Waterfall model is one of the most popular methodologies of Software Development Life Cycle (SDLC). It consists of a sequence of definite phases that starts once the previous phase gets completed and flow down steadily (Like Waterfall) towards the completion of the SDLC. These phases consists of - Requirement Analysis (Initiations), Requirement Specifications, Design, Implementation (Coding), Testing, Deployment, Maintenance. Each phases has an own timeline to get completed. Once the first phase is completed then only new phase gets started.


The waterfall model is a one way sequential method where there is no loop to go back. It is based on the principal that to complete each phase correctly once at the first time before moving to next phase. It hardly allows you to re-visit the previous phase. This is one of the most traditional approach of the water fall method.

To overcome this; an iterative waterfall method has been evolved which allows you to loop through the previous phase and hence allows you to re-visit the previous phase. This iterative model is one of the part of Rational Unified Process (RUP) and it is the modified version of traditional waterfall method.
The major advantages and disadvantages of waterfall model are:

Advantages
1. Waterfall method ensures each phase to be completed 100% correct with surety before
    proceeding to next phase. Thus each phase is completed with perfect understanding.

2. Since output is generated after each phase; it has high visibility over project progress.

3. In this model errors are identified at the initial stage and hence save time and money.

4. Waterfall model insist on lots of paper work and documentation. This helps the new
     resources to carry the work easily and thus aids in knowledge transfer to the next team.

5. Since each phase is done by the dedicated team it has better control over project
    deadline.

Disadvantages
1. In real project it is difficult to get the right requirement in first phase itself and hence it
    becomes a costly affair if any changes need to be done later on.

2. This model does not allow going back to previous phase nor does it allow starting more
    than one phases in parallel. In practical, real project hardly follows sequential approach.

3. This model can be time consuming if the requirement keeps on changing during
    each phases.

4. Since each phase is done by a separate team member; the chances are that the other
    team member may be sitting idle.

5. It has less flexibility. It is not suitable method if the client is not clear about the
    requirement in the initial stage or if the new requirement needs to be added frequently.