## 4 Classification and Core Components of DT Systems in Transportation Infrastructure
### Core Components of DTs in Transportation
#### 4.2.2 System Architecture  
Interoperability in the architecture of a DT should be designed to ensure smooth data exchange among different tiers and various subsystems [14]. Standard communication protocols, data formats, and application programming interfaces (APIs) enable seamless integration with external systems, which may include urban planning databases, intelligent transportation systems (ITS), and public transit networks. However, achieving a completely interoperable system is often an extremely complex effort that requires significant coordination among many stakeholders and technologies [62].  
The architecture of DTs in transportation is typically structured across several layers: data acquisition, data processing, modeling and simulation, and application. Each tier serves a distinct function within the DT, ensuring the seamless integration of physical assets with their digital counterparts.  
* Data Acquisition Tier: This tier gathers raw data from diverse sources, including sensors, IoT devices, GPS systems, and monitoring stations. For example, traffic cameras, road sensors, and weather stations continuously collect data, which are then forwarded to the data processing layer [38].
* Data Processing Tier: In this tier, raw data undergo filtering, cleaning, and conversion into analyzable formats. Sophisticated data processing methods, such as machine learning algorithms, are applied to identify patterns, eliminate noise, and pinpoint errors. Edge computing is sometimes employed to handle data locally, minimizing latency and enhancing system responsiveness [62].
* Modeling and Simulation Tier: This tier houses the core models of the DT, encompassing simulation, prediction, and optimization models. It executes simulations, analyzes scenarios, and generates predictions based on the refined data [3]. Continuous updates to the models maintain the accuracy of the DT.
* Application Tier: The final tier includes tools and applications for operators, planners, and maintenance teams. It features user interfaces, visualization tools, and control systems that enable stakeholders to interact with the DT, conduct simulations, monitor infrastructure conditions, and make informed decisions [60].  
The architecture of a DT must be interoperable. Seamless data exchange between different tiers and various subsystems can be ensured only when the basic design itself is interoperable [14]. Standardized communication protocols, data formats, and APIs integrate these with third-party systems, such as urban planning databases, ITS, and public transit networks. However, building a fully interoperable system is complex and often involves coordination at multiple levels among several stakeholders and technologies [62].  
Considering the core components of a DT, these could include data integration, simulation models, 3D visualization, and system architecture. At this point, it establishes a foundation for providing a well-functioning management and optimization platform for transportation infrastructure. A DT combines real-time data with advanced analytics, thereby offering proactive decision-making that keeps transportation systems safe, efficient, and sustainable [63].