Laser communications: fiber-optic cable vs. optical wireless

Nov. 1, 2000

Wireless optical-laser communications systems are gaining more popularity as an alternative to new fiber plants in metropolitan areas. How do the two technologies compare?

Dick Walter, LaserWireless Inc.

There are only two types of communications systems that can handle billions of bits of information per second: fiber-optic cable and optical wireless systems. The term "laser" is an acronym for light amplification by stimulated emission of radiation. In other words, a laser is a device that produces a specific wavelength of light, also known as coherent nondivergent light. Most systems that use laser light focus on the infrared spectrum.
One person can install LaserWireless's wireless optical system atop a building in four hours or less.

During the last 15 years, several small companies have been manufacturing optical wireless communications systems that can transmit through the atmosphere on a beam of light. It is only since Lucent Technologies (Murray Hill, NJ) announced their new venture with TeraBeam Networks (Seattle) that so many people began to understand how optical wireless technology could solve their many communications problems.

The need for higher bandwidth is growing to a point where some of today's legacy products cannot meet the demand. The best two ways to solve this bandwidth issue is by using fiber-optic cables, optical wireless technology, or both. A major advantage of optical wireless lasers is the ability to easily relocate the system. Fiber, on the other hand, once installed, is permanent for that location. With optical lasers, the installation can be completed in two to four hours and, should the need for relocation arise in a year or two, the equipment simply can be unbolted and reinstalled at the new location.

Business customers want to weigh any and all options before deciding what technology is best for meeting their particular communication needs. Fiber-optic cable may not be the most viable solution in terms of cost, ease of installation, and other considerations. Optical wireless technology provides an alternative option to fiber deployment.

First, there are advantages to using optical wireless, based on the fact that optical wireless systems have a distance limitation. In most cases, optical wireless is a more cost-effective solution when compared to digging and installing fiber-optic cables, especially at distances of more than 300-500 ft. In addition to ease of relocation, these systems can traverse physical barriers that may hamper the installation of fiber cable.

From purchase order to complete installation, the time required is much less with wireless optical systems and no license is required. Optical systems do not create nor are they affected by electromagnetic interference (EMI) or radio-frequency interference (RFI). They utilize very high levels of bandwidth and provide very private, secure transmission.

Without a doubt, all optical wireless systems are limited in distance. This limitation is based on the ability of light to travel through the air to a distant point, especially in fog. Two other very important elements to consider in optical laser systems are "alignment maintenance" (or tracking) and the sensitivity of the system.

Tracking is necessary to maintain alignment throughout the temperature cycles in a day and to compensate for building movement or sway. Without tracking, a system could easily move enough to be outside the active area of the beam of light coming to the unit, and communications would be lost.

Some manufacturers try to overcome this problem by opening up the divergence of the beam of light, which reduces the system's sensitivity at a given distance. Systems that open up the beam usually are limited to shorter distances due to the spreading of the beam of light. Systems with tracking can maintain alignment without opening up the beam, resulting in concentrated light that enables longer distances.

The term used to indicate sensitivity is "fade margin." Fade margin is defined by how much interference or signal loss is acceptable before the system is no longer able to communicate. Systems with the highest fade margin usually go longer distances. In systems with tracking, the light coming to each end of the system provides a small amount of light that is received and split off to electronics. Electronics can measure the increase or decrease of that light, and when a decrease occurs, they automatically begin a series of movements to reposition the system back into the most useful area of the beam of light. By equipping both ends of the network segment with this tracking technology, consistent alignment and a better fade margin is achieved. The signal used to achieve this tracking is independent of the customer's network information, allowing for a "protocol-independent" connection to the network.

The federal government must certify all optical wireless communication systems for eye safety. Each system must meet certain conditions before that product can be sold to the public. A system is certified when a person can look into the beam of light without suffering any eye damage. But as a rule, looking or staring into any beam of light is never advisable. The designations that identify a system as being approved are CDRH and UL. In Canada and Europe, certifications are CSA and CE.

Certain systems (the one manufactured by Laser Wireless Inc., for example) can be installed by one person. In a typical installation (see Photo), the system is mounted outdoors on any masonry material. Overhanging the roof edge, the unit requires a clear line of site to the other side of the link. With the tracking built in, the installation is very simple. Connections on the laser system are all weatherproof. Once installation is completed, a weather shield is installed at each end of the link. Normally, no regular maintenance is required, except annual checks on the optical lenses for dirt and inspection of the fiber connections. A typical installation may take one person up to four hours.

The LaserWireless systems have remote diagnostics-a remote status located in an indoor control room. By adding a terminal server that communicates directly to the network, the customer has access to the entire laser system performance data displayed on the remote-status monitor box. If customers experience a system failure and are unable to determine what caused the failure, they can connect any dedicated phone directly into the rear of the remote-status monitor. By simply dialing in, the problem can be quickly diagnosed. The system's diagnostic hardware also can be updated using the same dial-in method.

Let us assume there are many buildings in close proximity to each other, such as a campus or industrial park. Instead of individually purchasing expensive servers and/or switches, each tenant can be supplied access to a high-speed network by way of optical systems from a central source. The owner can then charge a fee for the service with payback in a few months.

This was actually the case at several large companies. The same technique can be used when an individual company outgrows its current facilities and obtains space in another building several blocks away. Here again, the cost savings are in not having to purchase a server and/or switch. Many times, the savings generated can more than pay for the optical system. Whenever a company has a need to relocate, its optical system can move along with it.

Recently, a local high school wanted to connect to a local university for network access. The decision was to dig and install a fiber-optic cable. Although the distance was very short (225 ft), it involved crossing a highway, bringing the total cost to $65,000. Previously, they had paid $21,000 for Internet connectivity. In the past few years, optical wireless systems have solved similar applications, and as the need for bandwidth continues to grow, so will the opportunities. Using an optical wireless solution, the high school could have completed its connection to the college for less than $25,000.

Optical systems have the ability to move a lot of information from point to point. A T1 (1.554-Mbit/sec) line has the ability to handle 24 individual telephone conversations at the same time. A 100-Mbit/sec network, such as Fast Ethernet, can handle 100 million bits per second of network data.

To illustrate the capacity of an optical system to move information, it is entirely possible to have 100 Mbits/sec and four T1s running at the same time on one optical wireless system. All that's required is the necessary peripherals that multiplex the different formats together. If a totally different requirement exists, such as the need for full teleconferencing capability, an optical wireless system with the proper selection of multiplexers can accommodate the required information transfer.

Dick Walter is president of LaserWireless Inc. (Lancaster, PA). The company's Website is