On September 11 traditional telephone providers did a heroic job of struggling to restore service. When the World Trade Center towers fell, they severely damaged a Verizon central office with 350,000 voice lines and 3.5 million data circuits carrying the financial information that is the lifeblood of Wall Street firms. Verizon employees and those of many other telecommunications carriers worked night and day, alongside the firemen, the police, and volunteers, at their own recovery job. In about a week they had rerouted some two million data circuits, restored switches, and installed temporary power supplies. The other 1.5 million circuits originated in buildings that no longer exist.
In the days after the attack the number of voice calls in the five boroughs of New York City doubled, from the normal 115 million a day to more than 230 million. For the next six days Verizon waived charges for its pay phones in Manhattan. On a single day following the disaster residents placed some 22,000 local calls free of charge from regular sidewalk pay phones below Canal Street, and Williams Communications switched five million voice calls in the metropolitan area—three times the average daily volume. AT&T's long-distance volume jumped from a weekday average of about 300 million domestic voice calls to more than 431 million on September 11, the busiest weekday ever across AT&T's domestic voice network.
But despite the efforts to keep them in operation, under the extraordinary pressure of September 11 the traditional voice-telecommunications systems in the New York area and the Washington, D.C., area—both wire and wireless—were significantly overtaxed. In East Coast cities cell-phone networks could not keep up with demand. Many long-distance calls inbound to New York City were blocked, in part to reserve circuits for outgoing calls. On that day the Internet proved its value as an essential part of the modern communications system.
More than half of America now uses the Internet. Globally, users number more than 300 million. Virtually all large businesses use the public Internet or private versions of the same technology to conduct their most important activities. So it was not surprising—although it was staggering—to see that on September 11 more than 1.2 billion instant messages were sent by AOL users alone. Slipping past the congested voice networks onto the PC screens of friends and family around the globe were the ties that bind us in the modern world: "R U OK?" "ALRIGHT?" "U THERE?"
As voice networks blocked incoming calls to New York in order to relieve congestion, some carriers pushed their voice traffic over the Internet. ITXC, which specializes in Internet voice services, saw its domestic wholesale business double on September 11 as carriers searched for new channels of communication; Yahoo's PC to Phone calling service increased by 59 percent. The performance of these voice-over-IP services suggests that in only a handful of years most voice traffic is likely to be carried on the Internet.
Why did the Internet work so well in the face of huge volume? Because its "distributed" technology is inherently robust. "Normal" phone connections, whether by means of wired line networks or by wireless cellular networks, open a specific circuit, or channel, connecting the person who is called and the caller. Just as if a superhighway lane were opened for one car only, the circuit remains dedicated to the conversation even if no one is speaking at the moment. If too many circuits are requested at one time, the system blocks calls.
In contrast, Internet messages don't travel on designated circuits. Instead the messages are coded in 1s and 0s, and then disassembled into packets of data. The packets go out from the PC down the phone line and into the maze of interconnected fibers that envelops every metropolitan area of every developed country in the world. Like cars on a superhighway, packets share lanes on the Net.
Each packet contains a destination address. As the packet moves into the maze, it encounters a router that selects the next step in the network. If the router senses congestion on one route, it selects another. The AOL instant-message packets could work their way around the jams and outages of the voice network and find their destinations in seconds.
One lesson from September 11 is that in order to maintain an effective communications system in the face of any calamity, we should promote and protect the Internet as a primary network, encouraging the private sector and using the resources of the public sector to make it faster, more robust, ubiquitous, and better integrated with other media. This policy would be consistent with the Internet's original development as an aspect of national security.