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Fiber
Optic Cable
Over
the last 20 years or so, fiber optic lines 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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