# Road-Side Unit Anomaly Detection
## 3 RSU Failure Detection  
In this section, we detail our failure detection technique mainly based on verifying whether the coverage which should be ensured by an RSU is effective. For this issue, we propose a new C-ITS message denoted \"Alarm Message\", which is sent by each vehicle entering a zone covered by an RSU. The idea is to insert in this message some information about the observed signals by the vehicle, mainly in the ID and the OD. The RSU collects all received data and runs an on-the-fly analysis in order to identify whether an anomaly has occurred. An antenna is a fundamental component of telecommunication systems such as radio, cellular networks, Wi-Fi, and satellite communications. It is a device that is used to transmit and receive signals in the form of electromagnetic waves. When used for transmission, it takes an electrical signal and converts it into electromagnetic waves that propagate through the surrounding space. When used for reception, it captures the electromagnetic waves from the environment and converts them back into electrical signals. Overall, there exist three kinds of antennas based on radiation patterns. An isotropic antenna is a hypothetical one and radiates over all directions in the horizontal plane as well the vertical one. Its radiation diagram resembles that of a sphere and can be seen in Figure 3. An omnidirectional antenna radiates over all directions in the horizontal plane, as can be seen in Figure 4. It is commonly used in applications wherein signals need to be transmitted and received in multiple directions; an example of the omnidirectional antenna is the dipole antenna. Meanwhile, a directional antenna radiates in the form of a concentrated blob over one direction, as can be seen in Figure 5. Examples of directional antennas include parabolic antennas and the phased array antenna. Figure 4: Radiation Diagram for an Omnidirectional Antenna: (**a**) vertical plane and (**b**) horizontal plane. Figure 3: Radiation Diagram for an Isotropic Antenna: (**a**) vertical plane and (**b**) horizontal plane. The used RSUs in this study are either omnidirectional or have two directional antennas each. Each antenna is directed toward a side of the road and has a relatively high directivity. As long as the road is relatively parallel to the antenna's lobes, the vehicle will be able to receive messages from the RSU. Our target is a specific failure which is when one of the two antennas fails, and the bidirectional RSU thus behaves as a directional antenna and radiates over only one side of the road. Within the C-roads and the Indid projects (two European--Connecting Europe Facilities-projects), we collected many datasets through real conditions on more than 300 km of highway roads in Italy, Germany, and France. More than 1 gigabytes of traces were collected in PCAP format (this format is the one used by network analyzers) produced by one driving vehicle and more than 50 road-side units, both in ITS-G5 and C-V2X. For each RSU, more than 20 megabytes were collected in each experiment. This work, however, only focuses on ITS-G5 RSUs, which have shorter ranges than their C-V2X counterparts. More than 10,000 messages were collected by the experimentation vehicle. In the experiment, each captured packet comes with a radiotap information layer. Radio is a protocol which is the standard for the 802.11 frame injection and reception. It does not belong to the original ITS-G5 protocol stack, but it is created at the moment of capture of a packet. The test vehicle that was used had the ability to generate the radiotap layer concerning the captured packet. The recorded information in the radiotap layer concerns the radio transmission, such as the signal intensity at the moment of capture. If the packet was sent from the vehicle, then the transmission power (in dBm) is stored, and if the packet was received then the signal intensity at the reception moment is stored. We are interested in the latter, since it provides us with the signal intensity of transmissions coming from the RSU.