used on Metropolitan Area Networks (MANs) and Wide
Area Networks (WANs). Although fiber is the best of all the
transmission media at carrying large amounts of data over long
distances, fiber is not without problems. When light travels
through fiber, some of the light energy is lost. The farther a
light signal travels through a fiber, the more the signal loses
strength. This attenuation of the signal is due to several
factors involving the nature of fiber itself. The most
important factor is scattering. The scattering of light in a
fiber is caused by microscopic non-uniformity (distortions) in
the fiber that reflects and scatters some of the light energy.
Absorption is another cause of light energy loss. When a light
ray strikes some types of chemical impurities in a fiber, the
impurities absorb part of the energy. This light energy is
converted to a small amount of heat energy. Absorption makes
the light signal a little dimmer. Another factor that causes
attenuation of the light signal is manufacturing irregularities
or roughness in the core-to-cladding boundary. Power is lost
from the light signal because of the less than perfect total
internal reflection in that rough area of the fiber. Any
microscopic imperfections in the thickness or symmetry of the
fiber will cut down on total internal reflection and the
cladding will absorb some light energy. Dispersion of a light
flash also limits transmission distances on a fiber. Dispersion
is the technical term for the spreading of pulses of light as
they travel down the fiber. Graded index multimode fiber is
designed to compensate for the different distances the various
modes of light have to travel in the large diameter core.
Single-mode fiber does not have the problem of multiple paths
that the light signal can follow. However, chromatic dispersion
is a characteristic of both multimode and single-mode fiber.
When wavelengths of light travel at slightly different speeds
through glass than do other wavelengths, chromatic dispersion
is caused. That is why a prism separates the wavelengths of
light. Ideally, an LED or Laser light source would emit light
of just one frequency. Then chromatic dispersion would not be a
problem. Unfortunately, lasers, and especially LEDs generate a
range of wavelengths so chromatic dispersion limits the
distance that can be transmitted on a fiber. If a signal is
transmitted too far, what started as a bright pulse of light
energy will be spread out, separated, and dim when it reaches
the receiver. The receiver will not be able to distinguish a
one from a zero.
Content 3.2 Optical Media
3.2.10 Installation, care, and testing of optical
fiber A major cause of too much attenuation in fiber-optic
cable is improper installation. If the fiber is stretched or
curved too tightly, it can cause tiny cracks in the core that
will scatter the light rays. Bending the fiber in too tight a
curve can change the incident angle of light rays striking the
core-to-cladding boundary. Then the incident angle of the ray
will become less than the critical angle for total internal
reflection. Instead of reflecting around the bend, some light
rays will refract into the cladding and be lost. To prevent
fiber bends that are too sharp, fiber is usually pulled through
a type of installed pipe called interducting. The interducting
is much stiffer than fiber and can not be bent so sharply that
the fiber inside the interducting has too tight a curve. The
interducting protects the fiber, makes it easier to pull the
fiber, and ensures that the bending radius (curve limit) of the
fiber is not exceeded. When the fiber has been pulled, the ends
of the fiber must be cleaved (cut) and properly polished to
ensure that the ends are smooth. A microscope or test
instrument with a built in magnifier is used to examine the end
of the fiber and verify that it is properly polished and
shaped. Then the connector is carefully attached to the fiber
end. Improperly installed connectors, improper splices, or the
splicing of two cables with different core sizes will
dramatically reduce the strength of a light signal. Once the
fiber-optic cable and connectors have been installed, the
connectors and the ends of the fibers must be kept spotlessly
clean. The ends of the fibers should be covered with protective
covers to prevent damage to the fiber ends. When these covers
are removed prior to connecting the fiber to a port on a switch
or a router, the fiber ends must be cleaned. Clean the fiber
ends with lint free lens tissue moistened with pure isopropyl
alcohol. The fiber ports on a switch or router should also be
kept covered when not in use and cleaned with lens tissue and
isopropyl alcohol before a connection is made. Dirty ends on a
fiber will cause a big drop in the amount of light that reaches
the receiver. Scattering, absorption, dispersion, improper
installation, and dirty fiber ends diminish the strength of the
light signal and are referred to as fiber noise. Before using a
fiber-optic cable, it must be tested to ensure that enough
light actually reaches the receiver for it to detect the zeros
and ones in the signal. When a fiber-optic link is being
planned, the amount of signal power loss that can be tolerated
must be calculated. This is referred to as the optical link
loss budget. Imagine a monthly financial budget. After all of
the expenses are subtracted from initial income, enough money
must be left to get through the month. The decibel (dB) is the
unit used to measure the amount of power loss. It tells what
percent of the power that leaves the transmitter actually
enters the receiver. Testing fiber links is extremely important
and records of the results of these tests must be kept. Several
types of fiber-optic test equipment are used. Two of the most
important instruments are Optical Loss Meters and Optical Time
Domain Reflectometers (OTDRs). These meters both test optical
cable to ensure that the cable meets the TIA standards for
fiber. They also test to verify that the link power loss does
not fall below the optical link loss budget. OTDRs can provide
much additional detailed diagnostic information about a fiber
link. They can be used to trouble shoot a link when problems
occur. Web Links A New Fiber-Optic Installation
Standard hhttp://www.ecmweb.com/ar/electric_ new_fiberoptic_
installation/
Content 3.3
Wireless Media 3.3.1 Wireless LAN organizations
and standards An understanding of the regulations and
standards that apply to wireless technology will ensure that
deployed networks will be interoperable and in compliance. Just
as in cabled networks, IEEE is the prime issuer of standards
for wireless networks. The standards have been created within
the framework of the regulations created by the Federal
Communications Commission (FCC). A key technology contained
within the 802.11 standard is Direct Sequence Spread Spectrum
(DSSS). DSSS applies to wireless devices operating within a 1
to 2 Mbps range. A DSSS system may operate at up to 11 Mbps but
will not be considered compliant above 2 Mbps. The next
standard approved was 802.11b, which increased transmission
capabilities to 11 Mbps. Even though DSSS WLANs were able to
interoperate with the Frequency Hopping Spread Spectrum (FHSS)
WLANs, problems developed prompting design changes by the
manufacturers. In this case, IEEE’s task was simply to create a
standard that matched the manufacturer’s solution. 802.11b may
also be called Wi-Fi™ or high-speed wireless and refers to DSSS
systems that operate at 1, 2, 5.5 and 11 Mbps. All 802.11b
systems are backward compliant in that they also support 802.11
for 1 and 2 Mbps data rates for DSSS only. This backward
compatibility is extremely important as it allows upgrading of
the wireless network without replacing the NICs or access
points. 802.11b devices achieve the higher data throughput rate
by using a different coding technique from 802.11, allowing for
a greater amount of data to be transferred in the same time
frame. The majority of 802.11b devices still fail to match the
11 Mbps throughput and generally function in the 2 to 4 Mbps