Light signals have been in use with communications systems for even longer than RF systems. Lanterns would provide a source of light to use with sending codes between ships at sea hundreds of years ago. Light guns are still in use today at many airports as a backup communication with aircraft having malfunctioning radio gear.
Wireless networks that utilize light signals, however, are not as common as these that use radio signals. Light signals generally satisfy needs for special applications, such as building-to-building links and short-range personal-area networks. Some wireless LANs and inter-building products use laser light to carry information between computers.
A light signal is analog in form and has a very high frequency that's not regulated by the FCC. Most wireless networks that use light for wireless signaling purposes utilize infrared light, which has a wavelength of approximately 900 nanometers. This equates to 333,333 GHz, which is quite a bit higher than RF signals and falls just below the visual range of humans.
Diffused and direct infrared are two main types of light transmission. Figure 3-6 illustrates these two concepts. Diffused laser light is normally reflected off a wall or ceiling, and direct laser is directly focused in a line-of-sight fashion. Most laser LANs utilize diffused infrared; inter-building modems and PDAs use the direct infrared technique.
Infrared light has very high bandwidth; however, the diffusing technique severely attenuates the signal and requires slow data transmissions (less than 1 Mbps) to avoid significant transmission errors. In addition, this technique limits wireless component spacing to around 40 feet, mainly because of the lower ceilings indoors and resulting signal path geometry. The advantage is relatively easy installation with inexpensive components.
The direct infrared approach, commonly referred to as free-space optics, intensifies the light signal power similarly to a directive radio signal antenna. This increases the range of low-power laser systems to a mile or so at data rates up in the Gbps range.
As with RF signals, the amplitude of light also decreases as distance between the sending and receiving stations increase. The range of an infrared light system can vary from a few feet with PDA applications to 1 mile with direct infrared systems. This is significantly less range than with RF systems.
As compared to RF signals, light signals have the characteristics defined in Table 3-2.
Light Signal Pros
Light Signal Cons
Extremely high throughput, up to the Gbps range
Variable, unreliable performance in the presence of significant smog, fog, rain, snow, and other airborne particulate matter
High inherent security because of narrow laser beam
Relatively short-range (1 mile) capability
Requirement for line-of-sight operation, free from obstructions such as buildings, trees, and telephone poles
Extremely low potential for RF interference from external systems
Issues dealing with alignment because of building swaying
These characteristics make the use of light signals most effective for specialized applications where extremely high performance is necessary. For example, a company can install an infrared communications link between two nearby buildings in order to facilitate high-speed server backups over a wireless network.
Light signal propagation is not free from difficulties. Impairments, such as interference and obstructions, limit the performance of the wireless network that uses light signals.
Light signals are free from RF sources of interference such as cordless phones, and microwave ovens. In fact, the FCC doesn't regulate light signals because of extremely limited potential interference among systems. Light signals have such a high frequency that their emissions are well outside the spectrum of RF systems, which means that the FCC doesn't regulate light signals.
Interference from other sources of light, however, can still be a problem for systems that use light signals. For example, the installation of a point-to-point infrared transmission system aimed in an easterly or westerly direction can receive substantial interference from infrared light found within sunlight because the sun is low to the horizon. This interference can be high enough in some cases to completely disrupt transmission of data on the infrared link. When installing these types of systems, be certain to follow the manufacturer's recommendations when orienting the antennae.
Obstructions such as buildings, mountains, and trees offer substantial amounts of attenuation to light signals as they propagate through the air. Most of these objects are composed of materials that readily absorb and scatter the light. As a result, be sure that the path between the end points of a light-based communications system are completely clear of obstacles.
Even if the communications path is open, weather can still impress large amounts of attenuation to light signals. The problem with weather is that it varies. For example, heavy fog might be present, and then the skies might be completely clear the following hour. This makes planning link budgets for light-based systems, especially those operating near the range limits, extremely difficult. Planners must be certain that the attenuation imposed by weather will not disrupt communications.