# The Latest Advances in Wireless Communication in Aviation, Wind Turbines and Bridges
## 3 Base of the SHM System  
Currently, wireless sensors are becoming very popular. They should not be treated as a kind of sensor but as a network of sensors. Structural Condition Monitoring (SHM) is based on using sensors for non-destructive testing (NDT). Thanks to the integration with commercial and military aircraft structures, it is possible to periodically or continuously monitor the initiation and growth of damage without decommissioning, disassembly, and manual inspections, which are costly and time-consuming [26]. On the one hand, the lack of cables and connection ports significantly reduces the structural weight, but it simultaneously requires a separate power source, weighing more. Furthermore, adding batteries to the SHM system may result in extra maintenance problems. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **WEP** & **WPA** & **WPA2** \\ \hline \multirow{3}{*}{**Purpose of security**} & \multirow{3}{*}{\begin{tabular}{c} Need to secure the sent \\ signals in open space \\ \end{tabular} } & The creation of new versions & \multirow{3}{*}{\begin{tabular}{c} Insufficient security in WPA. \\ \end{tabular} } \\  & & of wireless protocols forced & \\  & & the creation of a new type & \\  & & of security & \\ \hline \multirow{2}{*}{**Data encryption type**} & \multirow{2}{*}{\
\begin{tabular}{c} Rivest Cipher 4 (RC4) \\ \end{tabular} } & TKIP—Temporal Key & C.C.M.P.—A.E.S. based \\  & & Integrity Protocol (using RC4) & encryption protocol \\ \hline \multirow{2}{*}{**Authentication (types)**} & WEP-Open & WPA-PSK & WPA2-Personal \\  & WEP-Shared & WPA-Enterprise & WPA2-Enterprise \\ \hline \multirow{2}{*}{**Data integrity**} & \multirow{2}{*}{CRC-32} & Provided by Message & Provided by CBC-MAC \\  & & Integrity Code & (cipher block chaining \\  & & message authentication code) \\ \hline \multirow{2}{*}{**Technology weaknesses**} & Very vulnerable to DoS & Vulnerable to DoS attacks and & Vulnerable to DoS attacks and \\  & attacks. Now, this security & key reinstallation attack & key reinstallation attack \\  & is broken. & (KRACK.) & (KRACK.) \\ \hline \multirow{2}{*}{
\begin{tabular}{c} **Ease of** \\ **technology implementation** \\ \end{tabular} } & \multirow{2}{*}{Very easy to set up} & WPA-Enterprise requires an & WPA2-Enterprise requires an \\  & & authentication & authentication \\  & & server configuration & server configuration \\ \hline \multirow{2}{*}{**Protection against attacks**} & \multirow{2}{*}{No security} & Packet sequence counter & The new 48-bit vector IV acts \\  & & is implemented & as a frame counter \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of WEP, WPA, and WPA2 security [21]. There are many concepts and designs for SHM systems, with different costs and durability, e.g., Comparative Vacuum Monitoring (CVM) developed by Structural Monitoring Systems (SMS, Nedlands, Australia; Century City, CA, USA and Ashford, UK) and an Acousto-Ultrasonics (AU) technology called SMART Layer developed by Acellent Technologies (Sunnyvale, CA, USA) [26]. In general, SHM technologies can be divided into two categories: local techniques when the observed area is limited to the area underneath the sensor (this is the case, for example, with CVM and electro-mechanical impedance (EMI) techniques) and global techniques when damage can be detected at any point in the structure by a network of appropriately distributed sensors [27]. Real-time health monitoring is the basic principle, and it is the fundamental parameter of cost-benefit analysis investigations. In the case of, e.g., aircraft, it is continuous health monitoring using airborne wireless sensors and damage assessment employing a central station. In many studies [28], it has been stated that it is not practical to use wireless sensors to monitor an aircraft's whole structure because of the decrease in weight and reliability of batteries.