Sequential (Waterfall) Model – Advantages and Problems | Software Engineering

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waterfall modelIntroduction, advantages and problems of sequential model,
Software Engineering Notes | Sixth Semester,
BSc.CSIT | Tribhuvan University (TU)

Sequential (Waterfall) model:
The first published model of the software development process was Waterfall model. It was proposed by Royce in 1970s. Because of the cascade from one phase to another, this model is known as waterfall/sequential model. This model takes the fundamental process activities of specification, development, validation and evolution and represents them as separate phases such as requirements specification, software design, implementation, testing and maintenance.figure of waterfall model

Waterfall model phases are:

  • Requirements analysis and definition: The system’s service, constraints and goals are established by consultation with system users. They are often defined in detail and serve as a system specification.
  • System and software design: The system design process partitions the requirements to either hardware or software systems. It establishes overall system architecture. Software design involves identifying and describing the fundamental software system abstractions and their relationship
  • Implementation and unit testing: During this stage, the software design is realized as asset of programs and program units. Unit testing involves verifying that each unit meets its specification.
  • Integration and system testing: The individual program units or programs are integrated and tested as a complete system to ensure that the software requirements have been met. After testing, the software system is delivered to the customer.
  • Operation and maintenance: This is the longest life-cycle phase. The system is installed and put into practical use. Maintenance involves correcting errors which were not discovered earlier stages of the life-cycle, improving the implementation of system units and enhancing the system’s services as new requirements are discovered.


  • Easy to understand even by non-technical person, i.e customers.
  • Each phase has well defined inputs and outputs.
  • Easy to use as software development proceeds,
  • Each stage has well defined deliverables.
  • Helps the project manager in proper planning of the project.

Waterfall model problems

  • The drawback of the waterfall model is the difficulty of accommodating change after the process is underway because of sequential nature
  • Inflexible partitioning of the project into distinct stages
  • This makes it difficult to respond to changing customer requirements
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