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Fiber Optic
Cable
Over the last 20 years or so, fiber
opticlines have taken over and
transformed the long distance telephone
industry. Optical fibers are also a huge
part of making the Internet available
around the world. When fiber replaces
copper for long distance calls and
Internet traffic, it dramatically lowers
costs. To understand how a fiber optic
cable works, imagine an immensely long
drinking straw or flexible plastic pipe.
For example, imagine a pipe that is
several miles long. Now imagine that the
inside surface of the pipe has been
coated with a perfect mirror. Now
imagine that you are looking into one
end of the pipe.
Several miles away at the other end,
a friend turns on a flashlight and
shines it into the pipe. Because the
interior of the pipe is a perfect
mirror, the flashlight's light will
reflect off the sides of the pipe (even
though the pipemay curve and twist) and
you will see it at the other end. If
your friend were to turn the flashlight
on and off in a morse code fashion,your
friend could communicate with you
through the pipe. That is the essence of
a fiber optic cable.
Making a cable out of a mirrored tube
would work, but it would be bulky and it
would also be hard to coat the interior
of the tube with a perfect mirror. A
real fiber optic cable is therefore made
out of glass. The glass is incredibly
pure so that, even though it is several
miles long, light can still make it
through (imagine glass so transparent
that a window several miles thick still
looks clear). The glass is drawn into a
very thin strand, with a thickness
comparable to that of a human hair.
The glass strand is then coated in
two layers of plastic. By coating the
glass in plastic, you get the equivalent
of a mirror around the glass strand.
This mirror creates total internal
reflection, just like a perfect mirror
coating on the inside of a tube does.
You can experience this sort of
reflection with a flashlight and a
window in a dark room. If you direct the
flashlight through the window at a 90
degree angle, it passes straight through
the glass. However, if you shine the
flashlight at a very shallow angle
(nearly parallel to the glass), the
glass will act as a mirror and you will
see the beam reflect off the window and
hit the wall inside the room. Light
traveling through the fiber bounces at
shallow angles like this and stays
completely within the fiber.
To send telephone conversations
through a fiber optic cable, analog
voice signals are translated into
digital signals (see How analog and
digital recording works for details). A
laser at one end of the pipe switches on
and off to send each bit. Modern fiber
systems with a single laser can transmit
billions of bits per second -- the laser
can turn on and off several billions of
times per second. The newest systems use
multiple lasers with different colors to
fit multiple signals into the same
fiber.
Modern fiber optic cables can carry a
signal quite a distance -- perhaps 60
miles (100 km). On a long distance line,
there is an equipment hut every 40 to 60
miles. The hut contains equipment that
picks up and retransmits the signal down
the next segment at full strength.
FTTH, or Fiber To The Home, refers to
fiber optic cable that replaces the
standard copper wire of the local Telco.
FTTH is desirable because it can carry
high-speed broadband services
integrating voice, data and video, and
runs directly to the junction box at the
home or building. For this reason it is
sometimes called Fiber To The Building,
or FTTB.
Traditional copper telephone wires
carry analog signals generated by
telephone equipment, including fax
machines. Analog technology is by nature
a less precise signaling technology than
digital technology. Though multiplexing
has allowed digital signals to be
transmitted across multiple channels
over copper lines, fiber optic cable is
superior for relaying these signals and
allows for faster transfer rates and
virtually unlimited bandwidth. This
opens the door to better Internet speed,
streaming video, and other demanding
applications.
The Internet utilizes a backbone of
fiber optic cables capable of delivering
incredible bandwidth. This inherent
ability makes the Internet a prime
source for advancing network
technologies that can be brought to the
home or business. Most subscribers,
however, log on to this network through
copper lines with limited capacity. This
creates a bottleneck for advancing
technologies that increasingly require
greater bandwidth. FTTH bridges this
gap.
Fiber optic cables are made of glass
fiber that can carry data at speeds
exceeding 2.5 gigabits per second
(gbps). FTTH services commonly offer a
fleet of plans with differing speeds
that are price dependent. At the lower
end of the scale, a service plan might
offer speeds of 10 megabits per second
(mbps), while typical DSL (Digital
Subscriber Line) service running on
existing copper lines is 1.5 mbps. A
more expensive FTTH plan might offer
data transfer speeds of over 100 mbps –-
that’s about 66 times faster than
typical DSL.
FTTH is cost-prohibitive in many
cases. Installing FTTH can be expensive,
and the monthly charge for broadband
services thereafter can also be
off-putting, though these figures vary
widely. Expense is likely to drop with
time as FTTH becomes more common.
Because of the cost involved and the
logistic difficulty in replacing
existing copper lines in some
neighborhoods, FTTH is more often being
installed in newly built communities as
an added selling feature. Installing
FTTH raises the value of existing
property.
FTTH can be installed as a
point-to-point architecture, or as a
passive optical network (PON). The
former requires that the provider have
an optical receiver for each customer in
the field. PON FTTH utilizes a central
transceiver and splitter to accommodate
up to 32 clients. Optical electric
converters, or OECs, are used to convert
the signals to interface with copper
wiring where necessary.
FTTH differs from Fiber To The Curb
(FTTC) in that FTTC does not run
directly to the home or building.
Instead it runs to the curb, and the
last leg of wiring to individual
buildings remains copper wire.
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