Information Technology Reference
In-Depth Information
rounds the fiber and adheres to it. The tight buffer must be removed with a tool sim-
ilar to a wire insulation stripper before the fiber can be terminated in a connector.
Loose buffer cables use buffer tubes to surround and protect each fiber, which is
loose in the tube. A variation of tight buffer cable is intended for breakout of the
fibers without requiring a protection breakout box to safeguard the exposed fibers.
The type of cable encloses each fiber in a breakout jacket that contains its own
strength fibers. The entire assembly is covered with a protective outer jacket. Filler
cords may be used to make the overall cable round. A variation of the breakout
cable is the two-fiber duplex, or
zip cord
, fiber cable that is often used for jumper
or patch cords.
Fiber-optic cables are commonly available with 1 to 36 fibers. At two fibers
and above, even numbers of fibers are the most common (e.g., 2, 4, 6, 12) because
fibers are often used in pairs for full-duplex circuits.
Cables are also available with a combination of optical fibers and metallic
pairs. These special-use cables combine the properties of each type of cable, includ-
ing the advantages and disadvantages.
Fiber-Optic Sizes
The most common fiber size that is used in LAN wiring is multimode 62.5/125 µm
fiber. This number describes the diameter in micrometers, of the fiber core and the
cladding. The tight buffer surrounds the cladding and brings the overall diameter to
about 900 µm. One of the fibers that is specified in TIA/EIA-568-C is 62.5/125 µm.
It can be used for many applications, including Ethernet 10BaseF, 100BaseFX,
1000BaseF, FOIRL, optical Token-Ring, FDDI, and ATM. (See Fig. 5.6.)
Other fiber sizes that are commonly available include 50/125, 85/125, and
100/140 µm fiber. The 50/125 µm fiber was added in TIA/EIA-568-B because it
offers better performance in some applications, such as Gigabit Ethernet. It is
important to use the same type and size of fiber in an installation, since transitions
between sizes can cause excessive loss. Also, the light-emitting transmitters and
receivers are optimized for fiber of a certain size. Using the wrong size fiber will
cause some of the light signal to be lost because of poor coupling. This may result
in marginal operation or even circuit failure.
Fiber-Optical Characteristics
Unlike copper wire, which can carry electrical signals of any frequency from DC to
many megahertz, fiber optic cable is designed to carry light in a range of optical
wavelengths. These wavelengths are typically from around 800 to over 1500
