![]() Attenuation Because of Obstructions and Weather ![]() When installing these types of systems, be certain to follow the manufacturer's recommendations when orienting the antennae. This interference can be high enough in some cases to completely disrupt transmission of data on the infrared link. 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. Interference from other sources of light, however, can still be a problem for systems that use light signals. 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. In fact, the FCC doesn't regulate light signals because of extremely limited potential interference among systems. Light signals are free from RF sources of interference such as cordless phones, and microwave ovens. Impairments, such as interference and obstructions, limit the performance of the wireless network that uses light signals. Light signal propagation is not free from difficulties. 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. These characteristics make the use of light signals most effective for specialized applications where extremely high performance is necessary. Issues dealing with alignment because of building swaying Requirement for line-of-sight operation, free from obstructions such as buildings, trees, and telephone polesĮxtremely low potential for RF interference from external systems Relatively short-range (1 mile) capability High inherent security because of narrow laser beam Variable, unreliable performance in the presence of significant smog, fog, rain, snow, and other airborne particulate matter Comparing the Pros and Cons of Light SignalsĮxtremely high throughput, up to the Gbps range Light Signal Pros and ConsĪs compared to RF signals, light signals have the characteristics defined in Table 3-2. This is significantly less range than with RF systems. The range of an infrared light system can vary from a few feet with PDA applications to 1 mile with direct infrared systems. This increases the range of low-power laser systems to a mile or so at data rates up in the Gbps range.Īs with RF signals, the amplitude of light also decreases as distance between the sending and receiving stations increase. The direct infrared approach, commonly referred to as free-space optics, intensifies the light signal power similarly to a directive radio signal antenna. The advantage is relatively easy installation with inexpensive components. In addition, this technique limits wireless component spacing to around 40 feet, mainly because of the lower ceilings indoors and resulting signal path geometry. 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. ![]() Both Diffused and Directed Light Signals Offer a Basis for Wireless Networks Most laser LANs utilize diffused infrared inter-building modems and PDAs use the direct infrared technique.įigure 3-6. Diffused laser light is normally reflected off a wall or ceiling, and direct laser is directly focused in a line-of-sight fashion. Figure 3-6 illustrates these two concepts. This equates to 333,333 GHz, which is quite a bit higher than RF signals and falls just below the visual range of humans.ĭiffused and direct infrared are two main types of light transmission. Most wireless networks that use light for wireless signaling purposes utilize infrared light, which has a wavelength of approximately 900 nanometers. Light Signal AttributesĪ light signal is analog in form and has a very high frequency that's not regulated by the FCC. Some wireless LANs and inter-building products use laser light to carry information between computers. Light signals generally satisfy needs for special applications, such as building-to-building links and short-range personal-area networks. Wireless networks that utilize light signals, however, are not as common as these that use radio signals. Light guns are still in use today at many airports as a backup communication with aircraft having malfunctioning radio gear. Lanterns would provide a source of light to use with sending codes between ships at sea hundreds of years ago. Light signals have been in use with communications systems for even longer than RF systems.
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