Spiral model incorporates the elements of both prototype approach along with the classic software model. This model has been divided into four quadrants, each quadrant representing a major activity like planning, risk analysis, engineering, customer evaluation. The software process cycle begins with the planning activity represented by the first quadrant of this model. The second quadrant is associated with risk analysis activity. It used to evaluate different alternatives that are based on the objectives listed in the first quadrants. The third quadrant is about engineering activity, which actually involve the development of the software and uses various development strategies that resolves the uncertainties and risk. The last phase is customer evaluation phase. It involves a review of the preceding development stage. Based on the outcome of the development step the next phase is planned.

Some important characteristics of this model:

• It uses an iterative approach

• It allows the usage of prototype at any stage

• This model refines requirements & reduces risk element.

• It maintains a systematic approach

• This model is quite flexible