Primer: Free Space Optics

 
 
By David F. Carr  |  Posted 2005-03-07
 
 
 

What is it? Free space optics technology is wireless networking that uses light beams instead of radio waves; it's laser-based optical networking without the fiber optic cable. In corporate networks, the most common application is wireless campus networking—typically, a rooftop-to-rooftop connection between buildings. Or the laser beam might be shot out one building's window into the window of a building across the street.

What are the advantages? Wavelengths for these transmissions do not require a Federal Communications Commission license. While this is true of some wireless networking schemes based on radio-frequency transmissions, free space optics is immune to the radio interference that can sometimes sabotage those systems. Jeff Orr, a senior product marketing manager at Proxim, says radio-frequency products have erased the bandwidth advantage that free space optics once enjoyed. He concedes that his high-end products cost about twice as much as a free space optics unit with the same capacity (about $75,000 for a gigabit radio link versus $40,000 for an FSO alternative).

What are the disadvantages? Transmissions fade rapidly in certain kinds of weather—fog, in particular.

Isaac Kim, director of optical transport at MRV Communications, says fog droplets scatter the wavelengths of light used in free space optics. The power of the beam can't be increased because of concerns that the lasers might be dangerous to the eyes of anyone who happens to walk through a souped-up beam. Technical innovation has focused more on lowering costs than increasing the range of the optics equipment. To maximize availability, you can use a combination of free space optics and radio-frequency gear.

What's the real range? A conservative estimate is up to 500 meters for a free space optics-only solution, or 1 to 2 kilometers (about a mile) for free space optics with a radio-frequency backup. You can stretch this in dry areas.

Any other downsides? Atmospheric effects such as scintillation (the "waves of heat" pattern you sometimes see over dark surfaces) can have an impact on free space optics transmissions. Urban rooftop-to-rooftop setups can encounter problems because tall buildings sway slightly in the wind, throwing off the aim of the tightly focused lasers.

Who are the vendors? The most recognizable name is Canon, better known for cameras and copiers. LightPointe is a specialist in this niche, where many startups have long since come and gone. MRV Communications offers free space optics gear as an adjunct to its fiber optic and Ethernet solutions.

Who will vouch for it? Fred Murphy, associate director of information technology for Jazz at Lincoln Center, says free space optics equipment proved ideal for connecting the center's new auditorium with its administrative offices. "We were so much within that range that it's ridiculous," he says. "We're literally across a New York City street."

By shooting a laser out the window of one building into the window of another, Jazz at Lincoln Center established a network link that it owns, rather than paying the phone company for the bandwidth. Establishing a fiber optic link between the two buildings would have been prohibitively expensive because of the complication of digging up a New York street.

Paul Wolf, an engineering technology manager at CDI Business Solutions, was initially a reluctant customer of LightPointe's FSO technology when his company started using it to connect two buildings in Houston. He worried about how many complaints would be waiting for him the first time fog knocked out the link. But the setup failed-over smoothly to a redundant RF link when the light beam was interrupted, and in two years the connection has had zero downtime, he says: "Now, I don't even think about it."

Any other downsides? In addition to fog, you could run into problems with atmospheric effects such as scintillation (the "waves of heat" pattern you sometimes see over dark surfaces). Also, urban rooftop-to-rooftop setups sometimes run into problems because tall buildings sway slightly in the wind, throwing off the aim of the tightly focused lasers.