Should you suddenly find your entire house shaking for no good reason, even though you live thousands of miles from any place known for earthquakes, your first reaction once the rumbling stops might be to pick up the phone.
As East Coast dwellers learned after Tuesday afternoon's magnitude 5.8 quake, that's also the wrong reaction. And those of us in the Washington and New York areas should have known: While we may have zero acquaintance with seismic activity, almost 10 years ago we got a rough reminder about which communication systems fail when everybody wants to talk at once.
Both after 9/11 and during this week's vastly less eventful distraction, land and mobile phones were the first to stop working.
The phone system was never built for everybody to start dialing at the same time; each call requires a separate circuit on the line, and once those are tied up nobody else can get on. In return, "circuit switched" calling offers reliable quality–you shouldn't see your conversation fade into fuzz the way it can over a flaky Skype session.
Switching to text messages can work around overloaded voice circuits, because each text takes up far less bandwidth than a voice call. (That doesn't stop wireless carriers from charging exponentially more for text messages than voice calls.) But a flood of text messages can still swamp those services' infrastructure. Even in less exciting circumstances, researchers have found that the "store and forward" setup of texting, in which the carrier holds the message until it can locate the recipient's phone on its network, can lead to long delays (PDF).
That's not the case with the Internet. It's not true, as popular lore often has it, that the Net's architects built it to survive a nuclear war. But it was designed to route around outages and disconnections. It does this by breaking up each bit of data into small packets, each tagged with its destination, that every Internet router along the way sends along the quickest path available. No one machine needs to map out the entire route; it just needs to find the fastest hop to a closer waypoint.
So-called packet-switched communication may not let you watch a high-definition video clip online without it stalling or breaking down. But if you absolutely, positively need to send some information immediately, it's your last and best hope.
A land-based DSL, cable or fiber-optic broadband connection will work best, provided your power doesn't go out. But even smart phones can get by unless the airwaves get swamped by too many devices (for example, on the Mall in Washington during President Obama's inauguration in 2009 and during the Rally To Restore Sanity last fall).
I saw that demonstrated during 9/11, when I could not call anybody anywhere near New York but could engage in instant-messaging conversations with friends in Lower Manhattan. Much the same thing happened Tuesday.
Fortunately, this time around there was no real harm done. We could all straighten the pictures on our walls, inform the relatives that we were fine and get back to using the Internet for one of its more popular purposes: sharing jokes.