Software Engineering-System Engineering Heirarchy

Regardless of its domain of focus, system engineering encompasses a collection of top-down and bottom-up methods to navigate the hierarchy illustrated in figure below.

The system engineering process usually begins with a “world view.” That is, the entire business or product domain is examined to ensure that the proper business or technology context can be established. The world view is refined to focus more fully on specific domain of interest. Within a specific domain, the need for targeted system elements (e.g., data, software, hardware, people) is analyzed. Finally, the analysis,design, and construction of a targeted system element is initiated. At the top of the hierarchy, a very broad context is established and, at the bottom, detailed technical activities, performed by the relevant engineering discipline (e.g., hardware or software engineering), are conducted.

Stated in a slightly more formal manner, the world view (WV) is composed of a set of domains (Di), which can each be a system or system of systems in its own right.

                     WV = {D1, D2, D3, . . . , Dn}

Each domain is composed of specific elements (Ej) each of which serves some role in accomplishing the objective and goals of the domain or component:

                     Di = {E1, E2, E3, . . . , Em}

Finally, each element is implemented by specifying the technical components (Ck) that achieve the necessary function for an element:

                    Ej = {C1, C2, C3, . . . , Ck}

In the software context, a component could be a computer program, a reusable program component, a module, a class or object, or even a programming language statement. It is important to note that the system engineer narrows the focus of work as he or she moves downward in the hierarchy just described. However, the world view portrays a clear definition of overall functionality that will enable the engineer to understand the domain, and ultimately the system or product, in the proper context.

System Modeling

System engineering is a modeling process. Whether the focus is on the world view or the detailed view, the engineer creates models that 
Define the processes that serve the needs of the view under consideration.
Represent the behavior of the processes and the assumptions on which the behavior is based.
Explicitly define both exogenous and endogenous input3 to the model.
Represent all linkages (including output) that will enable the engineer to better understand the view.

To construct a system model, the engineer should consider a number of restraining

1. Assumptions that reduce the number of possible permutations and variations, thus enabling a model to reflect the problem in a reasonable manner. As an example, consider a three-dimensional rendering product used by the entertainment industry to create realistic animation. One domain of the product enables the representation of 3D human forms. Input to this domain encompasses the ability to specify movement from a live human actor, from video, or by the creation of graphical models. The system engineer makes certain assumptions about the range of allowable human movement (e.g., legs cannot be wrapped around the torso) so that the range of inputs and processing can be limited.

2. Simplifications that enable the model to be created in a timely manner. To illustrate, consider an office products company that sells and services a broad range of copiers, faxes, and related equipment. The system engineer is modeling the needs of the service organization and is working to understand the flow of information that spawns a service order. Although a service order can be derived from many origins, the engineer categorizes only two sources: internal demand and external request. This enables a simplified partitioning of input that is required to generate the service order.

3. Limitations that help to bound the system. For example, an aircraft avionics system is being modeled for a next generation aircraft. Since the aircraft will be a two-engine design, the monitoring domain for propulsion will be modeled to accommodate a maximum of two engines and associated redundant systems.

4. Constraints that will guide the manner in which the model is created and the approach taken when the model is implemented. For example, the technology infrastructure for the three-dimensional rendering system described previously is a single G4-based processor. The computational complexity of
problems must be constrained to fit within the processing bounds imposed by the processor.

5. Preferences that indicate the preferred architecture for all data, functions, and technology. The preferred solution sometimes comes into conflict with other restraining factors. Yet, customer satisfaction is often predicated on the degree to which the preferred approach is realized.

The resultant system model (at any view) may call for a completely automated solution, a semi-automated solution, or a nonautomated approach. In fact, it is often possible to characterize models of each type that serve as alternative solutions to the problem at hand. In essence, the system engineer simply modifies the relative influence of different system elements (people, hardware, software) to derive models of each type.

System Simulation

In the late 1960s, R. M. Graham [GRA69] made a distressing comment about the way we build computer-based systems: "We build systems like the Wright brothers built airplanes— build the whole thing, push it off a cliff, let it crash, and start over again." In fact, for at least one class of system—the reactive system—we continue to do this today. 
Many computer-based systems interact with the real world in a reactive fashion.That is, real-world events are monitored by the hardware and software that form the computer-based system, and based on these events, the system imposes control on the machines, processes, and even people who cause the events to occur. Real-time and embedded systems often fall into the reactive systems category.

Unfortunately, the developers of reactive systems sometimes struggle to make them perform properly. Until recently, it has been difficult to predict the performance, efficiency, and behavior of such systems prior to building them. In a very real sense, the construction of many real-time systems was an adventure in "flying." Surprises (most of them unpleasant) were not discovered until the system was built and "pushed off a cliff." If the system crashed due to incorrect function, inappropriate behavior, or
poor performance, we picked up the pieces and started over again.

Many systems in the reactive category control machines and/or processes (e.g., commercial aircraft or petroleum refineries) that must operate with an extremely high degree of reliability. If the system fails, significant economic or human loss could occur. For this reason, the approach described by Graham is both painful and dangerous.

Today, software tools for system modeling and simulation are being used to help to eliminate surprises when reactive, computer-based systems are built. These tools are applied during the system engineering process, while the role of hardware and software, databases and people is being specified. Modeling and simulation tools enable a system engineer to "test drive" a specification of the system.
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