# Emergency Communication System Based on Wireless LPWAN and SD-WAN Technologies: A Hybrid Approach
## 4 A Hybrid Approach to Emergency Communication Systems
### High-Level Architecture  
In this section, we propose our hybrid approach to an ECS based on SD-WAN and LoRaWAN technologies. The combination of open-source protocol and 5G technology creates several difficulties for SDWAN-based virtualization and slicing management [45]. Thus, the proposed system consists of an autonomous wireless 4G/LTE base station and a LoRa network. To implement the on-premises LTE/4G network, we used the Open Air Interface (OAI) open-source software. OAI is available from University Eurecom and runs on Ubuntu Linux, and its purpose is not only experimental, as it actively participates in the research and development of 5G and other communication networks and is also used on IP Multimedia System (IMS) platforms [3]. We also deployed a private star-of-stars topology LoRaWAN network, in which gateways relay messages between the end devices and a LoRa network server [8]. The gateways transmit data to the LoRa network server installed on a Linux environment in our isolated 4G/LTE network. Figure 1 describes the implementation of this hybrid approach, while Table 4 defines the ECSs abbreviations.  
The 4G/LTE implementation uses the HSS database, OAI EPC (MME, S-GW, and P-GW), and E-UTRAN (OAI eNB and UE). The MME, HSS, and SPGW apps operate on an Ubuntu Linux generic Kernel PC, while the eNB application runs on an Ubuntu Linux low-latency Kernel PC (eNB PC) (EPC PC). Using Software-Defined Radio (SDR) platforms and commercial user equipment, the 4G/LTE network operates in real-time (UE) [3]. The UE can be a 4G/LTE commercial mobile phone or commercial 4G/LTE module, subscribed in the private 4G/LTE network via a registered sim card. Users holding mobile phones can use Voice over Internet Protocol (VoIP) applications, while 4G/LTE modules send geolocation and other data (e.g., health data, environmental data) to an IoT platform. VoIP is a beneficial innovation based on contemporary business and industry needs, as it offers the technology of transporting voices in the environment of a data network. The software implementation and internet connectivity have offered this service incredible flexibility when voice communication systems are to be deployed. It also provides real-time transmission of voice signals through the Internet Protocol (IP) across a private data network or the public Internet [46]. Figure 2 shows our experimental 4G/LTE base station.  
Figure 1: ECS Hybrid Approach—High-Level Architecture.  
\begin{table}
\begin{tabular}{l l} \hline \hline
**Acronym** & **Abbreviation** \\ \hline eNB & Evolved Node B \\ EPC & Evolved Packet Core \\ EUTRAN & Evolved Terrestrial Radio Access Network \\ HSS & Home Subscriber Server \\ MME & Mobility Management Entity \\ OAI & Open Air Interface \\ PGW & Packet Data Network Gateway \\ SGW & Serving Gateway \\ UE & Mobile User Equipment \\ Si-C, S1-U, S11, S6a, SGi, RJ45, USB3.0 & Communication Protocols \\ USRP B210 & Software Defined Radio Device \\ LoRa gateway & LoRaWAN Gateway \\ LoRa nodes & LoRa User Equipment \\ \hline \hline \end{tabular}
\end{table}
Table 4: ECS Abbreviations.  
Figure 2: 4G/LTE Base Station Setup.  
As far as the LoRaWAN is concerned, we have implemented a firm, low-power consumption, and scalable wireless sensor network composed of a LoRaWAN gateway, LoRaWAN nodes, and GPS sensors. We use GPS sensors for geolocation tracking, while users can exchange another type of information (e.g., health data, environmental data) with the IoT platform.