## 3 AIoT Architecture for Structural Testing
### Overall Architecture  
Figure 1 shows a schematic of the general architecture developed in this study, which includes the major operations executed at every layer of the architecture. Structural testing applications typically involve several processes in which the acquisition of the sensors must be coordinated in a precise way to meet the test requirements. Thus, the sensors in the edge layer collect data that are sent to a processing center in the fog layer. The collected information is processed and analyzed in the fog layer, close to the test facilities, to control the aeronautical component testing environment. Finally, the collected and processed information is sent to the cloud layer to be stored for in-depth analysis by means of advanced algorithms, typically using AI-based technologies. The AIoT architecture for structural testing implements mature IoT technologies at every layer that are commonly accepted in industrial applications. The experimental validation in an industrial environment has been carried out using medium-performance and affordable devices to sense the different processes of the structural test. Sensors, whether analog or digital, need to fulfill the technical specifications necessary for aeronautical structural testing  
Figure 1: General architecture of the proposed monitoring network. measurements. Also, these sensors must be compatible with the electronic boards utilized in the edge nodes, which include state-of-the-art programmable microcontrollers. The use of wireless communication technologies in structural testing provides flexibility and eases the deployment of the sensor devices, avoiding costly wiring. WiFi and Message Queue Telemetry Transport (MQTT) technologies have been identified for data transfer between the edge and fog nodes in less time-critical measurement systems. These technologies are common in industrial environments and relatively easy to deploy, proving their advantages in a broad number of industrial applications. In particular, WiFi covers relatively short distances, providing a high transfer speed. MQTT has been utilized due to its simplicity and ease of implementation, enabling its use in small microcontrollers while optimizing network bandwidth. Additionally, MQTT allows message dissemination to groups of nodes [42; 43]. However, if necessary, this architecture could be implemented with other technologies which provide better Quality of Service (QoS) parameters, such as WirelessHart or ZigBee, as discussed in [10; 44]. The communication between the fog and cloud layers was enforced by means of TCP/IP-based technologies. Typically, wired networks, mainly Ethernet-based technologies, are found inside industrial facilities, making it possible to reuse available infrastructures. When possible, wired technologies are preferred since they provide more efficient communication with a better QoS. The fog layer is responsible for providing centralized clock services to all edge nodes in order to synchronize the transmission of data acquired by the IoT devices. This approach, described in [44], implements a time-slotted media access (TDMA) mechanism that allows the timely transmission scheduling of the messages, reducing the interference caused by the IoT devices. This approach was successfully implemented over MQTT [45] and over XBee 900 MHz [44]. A first layer of pre-processing and simple data analysis is performed at the fog layer, near the edge of the network. The fog computing paradigm also allows implementing the control tasks of the actuators involved in the structural test. This approach allows responding to alarm situations in a very short response time. The data collected from the experimental test are stored persistently and analyzed in real-time to produce monitoring information about the test. The use of file sharing protocols between the fog and cloud layers, such as Server Message Block (SMB) or FTP, is also welcome, since it allows sharing information between the nodes in an efficient and simple way.