Understanding Light Signals
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.
Light Signal Attributes
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.
Light Signal Pros and Cons
As compared to RF signals, light signals have the
characteristics defined in Table 3-2.
Table 3-2. Comparing the Pros and Cons of Light
Signals
|
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 |
|
License-free operation |
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 Impairments
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.
Interference
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.
Attenuation Because of Obstructions and Weather
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.
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