Free-space lasers shining as obstacles are overcome

By Tim Greene, Network World |  Networking

Beaming data from point to point in free space using lasers is no longer the show-stopping trick it used to be, now that metropolitan service-provider networks based on this technology are becoming reality.

Dallas-based Tellaire is delivering 10M bit/sec transparent LAN services in Houston and Austin, Texas, Washington, D.C., and New York. It's pricing varies city to city, but the company says it aims to give twice the bandwidth for the same price as a conventional landline service.

And Terabeam Internet, the cooperative venture of Lucent and Terabeam, will offer data services to customers in Seattle and five other undisclosed cities beginning later this winter. Terabeam doesn't have pricing set for its LAN-interconnect services, but says it will be cheaper compared to the prices customers pay now for landline services. The slowest offering will be 5M bit/sec, a company spokesman says.

Bringing such services to market has required overcoming some of the inherent problems with the technology. One obstacle, fog, is being challenged head-on by Terabeam, which chose Seattle -- one of the country's foggiest cities -- to premiere its services. Free-space lasers must be arrayed so nothing blocks the path of the laser beam as it passes from transmitter to receiver. A heavy fog comprised of tiny water droplets that diffuse the light can weaken signals to the point that a connection is lost.

But makers of these laser systems can overcome this problem to some degree by boosting the power of the laser so it cuts through the fog. In addition, service providers can engineer their networks so the individual links between sites are short enough so even the most dense fog will not block the beam. AirFiber, which makes the OptiMesh Rooftop System of free-space laser devices used by Tellaire, says its gear reliably transmits at 622M bit/sec at 200 meters. Terabeam says it can deliver 1G bit/sec throughput.

But other factors could block a laser's path, from a hovering helicopter to a billboard placed on an intervening rooftop. Equipment failure can also bring down a link, so designers of free-space laser networks have had to find a way to build in redundancy.

That is being done by building meshed networks that use recovery schemes inherent in standard routing and switching. AirFiber, for example, uses ATM switching behind its laser equipment. That lets AirFiber networks switch around failed links so sites with redundant links don't lose service.

The laser equipment is distributed in a network that looks much the same as a traditional dedicated circuit network. Each site ideally is connected by more than one beam, each coming from a different direction.

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