Keep an Eye on Optical Wireless

For IT departments, an expanding campus presents special challenges, such as how to connect the new site to the corporate network. For years, the practical choices for ad hoc remote links were limited to common carrier services such as frame relay and ISDN. Users at a remote site had to settle for relatively slow links (56Kbps to 154Kbps) and a monthly service fee of several hundred dollars (depending on local telephone company rates). Higher-bandwidth circuits were available, but at a steep cost. Running your own cable was potentially even more expensive and rarely justifiable for connecting leased offices. Wireless solutions were still costly and fraught with problems (e.g., limited range, susceptibility to interference).

Today, although frame relay and ISDN are still popular, wireless solutions have evolved into legitimate contenders for the remote-connectivity market. Optical wireless has made great technological strides and improved its cost effectiveness, making it a viable option.

The clear benefits of optical networks are high bandwidth, portability, security, and independence from third-party service providers. With off-the-shelf speeds at the OC-12 (622Mbps) level, optical solutions can deliver the goods. If your offices move, you can take the equipment with you. Point-to-point infrared (IR) beams aren't susceptible to eavesdropping. And a backhoe can't accidentally dig up an optical link.

Optical wireless data transmission relies on the short-wavelength (850 to 950 nanometers) IR spectrum. Today's optical wireless devices use high-power LEDs or laser diodes and can reliably send data over distances—some vendors claim ranges as far as 2.5 miles—and customized high-end systems work over even longer distances. Several products offer transmission speeds from 10Mbps to 622Mbps. Typically, range and speed are inversely proportional, but some customized packages provide high levels of both. Both big-name vendors (e.g., Canon, PAV Data Systems) and smaller vendors (e.g., MRV Communications, LightPointe Communications) now offer similar wireless solutions, but price and features distinguish these solutions.

Optical networking's basic concept is simple. Imagine a standard fiber- optic network link between two buildings, then replace the cable with a line-of-sight IR beam that travels between a pair of transmitter/receiver heads. You can mount the heads on the buildings' roofs or behind the windows. The only catch is that no objects that might obstruct the IR beam can be in the path between the heads. An SC fiber connector on the back of each head typically provides your network interface.

Restricted range and susceptibility to interference limit optical network links. Advanced LEDs and laser diodes have increased the optical equipment's effective range and improved IR reliability under adverse weather conditions (e.g., rain, fog, smoke), but other environmental conditions can cause problems (e.g., building sway, vibration, thermal expansion) that can result in misalignment between the transmitter/receiver heads. Increasing the transmit-beam angle creates a larger beam diameter at the receiver to permit some offset, although it sacrifices range. A tracking motor is expensive but can dynamically keep the heads in alignment. Multiple laser diodes don't sacrifice range but aren't as effective as tracking motors, which constantly realign the beam. These methods have greatly increased the reliability of optical links. PAV, for example, claims that its installations in the less-than-sunny UK have a 99.897-percent availability rate.

Competition has brought prices down, but a short-range (i.e., 1 kilometer) 10Mbps system still costs about $16,000. For 622Mbps systems with advanced reliability features such as tracking motors, you'll spend at least $60,000. Nonetheless, I think optical wireless will soon be the primary choice for high-bandwidth connectivity solutions.

End of Article