# A Roadmap to Integrate Digital Twins for Small and Medium-Sized Enterprises
## 4 Discussion
### RQ1: Design of the Communication Framework  
The earlier studies have defined DT as an appropriately synchronised body of useful information. However, there are 'twinning' challenges of DT development [8, 15]. We sought to address the implementation of an ethernet-based connectivity between a physical asset and its DT model in the communication framework. The focal attribute of the physical asset (i.e., the servo motor) was its rotational motion. As such, the useful information in this situation was whether the servo motor is rotating or stationary at any given time. We demonstrated through the use of some off-the-shelf tools that it is possible to create an integrated system whereby data were acquired at the field equipment level and communicated to the cloud-based system for storage and processing, which includes data visualisation as presented in Section 3.4. To collect data, CPS was required at the field level. In this case, the CPS was the WAGO PLC, a common instrumentation tool that can provide a range of status information including measurements, temperature and position. Although the amount of data was neither vast nor complex, our experiment demonstrated a method of continuously collecting, processing and storing real-time data continuously in practice. The concept of value generation from the DT development lies with the user and will be specific to their circumstances. When needed, the proposed framework can be expanded easily with new sensors, communication devices and software toolboxes to meet the unique needs of SMEs. Upon examining the choices for communication protocol (i.e., Modbus, OPC UA and MQTT), the final decision was weighed against a set of criteria: versatility between hardware devices, scalability and security. OPC UA was the prime candidate for the following reasons. Firstly, OPC UA has industrial standardisation among hardware and software, giving it compatibility with most available tools. Essentially, much of the existing hardware and software related to industrial automation are readily compatible with OPC UA. Secondly, it has sophisticated security features in its application layer and transport layer of communication. We considered security a high priority and especially relevant to any industry users to ensure confidentiality and security of information. Thirdly, OPC UA provided opportunities for scalability in both its data representations and its minimal resource constraints. In other words, data modelling of objects through its attributes and relationship to other objects allows users to build a larger data model as new sources of data, which can be included and mapped without any large overhaul of SMEs' existing communication network (i.e., all the connected hardware and software). Furthermore, since OPC UA has a high compatibility, it allows devices with different capabilities to be interconnected. For example, OPC UA can be used for connecting field level devices to control level devices such as PLCs, as demonstrated in this paper. It can also be used at control level for PLC to operate in a supervisory control and data acquisition (SCADA) system at remote locations. The considerations for SMEs may vary from our results. For a system where comprehensive data models are the key to operations, a protocol such as OPC UA may be necessary. When the available hardware is restricted in its ability to handle long communication messages and operates on a network with low bandwidth, MQTT will be more appropriate. Regarding costs, most communication protocols are open source and free; however, implementation and customisation that suit each SME requirements may depend on their resources, in-house expertise and constraints. We used a structured database as the RTD sensor recorded a time-series of temperature measurements. This data posed a minimal complexity for harvesting and minimal variety in data types for ease in structuring the database. Similarly, the data regarding the present state of the 3D model could be represented in time-series.