Bridge the golden last mile.

Gain an introduction to broadband optical wireless networking.

While some consider optical wireless to be bleeding-edge technology, its origins date back to Alexander Graham Bell's photophone. Today, optical wireless systems typically use lasers in the invisible infrared (IR) range (780 to 1550 nm in wavelength) to send data between buildings. While this requires that the buildings have a direct line of sight and be within a limited range of each other, the potential applications for this technology are numerous. The bandwidths that can be achieved are truly broadband--anywhere from 10 Mbps to 1 Gbps.

The most common use of optical wireless technology is in campus LANs. Imagine a suburban office park, and a company within that park that has outgrown its space in building A and will be expanding to additional leased space in building B. The group being sent to building B just happens to be the company's engineering group--the power users of the resources.

The network manager would be asking for trouble by using a few T-1s to connect the buildings. Dark fiber is unavailable, and the cost of putting in new fiber is prohibitive due to a lake in the center of the campus. The investment in the new fiber would evaporate in three years, since the company also has plans to move then to a new corporate headquarters across town.

What alternatives does the network manager have? A wireless system--either radio frequency (RF) or optical--would allow the manager to keep the power users happy, since they would access the LAN at the 100 Mbps speeds to which they are accustomed. In addition, many of these systems are expandable to higher speeds, like OC-12 (622 Mbps). At the end of the three-year lease, the manager would have an asset that could be used elsewhere within the company or sold as used equipment.

If optical wireless has stricter line-of-sight requirements and limited range in relation to RF, why should this network manager consider it? There are two reasons:

1. Unlicensed spectrum: RF spectrum comes in two flavors--licensed and unlicensed. The number of licenses in a given area is limited, and time must be allotted for the licensing process. In using unlicensed spectrum, there are sometimes interference concerns. The infrared spectrum, however, requires no licenses. Because the footprint of the beam at the opposite transceiver is limited (between one and five meters), there is no need to worry about interference.

2. Indoor setup: Imagine that the network manager was unable to gain access to the roof at building B. Optical wireless systems can be installed indoors. The beams can be transmitted through office windows. The cost to run the fiber-optic cable to the router is also lower.

When comparing optical wireless to the cost of dark fiber (assuming it is available), the attraction to optical wireless is primarily financial. An entire OC-3 (156 Mbps) or OC-12 optical wireless system can be purchased for the cost of just a few months of the equivalent rental from the local phone company. If the eventual resale of the equipment is included in the payback calculation, the numbers become even more attractive.

Optical wireless technology has a variety of uses outside of campus LANs. CLECs and ISPs are currently deploying the technology to bridge the golden last mile. Because of the high bandwidth available, they can cross the last few blocks from the local point of presence to the office buildings in urban business districts to offer high-speed Internet access.

Some regional Bell operating companies have used the technology to provide access to customers who are waiting for the fiber construction to reach them. The ease of setup also makes optical wireless a candidate for disaster recovery applications. With more fiber in the ground and plenty of construction taking place, inevitably there will be accidents that cause denial of service.

How far can it transmit? Numbers vary among different vendors' systems, but two kilometers is a typical rating. When reviewing these ratings, one must consider the three-way trade-off that is a factor in all optical wireless systems-- weather, distance and link availability. In dry climates, the beam is not scattered by fog. Heavy rain and snow can also affect performance, but not as much as fog. This means that longer transmission distances can be achieved.

If the optical link is not the sole link (perhaps a slower-speed leased line or radio link is available), then the lower availability figures associated with longer distances is less of a concern. If the hypothetical power users had 622 Mbps access 99% of the time, would they be willing to settle for slower speeds the other 1% of the time when heavy fog occurs and blocks a two-kilometer link? Considering that many fog events occur in the early morning before typical working hours, the loss of the link may not even be noticed. Ask a vendor for an estimate of performance reflecting a specific application.

Is optical wireless technology a good fit for a network? Only a field trial with real network data can prove this. A vendor can provide this and monitor the system's performance over a number of days. The financial benefits of the technology should make the technical exercise worth the effort.

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Markowitz is with Canon USA, Englewood Cliffs, NJ.