Essential Steps Of System Architecting

Two earlier representations, Figures 1.1 and 2.2, have shown the development of an architecture as the first top-down design or synthesis of the system in question. It is an attempt to come to terms with the critical design choices for the system as a whole. Thus, for this author, the essence of architecting is defining these top-level design choices and placing them in a context that establishes a set of reasonable alternatives that can then be evaluated. Following this notion, consider the structure of Exhibit 9.2 [9.9].

Exhibit 9.2: System Functions and Design Choices for a

Communications System

Top-Level System Functions Design Choices (Alternatives)

1. Multiplexing/demultiplexing D11, D12

2. Modulation/demodulation D21, D22

3. Switching and routing D31, D32, D33

4. Encryption/decryption D41, D42

Top-Level System Functions

Design Choices (Alternatives)

5. Formatting/signal conversion

6. Control and monitoring

7. Recording and playback

8. Satellite/terrestrial communications

This table shows a set of eight top-level functions of a communications system. For each of these functions, the system architect considers one or more design approaches that satisfy the requirements, as stated for the function. Thus, for the first function, the two design approaches are represented as D11 and D12, where D11 is the first design approach for function 1 and D12 is the second design approach for function 1. To generalize, DIJis the Jth design approach for function I. Since we are operating at the top-level functional breakdown, these approaches are considered fundamental to the overall system design and represent true alternatives from which we will eventually construct a preferred architecture. As an example, for the mux/demux function number 1, D11 might be frequency division multiplexing whereas D12 could be time division multiplexing.

From the above discussion, we are able to immmediately see the combinatorial nature of the architectural design problem. If all of the design combinations were admissable, then in principle there could be as many as (2)(2)(3)(2)(3)(2)(2)(2) = 576 combinations, each of which represents a single architectural choice for the system. When we introduce the fact that not all combinations are internally compatible or interoperable, the number of combinations (admissable alternatives) narrows, and usually in a rather dramatic way. On this basis, two critical tasks of the architect are to (a) set forth the various design approaches, and (b) look for ways to reduce the number of alternatives (combinations) to be considered. Given that the architect is able to do part (a) of the above, the approach delineated here reduces the number of alternatives by placing them in a cost-effectiveness context, specifically the one shown in Figure 7.6. From that perspective, a practical number of alternatives is constructed and then evaluated by the systems engineering team, led by the chief systems engineer (CSE).

If we return to the thirty elements of systems engineering, as defined in Chapter 7 (Exhibit 7.1), we can describe the essential steps of system architecting by means of elements 3 through 10, namely:

• Requirements analysis/allocation (element 3)

• Functional analysis/decomposition (element 4)

• Architecture design/synthesis (element 5)

• Alternatives analysis/evaluation (element 6)

• Technical performance measurement (element 7)

• Life-cycle costing (element 8)

• Concurrent engineering (element 10)

This ''short list'' does not mean that some of the other thirty elements of systems engineering are completely neglected during the process of archi-tecting; rather, it suggests that these above eight elements are critical and must be part of the process. Each of these essential steps is briefly described in the following sections.

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