Fiber Optics - Cabletron Systems 100BASE-FX Cabling Manual

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Fiber Optics

Fiber optic cable is a high performance media constructed of glass or plastic that
uses pulses of light as a transmission method. Because fiber optics do not utilize
electrical charges to pass data, they are free from the possibility of interference
due to proximity to electrical fields. This, combined with the extremely low rate of
signal degradation and dB loss makes fiber optics able to traverse extremely long
distances. The actual maximums are dependent upon the architecture being used,
but distances up to 10 km (6.2 miles) are not uncommon.
Glass optical fiber is made up of a glass strand, the core, which allows for the easy
transmission of light, the cladding, a glass layer around the core that helps keep
the light within the core, and a plastic buffer that protects the cable.
There are two basic types of fiber optics: multimode and single mode. The names
come from the types of light used in the transmission process. Multimode fiber
uses inexpensive Light Emitting Diodes (LEDs) that produce light of a single
color. Due to the nature of the LED, the light produced is made up of a number of
differing wavelengths of light, fired outward from the center of the LED. Not all
the rays of light enter the fiber, and those that do often do so at an angle, which
reduces the amount of distance the signal can effectively cover. Single mode fiber
optics use lasers to achieve greater maximum distances. Since light from a laser is
all of the same wavelength, and travels in a coherent ray, the resulting signal
tends to be much clearer at reception than an LED signal under the same
circumstances.
Fiber optics of both types are measured and identified by a variety of means. The
usual means of referring to a fiber optic cable type is to identify if it is single mode
or multimode, and to describe the thickness of each strand. Fiber optics are very
thin, and the width of each strand is measured in microns ( m). Two
measurements are important in fiber optic identification: the diameter of the core,
where signals travel, and the diameter of the cladding, which surrounds the core.
Thus, fiber optic measurements will usually provide two numbers separated by
the "/" symbol. The first number is the diameter, in microns, of the core. The
second is the diameter of the cladding. Thus, a 62.5/125 multimode cable is a type
of fiber optic cabling with a 62.5 micron core and 125 micron cladding, which can
be used by inexpensive LED equipment, as it allows multiple modes of light to
pass through it. Incidentally, 62.5/125 m multimode cabling is a very common
type of fiber optics.
9-8
Cladding
Transmissive Core
Figure 9-3. Fiber Optic Cable Construction
PVC Buffer (Jacketing)
1845n07
Cabling Types