# Hybrid RF/VLC Network Architecture for the Internet of Things
## 3 Network Architecture  
The proposed hybrid network architecture employs three different models of communication: Zigbee, LoRaWAN (employing LoRa physical layer), and VLC. For the implementation of these three communication methods, it is necessary to use different technologies for the inter-operability between the protocols used by each of them. Figure 2 shows the global network architecture implemented for the assembly of the developed hybrid network. The first thing that can be observed in the network shown in Figure 2 is the set of sensor/actuator nodes. These nodes will collect the data of sensors that are connected and transmit these data to the access point, or receive data from it, to perform the response associated with the possible actuators connected. In order to establish the interaction between nodes, in addition to communication between nodes and devices external to the sensor/actuator network, it is necessary to include a device that controls and establishes these communications. Therefore, to the network formed by the sensor/actuator nodes that use the Zigbee communication protocol, it is necessary to add a ZigBee coordinator device. Thus, it will be in charge of controlling the entire network and establishing not only communication between nodes, but communication between nodes and external devices, which will probably use another communication protocol. The communication between cell nodes and the central network node is implemented using LoRa links, forming a star topology and allowing long distance communications between cells and the Internet network. One of the most important challenges of this work is the inter-operability between these two technologies. To achieve this, it is necessary to define a unique frame format which allows for node discovering, identification, and routing. This frame should also be compatible with the VLC link and should implement routing functions to redirect data or control messages between the different subnetworks. In this work, each technology is assumed as a subnetwork and each cell of the overall network. In order to achieve this, each of the access points implements a routing table with node identifiers and MAC directions. The cell access point translates frames arrived from each technology and redirects them inside the cell to the IP network or to another cell. One of the objectives of this work is to store digital content in the access point, which could be transmitted to a single user or to a node. For example, the central node could take a digital image and transmit it to the internet network using the LoRa backbone. To reach that objective, it is necessary to implement a segmentation system that takes into account the designed frame format, as will be seen later. Packets received by the access point from the LoRa backbone are redirected to one of the two subnetworks (optical or ZigBee), transparently to the transport network and the user. Packets from these subnetworks are sent in the same manner. With the inclusion of LoRaWAN in the coordinating modules of the sensor/actuator network, it is possible to create a LoRaWAN communication network. However, it is necessary to use a gateway forming a star topology at the backbone side, which, among other features, can establish communication between devices employing the same protocol, in addition to providing the user the possibility of uploading the data collected by the sensors to a cloud to facilitate its visualization, and allow for the possibility to interact with the actuators or devices located in the network. In many cases, it is possible to use a previously available gateway at the area where one is working. However, there are areas that do not have this infrastructure. That is why in these areas it is necessary to implement a proprietary device. However, it is not only enough to have an Internet connection through the gateway with LoRaWAN, but a user interface and a service that stores the data collected by the network are necessary.