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More and more telephone and cable systems are converting
to fiber optic cable rather than copper wire. Local
Area Networks (LANs), particularly for high-capacity
backbones, are also using more fiber than copper.
FIBER OPTIC CABLE ADVANTAGES OVER COPPER:
SPEED: Fiber optic networks operate at speeds up to
2.5 gigabits per second, as opposed to 155 megabits
per second for copper.
BANDWIDTH: Substantially larger carrying capacity.
DISTANCE: Signals can be transmitted further without
needing to be "refreshed" or strengthened.
RESISTANCE: Greater resistance to electromagnetic noise
such as radios, motors or other nearby cables.
MAINTENANCE: Fiber optic cables costs much less to maintain.
KINDS OF FIBER
There are three types of fiber optic cable: single
mode, multimode and plastic optical fiber (POF).
Single Mode cable is a single stand of glass fiber
with a diameter of 8.3 to 10 microns. (One micron is
1/250th the width of a human hair.)
Multimode cable is made of multiple strands of glass
fibers, with a combined diameter in the 50-to-100 micron
range. Each fiber in a multimode cable is capable of
carrying a different signal independent from those on
the other fibers in the cable bundle. POF is a newer
plastic-based cable which promises performance similar
to single mode cable, but at a lower cost.
While fiber optic cable itself is cheaper than an
equivalent length of copper cable, fiber optic cable
connectors and the equipment needed to install them
are more expensive than their copper counterparts.
Fiber optic cable functions as a "light guide," guiding
the light introduced at one end of the cable through
to the other end. The light source can either be a light-emitting
diode (LED)) or a laser.
The light source is pulsed on and off, and a light-sensitive
receiver on the other end of the cable converts the
pulses back into the digital ls and 0s of the original
signal.
Even laser light shining through a fiber optic cable
is subject to loss of strength, primarily through dispersion
and scattering of the light, within the cable itself.
The faster the laser fluctuates, the greater the risk
of dispersion. Light strengtheners, called repeaters,
may be necessary to refresh the signal in certain applications.
EXAMPLE OF FIBER IN THE HOME
Let us review the case of TV signal being delivered
via a fiber optic cable to a TV set-top box. An analog
TV signal is first converted to a digital signal by
a Digital-To-Analog (DAC) converter. A laser is pulsed
on and off rapidly in response to the (digital) signal
input, converting the electrical signal to a laser light
signal. The light travels down the fiber optic cable
toward its destination which, if the distance is great
enough, is strengthened by a repeater. The process is
reversed at the receiving end. The light from the end
of the fiber optic cable shines onto a light-sensitive
receiver which converts the light back into digital
electrical pulses. The digital information is then converted
back into an analog TV signal by another DAC before
being fed to the TV set.
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