No. 4 ESS: Evolution of Software Structure

No. 4 ESS, the largest-capacity electronic switching system ever developed by the Bell System, was developed to meet specific objectives of capacity and reliability.1 To meet these objectives, the No. 4 ESS was designed using new hardware technology and a comprehensive stored program. The primary objectives of the initial No. 4 ESS program design were: (i) real-time efficiency, (ii) simple human interface, (ill) defensive design, (iv) ease of modification. Since its initial service date, the No. 4 ESS has released a new generic software package approximately once a year incorporating major new hardware and software capabilities. Each new generic was built upon the previous generic. As the number of features provided by the No. 4 ESS grew, it became increasingly more involved in certain areas of software to accommodate new features without impacting existing features. The amount of time spent in regression testing had the potential for becoming an ever growing part of the software development interval, thus, increasing new feature development cost. New software development methodologies that used top-down de1167