# Toward a Practical Digital Twin Platform Tailored to the Requirements of Industrial Energy Systems
## 2 State of the Art
### Digital Twin Architectures
#### 2.5.2 Standardization Aspects  
We see that the available reference architectures for DTs and related concepts such as CPSs use either structured elements (e.g., building blocks, components, dimensions) or some kind of layers (also interfaces) to structure functionality and reduce complexity for DT implementation. These basic DT parts often have the same function even though they have different names, which impedes direct comparison. In an attempt to bring order to this topic, several standardization efforts have been made.  
The Asset Administration Shell (AAS) was introduced in the context of the Industry 4.0 initiative as a standardized digital representation of an asset [86, 87]. The AAS is used to uniquely identify assets, describe their functionality over the whole lifecycle and enable the communication among assets within a single factory and between companies. The AAS is still being developed, and the integration of models into the AAS is due to be added soon [88]. However, it is not in the scope of the AAS to provide simulations [20], hence missing basic requirements for a DT. For this reason, the AAS can rather be seen as the first step to a standardized DT solution providing the basic DT functionalities of resource description, discovery and access.  
The ongoing standardization initiatives by the International Organization for Standardization (ISO) under the code ISO 23247 and title \"Automation systems and integration--Digital twin framework for manufacturing\" are also noteworthy as are the DT implementations that are built on these [89] as well as other standards relating to the scope of DTs, such as ISO 22989 and ISO 10303. However, both the transferability of such standards to the energy sector and ultimately the acceptance of the norms are not foreseeable yet.  
In an attempt to establish a common view and terminology, the Reference Architecture Model Industry 4.0 (RAMI 4.0) was introduced in the context of Industry 4.0 [90]. RAMI 4.0 is a guidance framework for implementing Industry 4.0 aspects and developing a common understanding of standards, tasks and use cases. Thus, RAMI 4.0 provides a very useful system for localizing the parts of a DT.  
To connect RAMI 4.0 with DT concepts, the Generic Digital Twin Architecture (GDTA) was proposed by [PERSON] et al. [63]. In it, the 5D-DT concept [72] was used as a basic conceptual model and aligned with the six interoperability layers of RAMI 4.0 (Business, Functional, Information, Communication, Integration and Asset Level [63]) in an attempt to achieve a consistent naming and understanding of the layers. The GDTA targets the \"instance-phase\" of the RAMI 4.0 lifecycle, i.e., the operational phase as opposed to the \"type\" or engineering phase. A DT can be located at various hierarchy levels within RAMI 4.0, depending on the physical entity for which it is designed for, potentially covering all levels [63]. Defined data models within the AAS can be semantically lifted to a knowledge representation based on Resource Description Framework (RDF) [91], enabling the representation of the AAS inside the shared knowledge base of a EDTA-based DT.