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Through the listening glass

Written by Ced Kurtz on .

When you receive an e-mail from Japan or download a file from a server in Finland, you take international communications for granted. But did you ever think what that message has to go through to get to your computer?

Most likely, it is glass.

By the year 2000, 80 percent of international communications traveled through undersea fiber optic cables. Due to low-cost, high-density glass fibers, advances in technology and the explosion of the Internet, that figure had increased 40 times since 1988.

An average undersea fiber optic cable is about 2.7 inches in diameter and weighs about 7 pounds per foot. From the inside out it might be a bundle of glass fibers in petroleum jelly inside a copper or aluminum tube. The tube is coated in polycarbonate, wrapped in an aluminum water barrier, strengthened with stranded steel wires wrapped in mylar tape, and the whole thing is sheathed by polyethylene.

As of 2003, submarine cables linked all the continents except Antarctica. The northern branch of the Gemini cable, for example, connects Charleston, R.I., with Porthcurno, England. Gemini's southern branch goes from Manasquan, N.J., to Porthcurno. The ARCOS-1 cable connects Miami with 14 countries in Central and South America and the Caribbean.

One of the busiest communications facilities in the world is a small, nondescript building owned by AT&T in the seaside town of Tuckerton, N.J. It is a hub for undersea cables coming ashore. Myriad phone calls and data flow through that humble structure from Europe, the Caribbean and South America and are distributed to customers in the United States.

It has been suggested that these communications hubs make a juicy target for terrorists. In fact in the fictional thriller "Breaking Point" by Richard A. Clarke, the former Clinton administration anti-terror czar, an attack on the United States begins with destruction of such communications nodes in an attempt to bring down the Internet.

But I digress.

Fiber optic cables can be made of plastic, glass or a combination of the two, but glass is usually preferred for long-distance communications. Glass has less attenuation, a fancy word for loss of a signal as it flows through the fiber. All fiber optic cables have much less attenuation than copper wire.

Signals inside a fiber optic cable are carried by light. The light stays in the fibers because the glass is coated with a plastic that makes the walls act like mirrors that reflect light back into the center. Over long stretches, amplifiers pick up the light pulses and "push" them along.

Signals are sent into the fiber by lasers at one end flashing on and off, producing the 1s and 0s of digital communications. The lasers can turn on and off several billion times a second.

Of course, the system isn't foolproof. Undersea cables can be broken by fishing trawlers, anchors, undersea avalanches and even shark bites.

In December three cables on the floor of the Mediterranean Sea were broken and international phone and Internet service in parts of the Mideast and southern Asia were disrupted. The villain was thought to be an anchor. A break like this one can take about 10 days to fix.

So the next time you get an e-mail from a friend in South America and it seems to get to you in a flash, that is exactly how it got there -- in a bunch of laser flashes.

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