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The previous decade saw dramatic
growth in the use of optical fiber in premises cabling
from approximately 50,000 kilometers of installed fiber
in 1984 to about 500,000 kilometers today. And while
optical fiber was first used primarily in backbones,
it is quickly moving throughout buildings and right
up to the desktop.
So if you haven't worked with fiber
yet, you will soon. In order to participate and succeed,
you'll need knowledge and experience. You won't be tackling
some exotic technology requiring highly specialized
skills and time-consuming training. While the technology
behind optical fiber is complex, the product itself
is surprisingly user friendly.
With standard field practices and
termination equipment, the installation process is simpler,
faster, and less costly than ever; and testing after
installation is a breeze. The fact is that, today, optical
fiber technology surpasses that of copper.
Freedom From Worry
Optical fiber transmission involves
changing electrical signals into pulses of light, using
an optoelectronic transmitter, and sending the pulses
down the core of an optical fiber. Because the core
and its surrounding cladding glass have different compositions,
the light is trapped within the core. It has nowhere
to go but down the length of the fiber. At the opposite
end, a receiver changes the pulses back to electrical
signals.
When it comes to network installation,
optical fiber cable offers many benefits. First, its
small size and light weight actually make it easier
than copper to install. Despite some lingering misconceptions,
optical fiber : cable is quite strong. Its pull strength
far surpasses copper's 200 lbs. for a two fiber cable
versus 25 lbs. for Category 5 UTP copper cable. And
the bend radius for two fiber cable and four pair UTP
copper cable is the same. Besides, current cable designs
provide ample protection for fibers, so there's little
need to worry about damage during or after installation.
Perhaps most important, optical
fiber is reliable enough to be virtually worry free.
Because it is dielectric, fiber eliminates most of the
concern over factors affecting link performance. It
is immune to crosstalk, electromagnetic interference
(EMI), radio frequency interference (RFI), impedance
mismatches, transmission frequency variability, and
ground loops all pitfalls of copper based systems.
Fiber's immunity to EMI means installers
need have few concerns about where they run optical
fiber cable. No need to worry about coming too close
to electric motors or fluorescent lights, for example.
This means only four link performance
factors should concern installers. These four are bandwidth;
environmental effects, such as temperature dependence;
continuity (unbroken transmission of a signal from one
point to another); and attenuation (acceptable signal
loss over distance). Bandwidth is built in during fiber
manufacture and is not adversely affected by installation,
unlike copper crosstalk, which can be dramatically affected.
Because of the attributes of optical
fiber, environmental factors are, for all practical
purposes, not an issue for inside plant cabling. By
contrast, Category 5 UTP is sensitive to environmental
factors. Temperature changes will noticeably change
the performance of the cable. Once again, the highly
sophisticated technology behind optical fiber and cable
takes care of these matters for you.
Ultimately, installers need concern
themselves only with continuity and attenuation. Using
quality optical fiber, cable, and connectors helps to
minimize these concerns, or even prevent them altogether.
Besides, testing for continuity and attenuation (see
below) is very simple.
A Fiber Geometry Primer
The key dimensions of fiber geometry
are cladding outer diameter, core/clad concentricity,
and cladding non-circularity.
Outer diameter. Tight tolerances
in cladding outer diameter determine the precision with
which each fiber fits into ferrule type connectors.
If the fiber is too thick, it won't fit, and connectorizing
times are increased. If it is too thin, the cores won't
align properly, and power losses are increased. In effect,
tight tolerances on both the fiber and the connector
optimize link loss performance.
Core/clad concentricity. Core/clad
concentricity is a measure of how well the fiber core
is centered in the cladding glass. Because the outer
cladding is referenced when connectorizing and splicing
to align the two cores, tight centering tolerances translate
to closer alignments and less power loss.
Cladding non-circularity. The uniform
ovality of the cladding glass is known as cladding noncircularity.
Consistency in noncircularity along with cladding diameters
ensures successful connectorization of optical fibers.
You Can't Have Everything:
Some Testing is Necessary.
Optical fiber technology is formidable
and impressive a sort of "gee whiz" application,
and fiber is nearly perfect. But network installers
still have to test fiber for continuity and attenuation.
The good news is that field-testing fiber is simple,
quick, accurate, and inexpensive.
Continuity. Testing for continuity
confirms that each fiber is connected at the correct
place (the chances of a break are extremely small).
The test can be performed on multimode fibers with a
flashlight, a walkie talkie set, two people, and common
sense. You simply select the color coded fiber to be
tested, darken the closet, shine the flashlight at the
fiber's end, and wait for confirmation that your partner
sees the light. It's that simple really!
Attenuation. Installers should
confirm actual power loss against acceptable levels
for each optical fiber link segment. In the horizontal,
a typical link segment runs from the telecommunications
outlet to the horizontal cross connect. It should include
the cable, connectors, and adapters.
In practice, attenuation and continuity
testing can be combined, because if you can measure
attenuation, you must have continuity. Even with some
excess power loss, fiber provides plenty of "headroom"
above the power budgets of today's network protocols.
However, attenuation testing is
usually worthwhile as certification for the end user.
Customers have a right to know that installations were
performed to specifications and will support high data
rate transmissions. These are important assurances that
cannot be provided with a Category 5 UTP system.
The only equipment required to
perform attenuation testing is a light source, power
meter, test cords, and an adapter (this is in accordance
with Annex H of SP2804 & the ballot copy of IA568A
(Commercial Building Telecommunications Cabling Standard).
Remember that it is not necessary to measure attenuation
in both directions. You need measure in one direction
only, and for horizontal links, only at one wavelength.
Testing begins with cleaning connectors
and adapters and zeroing out the equipment to establish
a reference measurement. The power meter and test cord
are then moved to the far path panel, where the equipment
is assembled, and a power reading is recorded.
This power level is compared with
the reference measurement to obtain the end-to-end attenuation.
The process is simple, quick, and uncomplicated.
Select Fiber Carefully
For A Worry free Network
Optical fiber technology has clearly
come a long way. Today, working with fiber is routine.
Fiber cable, along with connectors, is as easy to install
as regular copper, and even easier than Level 5. Specifying
a good brand of fiber and cable ensures quality of product
and service. Choose wisely, and all the rest is taken
care of.
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