power low-rate wireless personal area networks (LR-WPAN) that meet the requirements of WSNs. [PERSON] et al. [33] summarized the advantages of using ZigBee in the underlying wireless communication technology of WSH, and also pointed out its drawbacks, such as the idle listening mechanism with additional energy consumption.