Free-space lasers shining as obstacles are overcome


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.

Optical Access and Terabeam base their networks on IP and use standard router protocols to route around failures. Terabeam uses an off-the-shelf router made by another vendor (it would not say which one), and Optical Access bought router technology from Nbase Xyplex.

In its Austin deployment, for example, Tellaire has six buildings networked. One of them has a single laser connection, and two others each have two or three. So if there is a failure, the network reroutes based on well-known spanning-tree algorithms, says Andy Obst, Tellaire's chief operating officer.

Other factors to consider

To make sure the lasers sstay aligned with the receivers, service providers also have to consider the sway of buildings and the fact that buildings actually change shape slightly with heat and cold. That is dealt with in two ways. First, the apertures on the lasers are set so the beams spread to hit a wider area at the target site. At a distance of one kilometer, a laser can spread to two meters in width, Obst says. In its Seattle deployment, Terabeam is adjusting the lasers to fill the 16-inch wide receivers regardless of the length of the link.

The widening of the beam lets the buildings flex and sway without drawing the beam off the receiver. Terabeam has also built in proprietary tracking devices that repoint its lasers to correct for these changes.

Link length is crucial

Link length is key to reliability. Tellaire has links up to one kilometer that should afford 99.1% reliability based on a study of 50 years of National Weather Service data for 64 U.S. cities, Obst says. That is below the 99.999% availability service providers can offer. In some cases, Tellaire customers opt for a radio-frequency backup that reduces throughput dramatically, but maintains a link, he says.

AirFiber recommends using shorter links. "You can't go a kilometer and do it reliably at the same time," says Janet McVeigh, vice president of marketing for AirFiber.

Laser power is federally regulated for safety, and the lasers used in these systems are safe for property, people and flights of pigeons, vendors say. A spokesman for Terabeam says that unlike the laser pointers you can pick up at the office-supply store its customer-site gear requires no safety warning.

This technology is particularly well suited to upstart carriers, says Christopher Nicoll, vice president of consultancy Current Analysis. First, the equipment is relatively inexpensive. AirFiber says that in a metropolitan network laser links can be set up for $30,000 per building.

It is also easy to deploy, which means fast setup times, Nicoll says. AirFiber says its gear can be set up in half a day. And service providers need no rights of way into buildings; the beam travels in free space. All service providers need is a spot on the roof to place their equipment. Terabeam says in many cases it doesn't even need that because most of its hubs are located in offices, and the lasers are beamed out the window to a receiver behind a customer's window. Obst says Tellaire shoots a service through a window to a customer in the Empire State Building.

Service providers using this technology are unlikely to overtake traditional fiber metropolitan network providers, but they will fill a niche, Nicoll says. For customers who need high-bandwidth access turned on quickly to sites that have not been linked yet to fiber networks, fiberless optics my be the way to go.

This story, "Free-space lasers shining as obstacles are overcome" was originally published by Network World.

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