Life Cycle Approaches
Waterfall model
The waterfall model is a classical life cycle approach of software development (SEM V 3.0 is implicitly based on this model). Each phase contains constructive and checking activities (e.g. producing and subsequent reviewing of a software requirements specification in the Definition phase). In principle, all phases are executed sequentially in this model. Only if the results of one phase are available can the next phase be started.

Other models supported by stdSEM:

The evolutionary development model is used in PSE to execute all maintenance projects and the major version developments. Each follow-up product version is based on the preceding version. The specifications, plans and implementations produced in follow-up versions cover only those features that distinguish the new version from the old one.
While the prototyping model is based on a software requirements specification (important for contractual purposes!), the design and implementation are executed using appropriate development tools. Product development can take place in close cooperation with the customer using the resulting prototype (described in stdSEM as a separate Prototyping "phase" in parallel to the traditional Design and Implementation phases).
In the case of the incremental delivery model, development is based on a uniform software requirements specification and architectural design specification. The required overall solution is then subdivided into a series of independent subprojects, however (e.g. for deadline reasons, to complete important product parts as early as possible, or to provide support to development teams working in parallel). Each of the subprojects then results in an operational product version.
The SEM spiral model is essentially a waterfall model with comprehensive accompanying measures which is useful for very large projects (new goal definition, risk analysis, simulation, etc., after each phase cycle). This model therefore tends to be used rarely in PSE.
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