# An Experimental Tethered UAV-Based Communication System with Continuous Power Supply
## 4 Results
### Prototype Testing in an Open Environment  
#### 4.2.1 IP Telephony Service Performance Analysis  
The voice-over IP communication system implemented in the Raspberry Pi was validated by configuring two softphones connected to the local network \"VoIP Service\" created in the device. The softphones were configured with the data specified in Table 13. Figure 28 illustrates how this data was entered in the Zoiper application. The results show the successful establishment of the calls, as shown in Figure 29. The logs stored on the device that used the MizuDroid softphone for Android recorded two calls made: (i) one with a duration of 3 min and 12 s, and (ii) another with a duration of 11 min and 1 s. Additionally, the quality of the voice transmission was optimal, characterized by clear and uninterrupted audio throughout the communication, which confirms the functionality of the implemented communication system.  
To analyze the operation of the communications system, various parameters can be analyzed during the establishment and development of the VoIP call. These parameters included the behavior of the wireless interface wlan0, which is shown in Figure 30. This figure shows that the transmission and reception rates remain stable, with values of approximately 24 kbit/s (TX) and 20 kbit/s (RX), respectively. The total bandwidth consumed during the call is approximately 44 kbit/s, indicating low network resource demand, which  
Figure 27: Variation of voltage and current consumption in a closed environment during the interval from 20 to 90 s.  
Figure 26: Variation of voltage and current consumption in a closed environment during the interval from 280 to 319 s.  
is consistent with the use of voice compression codecs. This efficiency confirms the system's viability in scenarios with bandwidth constraints or reduced network infrastructure.  
For the proposed prototype, the Raspberry Pi 4B is used as the processing platform. This device features a quad-core processor, which enables concurrent handling of multiple tasks; therefore, a load average of 4.0 would indicate full utilization of all available CPU cores. Accordingly, Figure 31 presents the CPU load behavior during a VoIP call, showing the system load average over 1-, 5-, and 15-min intervals. These values represent the average number of processes either running or waiting for CPU access during the specified periods. Throughout the call, the load average remains below 0.3 in all intervals, which is considerably lower than the theoretical maximum of 4. This confirms that the processing demand imposed by the IP telephony service is minimal and does not represent a significant load placed on the system.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Username** & **Domain** & **Password** & **User Name/Extension** \\ \hline
**[PERSON]** & 192.168.4.1 & [PERSON] & 100 \\ \hline
**[PERSON]** & 192.168.4.1 & [PERSON] & 101 \\ \hline \hline \end{tabular}
\end{table}
Table 13: Extensions configured in the softphones for the communication system test.  
Figure 28: Configuration of [PERSON]’s extension on the Zoiper softphone.  
Figure 29: Call established on a Zoiper softphone for an iOS device and a MizuDroid softphone for an Android.  
Figure 32 shows the use of memory during the execution of the system. This figure shows that 16% of the RAM was used directly, while 3% was allocated to buffers and 17% to cache. The remaining 65% remained free, indicating ample availability of memory resources. The analysis of memory utilization revealed that no swap memory consumption was recorded, indicating that the system did not require secondary storage. Figures 31 and 32 reveal that the proposed communications system operated efficiently with the established CPU and memory configuration, exhibiting stable and adequate operating behavior.  
Figure 31: CPU usage behavior during the VoIP call.  
Figure 30: Traffic monitoring on the wlan0 interface during an active VoIP call.  
Figure 32: Memory usage during system operation.