Developed by Massachusetts Institute of Technology (MIT) engineers and scientists, SAGE monitored North American skies for possible attack by manned aircraft and missiles for 25 years. The heart of the system, the AN/FSQ-7 computer, was the first computer to have an internal memory composed of "magnetic cores," thousands of tiny ferrite rings that served as reversible electromagnets. SAGE also introduced computer-driven displays, online terminals, time sharing, high-reliability computation, digital signal processing, digital transmission over telephone lines, digital track-while-scan, digital simulation, computer networking, and duplex computing.