Systems Analysis & Design: Topic 9
Entering the Design Phase
Systems design -- the process of determining the components
and the interconnection among those components in the best possible
way in order to solve a problem (i.e., the problem that was explained
in the requirements specification.)
- No concrete demarcation between analysis and design.
- Some view design as beginning immediately after the current
logical system has been defined and the domain of change identified.
- Others end analysis with the future logical system DFD but
before the logical specifications.
- During the design phase of the system development life cycle,
the analyst uses the requirements gathered during the analysis
phase to create the specifications for the proposed system.
- The design phase is characterized by the analyst's focus on
how things must be done, rather than on what must
|Phase Name: ||DESIGN
|Major Function:||To design a new system that fulfills the requirements of the users and management.
|Input||The objectives, functions, and boundaries as determined in systems analysis.
|Output||The design specification.
|Principal Tools||data dictionary
|data flow diagrams|
|input and output design forms
|Personnel and Tasks:||
- evaluates and orders all necessary hardware and software.
- transforms the functional diagrams of the analysis phase into the hierarchical diagrams of the design phase.
- designs the user interface, including input and output formats.
- incorporates system security measures into the design.
- determines staffing requirements and design procedures workflow.
- designs the required database.
|Users, managers, and team members:||
- review the design specifications for accuracy and completeness.
APPROACHES TO SYSTEMS DESIGN
Systems have two major parts which designers must cope with: functions
Functions manipulate data in order to produce the desired results.
Process-oriented Design Approach
- focuses on functions or processes
- data is dependent on the needs of the functions
Data-oriented Design Approach
- focuses on data
- functions are necessary to manipulate data
- Much current systems development is closely associated with
structured programming, which is a process-oriented approach,
so the modularity associated with structured techniques is often
a part of traditional SA/D techniques.
- Most common applications are implemented with a main routine,
which calls subroutines, which in turn call lower-level subroutines,
and so on. This hierarchical arrangement is common in most business
While data flow diagrams are a useful analysis tool, they are
a network-style model and not sufficient for the hierarchical
- The analyst must translate the data flow diagrams into structure
charts, which accurately represent the supervisor-to-subordinate
relationships of the routines.
- Structure charts will be discussed in a future section.
USER INTERFACE DESIGN
- While the analyst creates structure charts, he or she must
also be designing the formats of all system inputs and outputs
such as screens, reports, and source documents, i.e., the user
- Since the user interface is the only part of the system with
which most users come into contact, its design is critical if
the system is to please the users.
- An interface that is incomprehensible or awkward will convince
the user that the entire system is incomprehensible or awkward,
so the analyst must take extreme care to ensure that the interface
is clear, self-explanatory, and efficient.
- To create a display screen, an analyst might manually fill
out a screen layout form to show the placement of fields, or she
or he might instead prototype the screen using a fourth-generation
language or CASE product.
- When designing a report, he or she would fill out a printer
spacing chart or, again, create a prototype.
- The analyst also creates the source documents (input forms)
that are to be filled out by a user and later entered into the
system by the data entry staff.
- The user interface will be discussed in more depth later.
FAULT TOLERANCE AND SECURITY
- Computer systems are vulnerable in several ways.
- Computer equipment is subject to failure.
- Software defects can wreak havoc on an organization's database.
- Distributed systems are vulnerable to misuse, including theft
of financial assets, information, and computer time.
- Systems must be designed on the assumption that some type
of fault will inevitably be encountered in the future.
- When a fault occurs it must be possible to restart the system
and to recover all data.
- Efficient restart and recovery will be possible only if failures
have been anticipated.
DATABASE DESIGN AND OTHER ACTIVITIES
- The analyst must select an appropriate method of database
implementation, as discussed in both CIS-480 and in a future
- The analyst must devise procedures and workflow for the personnel
who will be using the system.
- The analyst must determine staffing requirements.
- Together with users, managers, and other team members, the
analyst must review the design specifications for accuracy and
- The analyst must oversee the project, making sure that it
is on time and within budget.
THE DESIGN SPECIFICATION
- The design specification is the primary document of the design
phase; it should contain everything needed by the programmers
in order to program the system.
- The design specification is both the documentation of the
finished system design and the tool used to create the design.
- This means that the documentation is not created after the
fact, but instead is a result of the design itself--when the design
is complete, so is the documentation.
- Although the design specification contains the updated data
dictionary and modified versions of the process descriptions,
the primary design and documentation tool is the structure chart.
- Analysts must incorporate a hierarchical view into the system
design before turning everything over to programmers.
Contents of the Design Specification
- Table of contents
- Structure charts
- Process descriptions
- Screen, report, and source document design
- Detailed hardware and software specifications
- Security requirements
- Programmer's handbook
- Guide to using the design specification