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How Do You Fix an Undersea Cable?Electrical repairs on the ocean floor.

By Lindsay Goldwert
Posted Tuesday, Feb. 5, 2008, at 11:57 AM ET

Four undersea telecom cables have broken in the past week, cutting off or severely slowing Internet traffic in countries along the Mediterranean and the Persian Gulf. In an "Explainer" column first published last year and reprinted below, Lindsay Goldwert looked into how exactly they fix cables thousands of miles long and located thousands of feet under the waves.

Powerful earthquakes near Taiwan shut off international phone and Internet service for the Far East just after Christmas. Five repair ships have been working on the damaged undersea cables since late last week. How do you repair a cable that's lying across the ocean floor?

Drag it to the surface. Earthquakes—like ships' anchors and fishing trawls—can cause undersea fiber-optic cables to malfunction or break many miles below the surface of the water. When this happens, a telecom operator has to find the location of the accident, hoist up the damaged part, and replace it with a new stretch of cable.

First, the telecom operator has to locate the part of the cable that's no longer working. Cable engineers can figure out the general neighborhood of the problem based on the reported phone or Internet service outages. From terminal stations on shore, they can zero in on more specific coordinates by sending light pulses along the fibers in the cable. A working fiber will transmit those pulses all the way across the ocean, but a broken one will bounce it back from the site of the damage. By measuring the time it takes for the reflections to come back, the engineers can figure out where along the cable they have a problem.

Once they know that, the company can send out a large cable ship with a few miles of fresh fiber-optic lines on board to make the repairs. If the faulty part of the cable is less than about 6,500 feet down, the crew will send out a submersible tanklike robot that can move around on the sea floor. A signal can be sent through the cable to guide the robot toward the problem spot. When the robot finds the right place, it grabs ahold of the cable, cuts out the nonworking section, and pulls the loose ends back up to the ship.

The robot doesn't work in very deep water (with very high pressure). In those situations, the technicians aboard the cable ship use a grapnel, or a hook on a very long wire, to snatch up the cable from the sea floor. The grapnel uses a mechanical cutting and gripping device that can split the cable on both sides of the break and drag the loose ends to the surface. One end is hooked onto a buoy so it won't sink, and the other is hauled on board.

The malfunctioning cable section can be fixed onboard the ship. A skilled technician or "jointer" splices the glass fibers and uses powerful adhesives to attach the new section of cable to each cut end of the original—a process that can take up to 16 hours. The repaired cable is then lowered back to the seabed on ropes.

Got a question about today's news? Ask the Explainer.

Explainer thanks Bill Burns of Atlantic-Cable.com, Victoria Dillon of Tyco Electronics, Paul Kravis of International Telecom Inc., and Anne Smith of Global Marine Systems.

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Lindsay Goldwert is a writer based in New York City.
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