Logical description and physical description of system

Logical description and physical description of system

The system model must address both the logical and physical dimentions of the proposed system.for example,suppose the system developers are considering the type of  information inputs a mortgage loan manager needs during his or her dealings with a loan client. An input specification such as "Credit report* conveys a logical description of information that must be obtained. If bank policy will accept "written, oral, and electronically generated credit reports," then we have a physical description of the input. As Figure  indicates, the logical description of the design element is usually specified first; then, the physical description.
The level of detail required in the design specification often depends on the application. In the credit report example, the design specification might include the identity of the loan applicant, the applicant's address and phone number, credit references, assets and liabilities, salary, and so forth. The system developers must be careful not to over specify or under specify the design. Under specification may result in the design's ultimate failure. The ideas in the design may be sketchy, giving people the wrong impression of the system—or they may be underdeveloped and just not workable. Over specification (excessive detail), on the other hand, may result in a costly waste of time. Also, over speciticatioacan cause intimidation or information overload when users review the project team's design work.
The system components shown on the left side of Figure  are also usually considered in the order given. For example, outputs are considered  first because nothing makes much sense until the developers know what the system is supposed to produce. The nature and quality of the outputs desired by users determines What inputs, are needed, what types, of hardware and software are re- and so on. Similarly, storage needs cannot be fully determined until output, input and processing requirements are established. If, for instance, users want an image-processing system, then appropriate scanning, optical storage, and laser printing equipment are needed. Also, personnel needs are difficult to estimate until every other requirement is specified and the develop-
»can see how mu ch work will be involved.
The numbers 1 -12 in the middle part of Figure indicate the overall order in which design elements should usually be considered. Note that all of It logical system design elements are considered first; then, the physical design elements.  
finally, you may wonder how software and hardware fit into all of this. Specific types of software and hardware are generally physical design matters ' that cut across all areas in Figure .
 An application software package, for example, will often include output, input, processing, or storage features. For in- stance, a database? management system allows users to design reports (output), prepare template s (input), and create different views of the database (processing).
It may also contain utilities that allow computer professionals to place data efficiently onto disks (storage). Hardware, however, is normally acquired for a single function; for instance, a printer for output, a keyboard for input, or a computer for processing. Some devices, such as display terminals, perform two more functions (such as display terminals that are used for both output and input). Procedures and personnel are required to operate the software and Hardware.