By definition, a Work Breakdown Structure (WBS) chart is a hierarchical breakdown of a project into successive levels. Each level contain more detail than the previous.
Basically, A WBS is a list of tasks that project members have to do and when they are completed, the software product will also be done. There are many ways the project work can be broken down into smaller tasks. You can broken down by function (Identify how many functions in the project then decompose each function into smaller tasks) or by phases (requirements tasks, design tasks, programming tasks, testing tasks, support tasks (SCM, SQA tasks, etc.).
You will continue to decompose these tasks into smaller tasks until you get into a task that a single person can complete it within a week. At this level, you can add all the tasks together and come up with the number of resource (man-week) for the project.
Reference
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