Functional paleontology: system evolution as the user sees it

It has long been accepted that requirements analysis should precede architectural design and implementation, but in software evolution and reverse engineering this concern with black-box analysis of function has necessarily been de-emphasized in favor of code-based analysis and designer-oriented interpretation. We redress this balance by describing 'functional paleontology': an approach to analyzing the evolution of user-visible features or services independent of architecture and design intent. We classify the benefits and burdens of interpersonal communication services into core and peripheral categories and investigate the telephony services available to domestic subscribers over a fifty-year period. We report that services were introduced in discrete bursts, each of which emphasized different benefits and burdens. We discuss the general patterns of functional evolution that this "fossil record" illustrates and conclude by discussing their implications for forward engineering of software products.