## 3 Methodology: Requirements Analysis, Selection of Buildings, and Technologies
### Deriving Technologies and Instrumentation Variants
#### 3.3.3 Selected Variants  
For equipping buildings on the room level, we chose the approach of using one IoT gateway per building and connecting the different devices to it as a general communication architecture. On the centralized-decentralized continuum, this is an intermediate approach, tending towards the centralized end. As shown in Table 4, we use an industrial PC (IPC) by Beckhoff as a central IoT gateway or automation device per building. A huge advantage of such an IPC per building is its extendability: by adding hardware clamps to the same central CPU unit and adapting the freely configurable code on the IPC, it is possible to include several protocols and devices in a similar manner. Moreover, the need for several IP addresses for connecting different gateway devices to the cloud can be reduced significantly to one per building. For different circumstances in the selected buildings, we use different solutions regarding the protocols used, which we describe separately in the following paragraphs. Solution with EnOcean  
We decided to implement a basic variant which uses EnOcean [71] devices in combination with the central IPC per building. The IoT gateway for the building connects to several EnOcean transceivers by cable, and those communicate wirelessly with the field devices (EnOcean sensors/actuators). In this way, it is possible to flexibly place field devices without the need for cabling, which then reduces the installation effort. To the best of the authors' knowledge at the time of the selection, EnOcean was the only wireless technology which had devices to actuate a heater valve without the need for external power supply or frequent battery replacement. Recently, a wireless KNX [72] (KNX-RF) device for actuating heater valves has become available. Figure 3 shows an excerpt of a conceptual floor plan, indicating EnOcean field devices as well as the positioning of the central IoT gateway (the IPC). The devices used can measure temperature, relative humidity, CO\({}_{2}\) concentration, and the status of windows and doors. Measurements of temperature, relative humidity, and CO\({}_{2}\) concentration are used as indicators for IAQ and user comfort. To gain knowledge about the status of windows, different sensor devices can be used. Whenever possible, the window handle is replaced by a \"smart\" window handle. A second option is the installation of an additional sensor between the window handle and frame. In case the window frame does not not allow for any of the before-mentioned options, the windows are equipped with two window contacts. All three variants allow for the differentiation of the three possible states: tilted, opened, and closed. The valve actuator can control a heater's valve and reports several aspects of its status. A list of typically used device models is given in Table 4. Note, however, that the table does not contain every device model tested or used for comparison. The \"kill-switch\" aspect for the automation of pure EnOcean actuators is built into the IPC program as well as the chosen devices: with regard to the first aspect, we wrote the program code in a way which easily allows one to switch between different control modes for sending commands to EnOcean actuators. Moreover, the valve actuators allow for an independent control when no command is set for a specific amount of telegram cycles. Figure 3: Equipment concept for a floor and wing of an examplary building layout using EnOcean. Solution with EnOcean and KNX  
In addition to the fully EnOcean-based solution, there will be parallel KNX installations in some of the buildings investigated. In close coordination with the infrastructure manager, it was decided to make use of KNX for the control of blinds and lights, for the sake of the continuous availability of a HMI in all control modes. At the same time, existing switches will be replaced by a HMI for all interactions with the room.