# An Experimental Tethered UAV-Based Communication System with Continuous Power Supply
## 3 Methodology
### Elements for the Prototype
#### 3.2.3 Implementation of IP Telephony System  
This section describes the procedure for installing and configuring an IP telephony service on a Raspberry Pi 4B, explaining each step necessary to ensure proper configuration and operation. The steps to follow are detailed below. 1. **Installation of the RasPBX operating system**: A microSD card with an SD adapter is required to perform the installation. The minimum requirement for the card is a capacity of at least 8 GB, which is necessary to guarantee the correct functioning of the service to be installed. The microSD card must then be inserted into a computer. Next, the RasPBX operating system image must be downloaded. The latest version available is raspbx-10-10-2020, compatible with Raspberry Pi 4, 3, and 2 [34]. To begin with the configuration, follow the steps below: 1. Download the RasPBX image at [34]. 2. Use the Win32 Disk Imager tool to copy the image to the SD card. 3. Create an empty text file with the name'ssh' and save it on the SD card. Once the image has been written to the SD card, it is inserted into the Raspberry Pi and connected to the power supply. The Advanced IP Scanner tool is then used to identify the network generated by the Raspberry Pi (specifically, its IP address), which is available for download at [35]. With this information, the computer terminal is accessed, and a remote connection to the Raspberry Pi is established using the SSH protocol, which facilitates remote administration of the device. During the SSH connection process, a password is requested; the default password is _raspberry_. 2. **Operating system upgrade**: To avoid unwanted updates in the _PHP_ repository and prevent possible errors during the system update, the command Listing 1 must be run. With the commands shown in Listing 2, the system is updated correctly without generating problems. 3. **Creation of the access point**: The Raspberry Pi must establish its wireless network in the proposed prototype. The programs _hostapd_ and _dnsmasq_ are used to do this. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Features** & **3 CX** & **FreePBX** & **Asterisk** \\ \hline \multirow{4}{*}{**Main Features**} & Call management, & Advanced call & Flexible platform with full \\  & extensions, video calls & management, extensions, & support for extensions, \\  & (limited in free version) & IVR & IVR, and advanced \\ \hline \multirow{4}{*}{**Configuration Type**} & Requires downloading the & The software image is & The Raspberry Pi OS image \\  & Raspberry Pi OS image; & directly stored in memory & is downloaded, and the \\  & installation is complex and & and configured via & softphone is installed using \\  & achieved via GUI or & commands & manual commands \\ \hline \multirow{2}{*}{**Scalability**} & Moderate (limited free & & \\  & version) & High & Very high \\ \hline \hline \end{tabular}
\end{table} 
Table 4: _Cont.__Hostapd_ allows the Raspberry Pi to function as an access point. In contrast, _dnsmasq_ provides the services of _DNS_ and _DHCP_, taking care of assigning IP addresses to the devices that connect to the network [36]. The commands in Listing 3 must be executed to install both programs. ```
1 suboapptinstallhostapdnsmasq
```  
Once _hostapd_ is installed, it must be configured to function as an access point and allow the Raspberry Pi to provide a wireless network. To perform this configuration, the _hostapd_ configuration file must be accessed using the command in Listing 4. ```
1 subonano/etc./hostapd/hostapd.conf
```  
Inside the configuration file, it is necessary to define the specific parameters to establish the network access point. For example, a user can assign the network name (SSID) as \"ServiceVolp\" and set the password as \"veronicar\". To do this, the following lines should be added to the file as shown in Figure 6. Furthermore, the configuration file must be configured at startup to ensure that hostapd uses the _/etc/hostapd/hostapd.conf_.