Logical And Phycal Design

Systems design goes though two phases of development: logical and physical design. As we saw in Chapter G, a data flow diagram shows the' flow of a system and defines the boundaries of the system. For a candidate system it describes the inputs (source), outputs (destination), data (data stores), and procedures (data flows)-all in a format that meets user's requirements. When analysts prepare the logical system design, they specify the use" needs at a level of detail that· virtually determine information How into and out of the system and the required data sources. The design covers the following:

   1. Reviews the current physical system-its data flows, file content volumes, frequencies, etc.

   2. Prepares output specifications-that is, determines the format, content and frequency of reports, including terminal specifications and locations.

   3. Prepares input specifications-format, content, and most of the input functions. This includes determining the flow of the document from the input data source to the actual input location.

   4. Prepares edit, security, and control specifications. This includes spec­ if) ring the rules for edit correction, backup processing, and the controls that ensure processing and file integrity.

   5. Specifies the implementation plan.

   6. Prepares a logical design walkthrough of the information flow, output, input, controls, and implementation plan.

  7. Reviews benefits, costs, target dates, and system constraints.

      As an illustration, when a safe deposit tracking system is designed, system specifications include weekly reports, a definition of boxes rented d boxes vacant, and a summary of the activities of the week-boxes -ed, boxes drilled, and so on. The logical design also specifies output, at, file, and screen layouts. In contrast, procedure specifications show \ data are entered, how files are accessed, and how reports are produced.

Following logical "design is physical design. This produces the working system by defining the design specifications that tell programmers exactly the candidate system must do. In turn, the programmer writes the necessary programs or modifies the software package that accepts input from the user, performs the necessary calculations through the existing or data base, produces the report on a hard copy or displays it on a sc and maintains an updated data base at all times. Specifically, physical system design consists of the following steps:

1. Design the physical system.

   a. Specify input/output media.

   b. Design the database and specify backup procedures.

   c. Design physical information flow through the system and a Physical design walkthrough.

2. Plan system implementation.

   a. Prepare a conversion schedule and a target date.

   b. Determine training procedure, courses, and timetable.       

   c. Devise a test and implementations plan and specify any new hard\software.

   d. Update benefits, costs, conversion date, and system constraints (legal financial, hardware, etc,)

The physical design for our safe deposit illustration is a software age written in Pascal (a programming language). It consists of program s that accept new box rental information; change the number of boxes a able with every new box rental; print a report by box type, box size, and location; and store the information in the data base for reference. Analyst instructs the software programmer to have the package dips. Menu that specifies for the user how to enter a new box rental, produce report, or display various information on the screen. These and 0 procedure specifications are tested and implemented as a working model the candidate system.