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The Semi-Automatic Ground Enviroment (SAGE) system. Required by DoD. Built in 1957 by IBM. Each Direction Center a computer the size of a 4-story bunker building. 700 m2, 2 CPUs in parallel, 60k vacuum tubes, 175k diodes, 13k transistors, 256KB RAM. 250 tons. 3 Megawatts!
Capable of executing 75k instructions per second. Interconnected by a nation-wide network of bleeding-edge 1,300-baud modems. By far the world's largest computer. There were more than 20 Direction Centers in operation. 100 billions dollars!!! x3 Manhattan Project. But why?
SAGE was the pinnacle of the United States Cold War radar and missile air defenses. The reaction time for a surprise nuclear attack by Soviet Union with supersonic bombers should be less than 15 minutes.
Hundreds radars and sensors throughout the territory produce an enormous amount of continuous data. Initially this info was processed by on-site intelligence personnel inside each radar. But this allowed a slow and limited view. To scale up, everything had to be rethought.
A REAL-TIME GLOBAL VISION The US DoD determined that it was necessary to process and unify all the vast info. But national security could not depend on a single central datacenter. Moreover, such computing capacity was unimaginable. So...
All sensors were connected by telephone cable to a Direction Center. And each Direction Center was connected by a NORAD Combat Center from where air defense decisions were made. The best of the tech of the time. An almost global vision, divided into 7 independent sectors.
SAGE was the world's most powerful computer for a decade. Thanks to the SAGE system it became clear that the next level radar system required a fully flexible packet routing, to achieve a complete global vision. The next level was ARPANET. SAGE --> ARPANET --> INTERNET
Yes, everything was a radar system!! Unified but decentralized. Which is a huge engineering challenge even today, requiring network computing, packet switching, real-time processing, sophisticated user interfaces... Necessary to defend against supersonic nuclear attacks.
This is the first INTERNET use case!
ARPANET packet-switched network became necessary because SAGE Direction Centers —despite duplicate CPUs— were a Single-Point-of-Failure for an entire airspace region. And to meet this challenge meant creating a decentralized network so resilient that control was lost forever.