## 2 Sensor Node Architecture for SHM and Communication Architecture  
The virtualization and deployment of VMs onto tiny micro-controllers was inspired by the wireless material-integrated sensor node [11; 12], which is embedded between two layers of a fiber-metal laminate plate and developed in the DFG research group 3022 for automated diagnostics of hidden damages in fiber-metal laminates using guided ultrasonic waves (GUW). The sensor node is supplied with power via RFID/NFC communication technology only [13]. The energy harvester is able to deliver up to 15 mW of continuous power, significantly constraining the selection and operation of the electronic parts, as shown in Figure 1. After the sensor node is integrated into the plate, no software updates or maintenance can be applied. The communication with the micro-controller takes place only via the NFC tag. The communication is bidirectional, originally via the NFC tag's EEPROM (code and data). Alternatively, wireless communication can be realized directly with a write-through mode (because the lifetime of the EEPROM is limited to about 1000-10,000 write cycles), writing message data directly to the micro-controller. The sensor node uses an ARM Cortex STM32 L031 device, an NFC tag with a power supply, and a pre-amplifier for piezoelectric sensors. The features are summarized in Table 1. More details and descriptions of the sensor node can be found in [12]. \begin{table}
\begin{tabular}{l l} \hline \hline
**Feature** & **Value** \\ \hline Size & 17 \(\times\) 17 mm (without antenna and sensor) \\ Sensors & Piezoelectric transducer, MEMS sensor, temperature, radio \\  & field strength \\  & ADC (1 MSPS, 8–12 bit resolution), pre-amplifier, power regulator, NFC \\ Components & tag, ARM Cortex M0 STM32L031 micro-controller with 8 kB RAM and \\  & 32 kB ROM \\ Communication & Wireless, NFC (13.56 MHz), up to 100 kb/s \\ Energy & Energy harvesting by NFC tag, up to 15 mW continuous power \\ Software & REXA VM (CS = 1024, DS = 512, RS/FS = 32, only single-precision data) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Features of the material-integrated wireless sensor node. Although this work focuses on the design and implementation of a universal VM suitable for implementation on very low-resource embedded systems and providing ML as a programmable service, communication aspects are relevant and must be considered as constraints, both for the design and operation of the VM and the communication design. Communication with material-integrated sensor nodes is commonly performed by using wireless technologies, but in the presence of metals and dielectric materials, wireless communication is a challenge. Low- and mid-frequency RFID technologies are widely used. In this work, it is assumed that the REXA VM is accessible by RFID/NFC communication, as shown in Figure 2. Sensor nodes communicate wirelessly via RFID/NFC tag circuits with a reader, which is connected short-range to a \"remote\" VM instance. The reader nodes are connected long-range (wired or wireless) to establish a distributed network. The reader nodes are communication end-points and message routers. Figure 1: ARM Cortex M0-based sensor node (STM32L031) implementing the REXA VM for material-integrated GUW sensing with NFC for energy transfer and bidirectional communication with only 8 kB of RAM and 32 kB of ROM. Figure 2: Principle REXA VM network architecture using different wired and wireless communication technologies. The communication capabilities have an impact on the ML models that can be implemented with respect to the code and data size. It can be expected that the typical message size will not exceed 1k Bytes. The transfer of 1k Byte of payload data requires about one second of transfer time, including packet fragmentation. ML models, including the forward prediction functions, are submitted to the VM via text, including the fixed model parameters.