What are the principal stages of the waterfall model that reflect the fundamental software development activities?

The principal stages of the waterfall model directly reflecting the fundamental software development activities are:
1. Requirements analysis and definition: The system’s services, constraints, and
goals are established by consultation with system users. They are then defined
in detail and serve as a system specification.


2. System and software design: The systems design process allocates the requirements
to either hardware or software systems by establishing an overall system
architecture. Software design involves identifying and describing the fundamental
software system abstractions and their relationships.


3. Implementation and unit testing: During this stage, the software design is realized
as a set of programs or program units. Unit testing involves verifying that
each unit meets its specification.


4. 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.


5. Operation and maintenance: Normally (although not necessarily), this is the
longest life cycle phase. The system is installed and put into practical use.
Maintenance involves correcting errors which were not discovered in earlier
stages of the life cycle, improving the implementation of system units and
enhancing the system’s services as new requirements are discovered.