23.3 Example of AFD and AID Mapping
The allocation of system requirements into an architecture must take into account all the considerations discussed in Chapter 22. As an illustration, we allocate to architecture modules the enhanced DFD and CFD for the Automobile Management System, developed in the previous chapter. We also create the AFDs and AIDs for this allocation. Seven modules are needed: one each for the shaft, gear, engine, brake interface, user interface, throttle interface, and the automobile management computer. The AFD in Figure 23.3 is based on these modules, and on the flows from the enhanced requirements model.
Figure 23.3. AFD 0: Automobile Management System.
The next step is to build the AID. To do this, we will need to select the channels by which information flows between the modules. Usually, we could just as well build the AID first, but in this case, the architecture modules were already available for building the AFD, and so we chose to build the AFD first and the AID second. Selecting the channels is one of the two major decisions being made as the AFD and AID are created, the other being the allocation of functions to architecture modules. The channel decisions for the Automobile Management System are documented in Figure 23.4.
Figure 23.4. AID 0: Automobile Management System.
The combination of the processing required in the enhanced model, and the architecture modules chosen, may result in some modules being unable to fulfill the requirements for system reliability or safety. A solution to this problem might be to duplicate some of the modules to obtain additional reliability through redundancy. In this sense, the enhanced requirements model only provides a partial view of the system’s technology dependence.
The introduction of redundant architecture modules may require some processing in the maintenance or self-test buffer of the architecture template to arbitrate the selection of the redundant processes. In our example of the Automobile Management System, we might allocate this processing to the automobile management computer. Now we can build architecture module specifications, architecture interconnect specifications, and the architecture dictionary for the allocation shown in the AFD and AID. The architecture module specifications will document the allocation of processes, and may contain a top-level functional description of the architecture modules. The AMS for the Automobile Management System is shown as a traceability matrix in Figure 23.5, the AIS in Figure 23.6, and the enhancements to the dictionary in Figure 23.7.
Figure 23.5. Architecture module specification for the Automobile Management System.
Figure 23.6. Architecture interconnect specification for the Automobile Management System.
Figure 23.7. Architecture dictionary for the Automobile Management System.