# Wireless Sensor Networks for Enabling Smart Production Lines in Industry 4.0
## 2 Methodology
### System Architecture  
The overall system architecture is shown in Figure 1. We consider a hybrid wireless network in which multiple technologies with different levels of customization co-exist. We categorize all devices into either the access point (AP) or client, irrespective of the used technology (Client and access-point device types originate from WiFi terminology). In IEEE 802.15.4 ZigBee, the client is called the _end device_ and the access point is called the _coordinator_. In Bluetooth Low Energy (BLE), the terms _peripheral_ and _central_ are used. In LoRa, the terms _end device_ and _gateway_ are used. For simplicity, we use the terminology access point and client for all technologies.). Both AP and client contain an RF transceiver device and we categorize the transceivers according to the wireless technology that they support. Moreover, one physical device can implement multiple RF transceivers and act as an AP for one technology and as a client for another technology. APs are connected to the intranet via wired connections, or via a gateway using another technology. An example of the latter is a ZigBee gateway that acts as an IEEE 802.15.4 AP and as an IEEE 802.11 client, that is, data from the ZigBee clients are gathered and sent to another AP via WiFi. This is shown in Figure 1, where the APs for Technology 2 at the right of the floor plan acting as clients for Technology 1, and connect to the intranet via AP1.  
Wireless communication links are categorized according to the required QoS, that is, maximum delay and latency, and minimum throughput. Jitter and packet loss are QoS parameters that relate to MAC-layer network configurations or are caused by interference. Packet loss can also be caused by RF blockage, which we handle from a throughput perspective.  
During the initial network-planning, APs are placed on the floor plan based on an initial set of requirements and based on an initial floor plan layout. In this work, we monitor the link quality on a per-packet basis. Once the networks are operational and data are being sent, monitoring data are used to calibrate the simulated parameters, such as received signal strength and throughput. In a continuous monitoring process, a centralized local network controller can reconfigure PHY and MAC layer configuration settings to allow an adaptive system that can be used in a dynamic environment. Indeed, the floor plan layout of the factory can change, for example, due to adjusted assembly lines or added machinery, but the application requirements may also change. The network controller is similar to the gateway that is proposed in [30], that is, it keeps track of the current state  
Figure 1: Overall system architecture with multiple wireless technologies. Clients wirelessly connect to an access point, and access points are connected to the network controller via a wired connection, or via another (1-hop) access point. Wired connections are represented by a straight line, whereas wireless connections are represented by a dotted line.  
of the network, can provide local processing with low latency, and can also send data to the cloud. The different phases, from initial network-planning to the monitoring loop, are visualized in Figure 2.