How the World Got Wired · Packets

Why the Phone System Was Fragile

When Paul Baran started his work in 1960, the only big communication network in the world was the telephone system. If you wanted to call your aunt in another city, you picked up your phone, told the operator who you wanted to talk to, and the operator plugged a wire into a hole on a big board that connected your phone to a wire that led to your aunt’s phone. Later, the wire-plugging was automated, but the idea was the same. A call was a circuit — an unbroken wire, all the way from you to your aunt — held open for the whole conversation.

This system worked. But it had a weakness Paul Baran could not stop thinking about.

To make the system work efficiently, the telephone company had built a small number of enormous switching centers, where most of the wires came together. Almost every long-distance phone call in America passed through one of about a dozen of these centers. They were huge buildings, full of switches and operators, and they were the heart of the network.

The trouble was, if you wanted to destroy the American phone system, you didn’t have to destroy every phone or every wire. You only had to destroy those dozen buildings. Twelve bombs and the country would go silent.

The Soviet Union knew where the buildings were. So did everyone.

Baran sketched this out on paper. He drew a picture of three different kinds of network.

The first kind he called centralized. It looked like a star — one hub in the middle, with lines going out to every other point. Destroy the hub and the whole network was dead.

The second kind he called decentralized. It looked like several stars joined together — a few hubs, each connecting to many points, with the hubs connected to each other. Destroy enough hubs and most of the network was dead. This was, roughly, the American phone system.

The third kind he called distributed. It looked like a fishing net, or the cracks in old pavement, or the veins on the back of your hand. There was no center at all. Every point was connected to several of its neighbors, and from any point you could reach any other point by many different paths. If part of the net was destroyed, messages could simply go around the hole, taking a different route. There was no critical building you could bomb.

The distributed network was the answer to the Air Force’s question. But there was a second part to Baran’s idea, and it was the part that turned out to matter the most.

All 24 chapters of How the World Got Wired · Library · All courses