FIBER OPTIC CABLE Fiber cable
comes in two forms: multimode and single-mode. A mode in optical transmission
is a ray of light entering the core at a particular angle. Modes can therefore
be thought of as bundles of light rays of the same wavelength entering the
fiber at a specific angle. Single-mode and
multimode fiber have differences that range from structural to use in
structured cabling systems. Single-mode fiber is capable of higher bandwidth
and greater cable run distances (up to 3000 meters) than multimode fiber (up to
2000 meters only). Because of these characteristics, single-mode fiber is often
used for interbuilding connectivity or WANs (for
example telephone company switch-to-switch connection). Multimode fiber is more
commonly used in LAN backbones within buildings. Multimode fiber
uses LEDs as the light source, while single-mode
fiber generally uses laser light sources. Furthermore, single-mode fiber is
typically more expensive than multimode. The reason is because the almost
hair-size glass fiber in single-mode is more fragile and it needs added
protection (coating and buffering materials) to make it manageable. Multimode A standard
multimode fiber-optic cable (the most common brand of fiber-optic cable) uses
an optical fiber with a 62.5-micron core and 125-micron cladding diameter. This
is commonly designated as 62.5/125 optical fibers. Because the diameter of the
cladding is considerably larger than the wavelength of the light being
transmitted, the light bounces around (reflects) inside the core as it is
propagated along the transmission line. Multimode fiber
uses LEDs as the light-generating device. LEDs are cheaper to build, require somewhat less safety
concerns, and are effective for shorter distances than the lasers used in
single-mode. Multimode (62.5/125) can carry data over distances of up to 2000
meters (6,560 ft.). It is mainly used in LAN applications including backbone
cabling. Single-mode The core in
single-mode fiber is only approximately 10 times larger than the wavelength of
the light it is carrying. This leaves very little room for the light to bounce
around. As a result the data carrying light pulses in single-mode fiber are
essentially transmitted in a straight line through the core. Typically
single-mode uses a laser light source, which is more expensive to produce,
requires higher levels of safety awareness, and can transmit data further than
multimode. Single-mode (such as a 9/125) can carry data up to 3000 meters
(9,840 ft.) according to the existing standard (note that the standard in this
case may not reflect the physical limitation). Single-mode is often used in
exterior segments and to connect buildings in larger campus environments. There are
several advantages that have lead to the ever-increasing development and
implementation of fiber-optic cable systems. Compared to copper, fiber-optic is
proving to superior in the following categories:
Since
fiber-optics use light to transmit a signal, it is not subject to EMI, RFI, or
voltage surges. This can be important when laying cables near sources of these
forces like motors, fans, some light sources (sodium vapor, mercury vapor,
neon, and florescent), pumps, transformers, power lines, and so on. In some
factory or industrial environments, these factors can be so great as to make
any other communication media virtually worthless. Since fiber does not use
electrical impulses and therefore cannot produce or transmit electric sparks,
it becomes the logical solution for passing through flammable environments like
paint rooms, solvents facilities, or even fuel tanks. Furthermore, the
non-conductivity nature of fiber-optic makes it great choice for areas of high
lightning-strike incidence and even running through liquids, such as running
under the oceans. Finally, a fiber-optic connection avoids the problem of
differing ground potentials and eliminates the danger ground loops pose to
personnel and equipment. Fiber in effect isolates devices connected to either
end of it, making it a good choice where completely separate systems are linked
together, such as two LANs in different buildings. Unlike
metallic-based systems, the use of light in optical fiber makes it impossible
to remotely detect the signal being transmitted within the cable. Signals sent
on copper wires can be intercepted by devices placed in close proximity to the
cable. The only way to tap a fiber circuit is to actually access the optical
fiber itself, which requires intervention that is easily detectable by security
surveillance. As a result, fiber is usually the choice of cable used by
governments, banks, and other organizations with major security concerns. An
optical repeater is used to boost the light pulse in a fiber-optic cable. The
advantage of optical fiber is that it performs better with respect to attenuation.
Fiber-optic cable needs fewer boosting devices than copper cable. Long,
continuous segment lengths of fiber-optic cable also provide advantages for
manufacturers, installers, and end-users. Currently, the
fiber circuits used in trunk connections between cities and countries carry
information at up to 2.5 gigabits per second (Gbps).
This is enough to carry 40,000 telephone conversations or 250 television
channels. Industry experts predict larger bandwidths than this as light
frequency separation becomes available. Private communication systems are
already using much higher bandwidths. Compared to
copper, optical fiber is relatively small in diameter and much lighter in
weight. These characteristics have made it desirable as intra-floor conduits
and wiring duct space has become increasing plugged with expanded copper cable
installation. It is even becoming commonplace to install new fiber cabling
within existing duct systems to replace many copper circuits and free up much
needed duct space.
While increased
demand for optical fiber has brought the prices down to be more competitive
with copper, it is still true that new fiber installations cost more than
copper installations. This imbalance shifts more towards copper when
considering extending existing copper networks. In the short term it is often
less expensive to continue using copper cabling for covering expanded
communication needs. By simply adding more wire to an existing system, expanded
needs can be covered. Since transmitters, converters, optical repeaters, and a
variety of connecting hardware will be needed; the initial cost of changing
over to fiber can be quite expensive. The most often
identified disadvantages of fiber-optics include:
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