3.4.3 Architectural Design Diagrams It is a common practice to construct application models and system models during system design. These models help the team understand and analyze the application domain, business processes, and workflows to identify problems, and develop and evaluate design solutions. Various diagrams are used to depict different aspects of the application and the system. Block diagrams, Unified Modeling Language (UML widely used during system modeling and design and its extension, System Modeling Language (SysML), and data flow diagrams are Block diagrams use rectangles to represent subsystems and components and directed edges to denote workflows between the subsystems and components. As an example, Figure 3.5 shows a high-level block diagram for the ABHS. The block diagram indicates that an ABHS consists of terminal subsystems, connected to a high- track subsystem. Passengers check in luggage with the terminal subsystem. The high-speed track subsystem transports luggage between the terminals. Both of these s interact with the ABHS control software, which interacts with the flight information system. Figure 3.6 shows a refinement of the terminal subsystem. In the subsystem FIG figure, hardware, software, and human subsystems are displayed. UML was initially created for modeling software applications and systems. Its generality makes it a useful tool for system modeling as well. This is achieved by using the stereotype mechanism provided by UML. For example, Figure 3.7 shows a component diagram that models the hardware and software of a radio communication system (RCS). It uses the stereotype"subsystem> >" to extend the component modeling construct to model a subsystem. The diagram shows that an RCS consists of three subsystems: a base station subsystem, an account management subsystem, and a mobile units subsystem. The working of an RCS is similar to a cellular network except that it has only one base station with high-power transceivers. The high- power transceivers can service a much larger area than a single base station of a cellular network. Through the relay of the high-power transceivers, mobile units can Airport Baggage Handling System (ABHS) High-Speed Tracks Terminal Luggages, Subsystem Luggages Luggages Passen ger ticket, ID Subsystem ev Data th ai ABHS Control Instructions Software Instructions Data Luggage & flight data Flight Information System he FIGURE 3.5 Block diagram for an airport baggage handling system