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This paper describes the software and the software structure used in a family of high-capacity digital telephone switches. The call processing task is complex, and has great diversity. It must support existing and future features, including some not yet envisioned. The software must also provide man-machine interfaces, maintain the office database, and offer automatic recovery from hardware and software errors. It must also meet critical real-time performance specifications. The techniques adopted to meet these requirements include the use of a strongly typed high-level language, independent design and compilation of software modules, and the provision of an automated program library system and problem reporting system. Considerable experience in the development and support of large software systems has been accumulated, and is described in the paper.