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THE THIRD OPTION: INFRARED

A Free- Space Infrared Local Area Network (FIRLAN) system is based on infrared (IR) technology and can be used to build or replace a traditional hardwire network. The FIRLAN provides point-to-point or point-to-multipoints transmission of Ethernet signals between segments or stations, as well as T1 line signal transmission, as shown in Exhibit 6.

IR technology replaces cables with wireless optical links using line-of-sight JR transmission. Some systems use lasers as a basis for optical transmission. This is superior to standard JR transmission devices, especially under high ambient lighting and poor weather conditions.

It is recommended with IR systems that the installer have an IR viewer for alignment accuracy. The units are mounted on the corner, at the windows, or on the roof of the building to which they are attached and preferably are mounted to masonry construction. The supporting structure for the viewers cannot be wood or sheet metal.


Exhibit 6.  The Typical T1 Connection Setup.

JR systems offer industry- standard LAN and WAN interfaces, enabling the use of standard network interface cards and network software. FIRLAN provides seamless Ethernet integration, setting the required bit rate at 10M bits, and defines the physical interfaces and operating characteristics for the hardware, as shown in Exhibit 7.

Conditions Affecting Effectiveness. The three most significant atmospheric conditions that affect laser transmission include absorption, scattering, and shimmering. All three conditions can reduce the amount of light energy received by the receiver. The phenomena affect the laser transmission to varying degrees.

Absorption along the transmission path is caused mainly by the water vapor and carbon dioxide content in the air, which in turn depend on the humidity and altitude. The gases that form in the atmosphere have many resonant bands (i.e., transmission “windows”) that allow specific frequencies of light to pass. These windows occur at various wavelengths including the visible range. If a system uses a near IR wavelength of light (820 wavelength, or 820 nm) for laser transmission, however, absorption is not a great concern.


Exhibit 7.  The Typical Ethernet Setup.

Scattering has a greater effect than absorption, because a smaller percentage of the transmission beam reaches the receiver. The atmospheric scattering of light is a function of light’s wavelength and the number and size of scattering particles in the path.

Specific conditions that cause scattering problems include the following,

1.  Fog. The worst condition, fog, produces a scattering effect in all directions. This is attributed to water drops with a radius less than a few microns. Smog has a similar effect.
2.  Snow. Its effect varies depending on its water content. A very wet snow is closer to rain, which has less of an effect on visibility than fog. An extremely dry snow, on the other hand, is closer to fog and has a similar effect. The severity of the problem is in direct correlation to the radius of the particles; therefore, snow’s scattering effect vanes.
3.  Rain. Rain-related attenuation is one hundred times less than that of fog. Although the liquid water content of a typical heavy shower is 10 times that of a typical dense fog, the raindrop radius is about one thousand times larger than that of a fog droplet. This causes less scattering effect.

There is a way to minimize the effects of scattering, too. A product designed with a fade margin of 17 Db, for a 1,000-meter link would activate an automatic turn off when visibility drops below 800 meters.

Shimmer, also known as mirage or atmospheric turbulence, is the visual distortion of images in heat situations and imposes a low-frequency (normally below 200 Hz) variation on the amount of light detected by the receiver. This low frequency variation can result in excessive data error rates or video distortion on a laser communication system.

The shimmer effect is a combination of time of day, terrain, cloud cover, wind, and height of the optical path above the source of shimmer. Localized differences in the air’s index of refraction cause fluctuations in the received signal level by directing some of the light out of its intended path. Beam fluctuations may degrade system performance by producing short-term signal amplitudes. Signal fades below the threshold result in error bursts. Selection of an optical path several meters above heat sources, however, greatly reduces shimmer effects.

In addition to the atmospheric conditions mentioned, direct sunlight into the front of the transmission or receiving unit affects performance. Sunlight can saturate the receiver photo diode, resulting in outages that can last for several minutes, depending on the time of the year and the angle of the sun. In addition, direct sunlight can saturate the feedback diode in the laser, resulting in transmitter shut off.

Licensing Requirements

There are no licensing requirements. However, the Center for Devices and Radiological Health (CDRH) of the US Food and Drug Administration is the agency responsible for reviewing JR technology. The technology complies with the federal regulations covered by 21 CFR 1010 and 21 CER 1040 (HI-IS Publication FDA 88-8035).

Advantages

FIRLAN advantages include:

  Freedom from government regulation through licensing.
  Equipment ownership when a company changes locations.
  Connectivity ease when locations need to be added.
  Immunity to radio interference.
  Data security.

Disadvantages

Specific FIRLAN disadvantages include:

  The effect of atmospheric conditions on reliable data transmission, which may prohibit its use in some locals.
  High up-front equipment investment and installation support.
  Potential safety issue of retina damage caused by looking directly into the beam.

Cost

The basic cost to set up the system is as follows:

Materials Description Unit Price Quantity Total Price

Ethernet Link IEEE 802.3M bits to 10M bits. Includes Two Laser Sets with AUI Connection $15,000 1 $15,000
Mounting Cost $600 2 $1,200
Bridge Ethernet to Token Ring $2,000 2 $4,000
Converter $2,000 1 $2,000
Total for Basic System Setup $22,200


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