# Communication Manager for Hyper-Connected RPAS Environments
## 4 Validation: From the Lab to the Air
### Laboratory Ground Truth
#### 4.1.1 Choosing the Minimum RTT Path  
This first experiment proves the performance of the CIM following an SDN strategy to select the lowest latency (taking into account the RTT) communication alternative (among the available ones) in a scenario where the network topology frequently changes, which is quite common in small RPA operations. The experiment includes an RPS and two RPAs (RPA1 and RPA2). The RPS and RPA1 remain static, while RPA2 follows the predefined trajectory (dashed blue line) depicted in Figure 4 from _Source point_ to _Destination point_ and vice versa. Thus, at some point, (i) the RPA2 can directly communicate either using Wi-Fi with the RPS (green background), or (ii) using Wi-Fi with the RPS via the RPA1 (blue background), or (iii) it has to employ the 4G/LTE (yellow background) when the Wi-Fi is out of range. The RPA2 sends a continuous 1.2 Mb/s stream to the RPS to test the operation (1 Mb/s serves to depict a representative RPA video stream, while the remaining 200 Kb/s corresponds to telemetry information sent by the UAVs to the station). This value (200 Kb/s) has been estimated by analyzing the telemetry generated by the different RPAs used in the Labyrinth project in its use cases. In this experiment, one of the objectives of the CIM (it can be any other depending on the mission needs) is to manage the forwarding plane of the programmable virtual switches (i.e., the OvS switches in the RPA and the RPS in Figures 2 and 3) in order to select the shortest path based on the minimum measured latency. For this reason, whenever a Wi-Fi path becomes available (even if it has to go through RPA1), it will be the preferred one (it will always be faster than using the public 4G/LTE network). Thanks to the report received by the _network monitoring application_ installed in each RPA and RPS, this information is available at the SDN controller. As shown in Figure 5, during the first period of the experiment, the stream is received over the Wi-Fi interface as the RPA2 and the RPS are in range. From 38 s onwards, this information is no longer received over the Wi-Fi interface because the RPA2 goes out of range, and the one-hop communication is no longer available. However, two-hop Wi-Fi communication is available, although it will not be possible until the routing protocol (OLSR as explained in Section 3.2) discovers the path. While this process is in progress, the SDN controller installs a rule in RPA2 to force the transmission through the 4G/LTE interface. From 52 s onwards, the protocol enables the multi-hop path, and the Wi-Fi interface becomes available again (indicated by the SDN controller). From 65 s, the RPA2 is no longer in the Wi-Fi coverage range of the RPA1, so the only possible way to communicate RPA2 and RPS is through the 4G/LTE interface. Figure 4: Choosing the minimum RTT path scenario. From the 160 th second, RPA2 is back in RPA1's Wi-Fi coverage range. However, the change does not occur until 170 s, corresponding to when the OLSR protocol converges and enables the path. From the 180 th second, RPA2 is back in range of the RPS and communicates using the Wi-Fi interface with one hop. Unlike the previous case (when the RPA2 is moving away), there are no communication breaks in this case. This behavior is because, in this case (when approaching), alternatives do not disappear, but the process is to move toward the optimal option according to the RTT. As mentioned above, the interface changes are managed by the SDN controller. RTT measurements have been taken from the RPA2 to the RPS using the _ping_ tool to verify this behavior. This value is reflected in the background of Figure 5. When the figure is shaded with green, the average RTT value is 10 ms. This period corresponds to one-hop Wi-Fi connectivity. When the figure is shaded with blue, the average RTT value is 21 ms. This period corresponds to two-hop Wi-Fi connectivity. When the figure is shaded with yellow, the average RTT value is 98 ms. This period corresponds to 4G connectivity.