Since the founding of the Bell Telephone System, the public switched telephone network
(PSTN) has evolved into a highly successful, global telecommunications system. It is designed
specifically for voice communications and provides a high quality of service and ease of use. It is
supported by sophisticated operations systems that ensure extremely high dependability and
availability. Over the past 100 years, it has been a showcase for communications engineering and led to
groundbreaking new technologies (e.g., transistors, fiber optics).