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Architecture Definition


When using SDL to design a system the architecture of the system is defined by the block diagrams. They define how the system is decomposed into blocks and how these blocks either form the leaves of the block hierarchy or are further decomposed into smaller blocks. Essentially this block structure is a formalization of the logical architecture from the system analysis.

As an example, consider once again the access control system. The system controls the doors of a building to unlock the doors when an authorized user wants to enter or exit the building. The task in system design is to define how to structure this system. One natural choice is a distributed structure where the control of each door is localized close to the door and a central controller keeps all common information about authorized users, cards and codes. Furthermore one special block is responsible for the handling of an operator panel. A logical architecture that described this was illustrated in Figure 650 on page 3875 in chapter 72, System Analysis. The SDL diagram that shows a beginning of a formalization of this architecture is depicted in Figure 656.

Figure 656 : The architecture of the Access Control system defined
by an (incomplete) SDL system diagram

In this diagram we can see the blocks CentralC, Door and OperatorC that are instances of the block types CentralCtrl, DoorCtrl and OperatorCtrl. CentralC contains the common information base about cards, codes etc. that are registered in the system. Door is the block responsible for the control of each door and OperatorC handles the operator communication. The diagram also shows how the blocks communicate using the channels DoorCh and OperatorCh. Note that there are five doors in the building in this case and that this is shown by defining Door to be a block instance set. The types DoorCtrl, CentralCtrl and OperatorCtrl are assumed to be defined in the packages BasicOperation and OperatorMan that are referenced in the USE clause in the top of the diagram.


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