## 2 Wireless Telecommunication Networks for Critical Infrastructures and Emergencies  
The primary advantages of a private telecommunication network are high reliability and security while maintaining high speed and lower latency. The well-designed private network addresses issues that are difficult to handle for public infrastructures or Wi-Fi networks installed by businesses. Private networks, which will also be 5G-based in the future, will hold a powerful position in such solutions [4]. Below we present some private-based communication systems and experimental platforms using either SD-WAN technology or VoIP services. We also include some emergency voice communication networks. [PERSON] et al. [5], trying to replace the traditional public switched telephone network (PSTN) with a more modern solution, created a private network by implementing the concept of a walkie-talkie using voice over IP (VoIP) services and existing telephony infrastructure that supports the service. This project aimed at utilizing VoIP between mobile phones and a simple router. From the central router of a house, IP packets were forwarded through the voice channel on the Internet, allowing data exchange to occur through interconnected mobile devices within the private network of the house. The flexibility of this project lay in the fact that there was no need for new device purchasing, as a simple Wi-Fi connection to the router could serve the implementation need. [PERSON] et al. [6] designed a system consisting of multiple communication nodes, like a walkie-talkie, using the P2 Pnet platform and portable electronic devices. During the critical early hours of a natural disaster, these nodes can communicate wirelessly with each other until they approach the area where rescue operations are accessible. During the system's design, it is mentioned that each IP packet, along with headers and frames, can be sent as a UDP packet without significant issues. The system is fully capable of receiving and converting all incoming and outgoing packets from the same node without filtering. Additionally, the use of PCM encoding was studied to address the scenario where more than two nodes communicate simultaneously, providing a solution to the aforementioned problem. PCM encoding reproduces data at a regular and relatively low rate, contributing to the system's functionality. Through [PERSON] et al. 's [7] SD-WAN solution, businesses can enhance the quality of voice services for VoIP calls. They support this option in their research, as the activation of SD-WAN on a device via the Internet allows many employees to work exclusively from their homes. This implementation ensures better resource management for each user adopting this idea, while the central SD-WAN management eliminates traditional issues related to configurations, changes, and scalability. During their research, they conducted various test scenarios. One scenario involved a single communication line where two computers were connected through a simulated broadband connection without SD-WAN functionalities. Additionally, the IP telephony was controlled by a remote PABX (private automatic branch exchange) system. This asynchronous broadband network was used as the connection for the two computers. Subsequently, they conducted tests using a dual link for SD-WAN, focusing on the primary and secondary routes created within the WAN between Router A and Router B. The primary route was separated by a WAN simulator, to meet WAN connection needs, while a 4G-LTE connection simulator was used for the broadband connection requirements. Another valuable reference on VoIP via the SD-WAN platform is presented in [8]. They propose a connection implementation between two central SD-WAN systems. This approach can lead to improved quality of service and increased bandwidth. They demonstrate that this solution can deliver better network traffic on VoIP calls while enabling more efficient management of network resources, resulting in the desired quality.