The author describes a novel software estimation modeling process as well as the important productivity factors and the productivity measurement metrics used in the 5ESS project, one of the largest telecommunication projects at AT&T Bell Laboratories. The 5ESS switch is a modern digital electronic switching system with a distributed hardware and software architecture. The model estimation approach has greatly improved the quality of estimates. The study of productivity factors has resulted in some significant productivity and quality improvement processes in the 5ESS development community. >