Media - pages 1 & 2
(Located at http://library.advanced.org/27887/gather/fundamentals/media.shtml)
Media is the material on which voice and data transmissions are
carried. The media is a very important part of telecommunications as the characteristics
of a particular media has a direct effect on the speed, accuracy, and distance at which
the exchange can be carried. The two most prevalent media within organizations are fiber
optic cabling and unshielded twisted pair (copper).
Copper
The electrical properties of copper cabling create resistance and interference problems
with transmissions. Voice or data signals sent over copper cabling weaken the further they
are transmitted. The resistance within the copper media slows down the signal or flow of
current since it is electric, limiting the speed of transmission on copper lines.
Signals sent over copper wire are direct current electrical signals. As such, any other
signals nearby, like magnetic signals or radio station transmissions, can introduce
interference and noise into transmission. This leads to people hearing radio station
programming on telephone calls, and hearing another person’s conversation due to
"leaking" electrical transmissions (called crosstalk). A way of protecting the
transmission from crosstalk and noise is to twist each copper wire of a 2-wire pair. The
noise induced would be canceled out at the twist in the wire.
The most common forms of unshielded twisted pair cabling used by businesses are rated by
the EIA/TIA (Electronics Industry Association/Telecommunications) as either category 3 or
category 5. Category 3 unshielded twisted pair is rated as suitable for voice transmission
and the Category 5 unshielded twisted pair, commonly referred to as Cat 5, for data.
Organizations usually save money on data communications by laying copper cabling to
individual telephones and PCs rather than installing fiber optics. They ordinarily install
Cat 5 cabling for both voice and data rather than pay technicians to lay one set of cables
for data and another for voice.
Fiber Optic Cabling
The other form of media, fiber optic cabling, is superior to traditional copper cabling in
many ways. Optical fibers, first developed in the 1970s, working either individually or in
bundles, can carry data from a few centimeters to more than 100 miles. Some of these
fibers also measure less than 0.001 inches in diameter – which means less duct space
is required, which is particularly significant under city streets where the underground
duct is at its capacity, filled with old copper cabling. A single conductor fiber also
weighs nine times less than a coaxial cable. Optical fibers have an extremely pure core of
glass or plastic surrounded by a plastic called a "cladding". Light from a
laser, a light bulb, or some other source enters one end of the optical fiber. As the
light travels through the core, it is constantly kept inside by the cladding, since the
cladding is designed so that it bends light rays that strikes its inside surface inwards.
At the other end of the fiber, a detector such as a wellphotosensitive device or the human
eye then receives the light.
There are two basic kinds of optical fibers – single-mode fibers and multi-mode
fibers. The single-mode fibers, used for long distance transmissions, have extremely small
cores, and accept light only along the axis of the fibers. As a result, single-mode fibers
require the use of special lasers as a light source, and they need to be precisely
connected to the laser, the other fibers in the system and the detector. Multi-mode
fibers, on the other hand, have cores larger than single-mode fibers do, and as its name
suggests, it accepts light from a variety of angles. Multi-mode fibers can therefore use
more types of light sources and cheaper connectors than can single-mode fibers They are,
however, bigger, carry signals slower than single-mode fibers and cannot be used over long
distances.
In fiber-optic communication systems, special lasers transmit coded messages by flashing
on and off at extremely high speeds (450 million times per second). The messages travel
through optical fibers to interpreting devices that decode the messages, converting them
back into the form of the original signal.
Fiber optics have a much larger information-carrying capacity than copper cabling.
Regularity and predictability makes it possible to interweave light signals representing
many conversations into a single stream of pulses, and as such, two strands of fiber
carries more information than a bundle of copper wires 4 inches in diameter, allowing for
24,000 calls at one time. This is because a pair of plain copper wires carries one
telephone conversation traveling as electrical impulses, while a pair of optical fibers
conveys light pulses at a rate of hundreds of thousands of millions of bits per second.
In addition to that, no electricity is present in transmission over fiber optics and the
signals carried on fiber optics don’t interfere with each other. As such, fiber can
be run in areas without regard to interference from electrical equipment. There is also no
sparking hazard, and thus fiber optic cabling can be set up in flammable areas. Signals
sent over long-distance fiber-optic cables need less amplification than do signals sent
over copper cables of equal length since there is less fading of signals over distances.
Furthermore, fiber-optic cables have greater security since they are virtually
tap-resistant. They do not emit electromagnetic signals, therefore to tap fiber strands,
the strands have to be physically broken and listening devices have to be spliced into the
break, which are easily detected. They are also suitable for new high-speed transmission
speeds such as SONET and ATM, since it has a high bandwidth.
However, nothing is perfect, and fiber-optic cabling is not without disadvantages. The
main disadvantage is, of course, the high costs.
Copper vs. Fiber Optic Cable
The termination, components, and connector costs are much higher than for copper wiring,
since specialized equipment is required to terminate fiber cables within buildings, to
test and splice fiber, and to convert electrical signals to light pulses. Specialized
technicians, who may be paid at higher levels, are also often required to work with and
test fiber cabling. When fiber is brought into buildings from telephone companies or to
the curb in residential areas, local electrical power is needed, leading to extra expense.
More care in handling the fiber is needed, and in particular, the fiber is not as flexible
as twisted pair in bending around corners.
Despite the disadvantages, the advantages have many communication companies replacing
their copper cabling with fiber-optic cables. The added expense of fiber, they feel, is
justified because of the greater capacity and speeds. In addition, telephone companies
need only lay a few strands of fiber to achieve the same capacity as heavier copper
cables. Moreover, signals can travel further in the range of 30 miles on fiber without the
use of repeaters to strengthen a faded signal, and as such, fewer repeaters are needed.
This translates into lower maintenance cost.
Technical improvements, such as ever-purer glass fibers and new methods of feeding light
pulses into the fibers, have produced constant increases in the carrying capacity of
fiber-optics systems. A fiber-optic cable laid under the Atlantic Ocean in the 1990s by
AT&T transmits 5 billion bits per second over each of 2 pairs of fibers – enough
for 320,000 simultaneous conversations.
In the 1980s, US companies started laying fiber-optic cable in earnest. By 1994, the
nation was criss-crossed by an optical fiber network. But since it costs too much, the
last mile or so uses copper cables. For example, it’s similar to a 6-lane highway
that links cities and towns, while local traffic still use normal roads.
Wireless Communication
Another medium being discussed for in-building local area networks (LAN) is wireless
communication. Wireless has advantages in the mobility it provides. Wireless LAN
connections are less messy - it enables computers to move and office space to be
reconfigured without LAN administrators having to physically move cabling. It also carries
data at higher speeds than any other media at 100 megabits and greater.
However, wireless, like fiber optics, involves a much higher cost as compared to
unshielded twisted pair and has the added problem of interference in some office
environments. For example, wireless cannot travel through 1-foot thick walls, since that
thickness impedes data from traveling through them.