Straight Talk By Laser 2/4/2002
Add bank-shots off skyscrapers to the list of telecommunication innovations devised and field tested in New York City in the wake of the Sept. 11 terrorist attacks.
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| (Photo by Tom Sawyer for ENR) |
When a long-time client of consulting engineer Cosentini Associates was forced out of its office near the World Trade Center, the firm was given a week to establish temporary voice and high-speed data links for 1,000 employees scattered about in seven buildings in Midtown Manhattan.
Help from the phone company was out of the question, so Cosentini turned to free-space optics—lasers—to carry both telephone and data at 100 megabits per second.
"There's a lot of issues with FSO," says Doug Smith, project manager in Cosentini's Information Technologies division. Lasers need line-of-sight and would normally use roof-top, all-weather installations. But there wasn't time, "so we shot through the windows," says Smith. The lasers can shoot through anything you can see though, but require very stable mounts and pinpoint accuracy, he says.
White-out fog disrupts lasers, so radio transmitters with directional antennas also were installed at each location. These have relatively slow transmission rates, but the antennas can be tightly focused, enabling three to aggregate for each shot, which brings the speed up to 30 mbp.
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| LASER TALK Smith diagrams links.(Photo by Tom Saywer for ENR) |
The network core was in a building at 53rd St. and Lexington Ave. The lasers, which look like surveillance cameras, shot out of the 38th floor, targeting other lasers in buildings in three directions. The hardest shots were at sharp angles, like one down to the third floor of a building close by, and one to a building that could not be seen and required a bank-shot through another pair of lasers in a building five blocks away.
Establishing the radio links was easy, but aiming the lasers required a mix of high and low tech. To spot the right window across town, installers resorted to car headlights and battery packs, flashing on and off at night to spot each other. The network has been disassembled, now that the client is back in its former home.
Click here to view: FSO / RF Communication Links
