CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 18 ppsx

Optical Media 139
Figure 3-18 Refraction Example
Total Internal Reflection
A light ray that is being turned on and off to send data 1s and 0s into an optical fiber
must stay inside the fiber until it reaches the far end. The ray must not refract into the
material wrapped around the outside of the fiber because such refraction causes the
loss of part of the ray’s light energy. A design for the fiber that makes the outside sur-
face of the fiber act like a mirror to the light ray moving through the fiber must be
achieved. If any light ray that tries to move out through the side of the fiber is reflected
back into the fiber at an angle that sends it towards the far end of the fiber, this is a
good pipe or wave guide for the light waves, as illustrated in Figure 3-19.
Figure 3-19 Total Internal Reflection
The laws of reflection and refraction tell how to design a fiber that guides the light waves
through the fiber with a minimum energy loss. Two conditions are needed to cause
light rays in a fiber to be reflected back into the fiber with out any loss due to refrac-
tion. These two conditions are as follows:
■ The core (the inside) of the optical fiber has to have a larger (a higher) index of
refraction than the material that surrounds it. The material that surrounds the
core of the fiber is called the cladding.
■ The angle of incidence of the light ray is greater than the critical angle for the
core and its cladding.
When both of these conditions are met, all the incident light in the fiber is reflected back
inside the fiber. This condition is called total internal reflection, which is the foundation
on which optical fiber is constructed. Total internal reflection causes the light rays in the
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140 Chapter 3: Networking Media
fiber to bounce off the core-cladding boundary and continue their journey toward the
far end of the fiber. The light follows something of a zigzag path through the core of
the fiber.
A fiber that meets the first condition (a core with a higher index of refraction than the
cladding) can be easily created. Also, the angle of incidence of the light rays that enter

the core can be controlled. Restricting two factors controls the angle of incidence:
■ The numerical aperture of the fiber. The numerical aperture of a core is the range
of angles of incident light rays entering the fiber that are totally internally
reflected, as illustrated in Figure 3-20.
■ The paths (called the modes) that a light ray can follow when traveling down a
fiber.
Figure 3-20 Numerical Aperture
Controlling conditions 1 and 2 creates a fiber with total internal reflection, consequently
giveing a light wave path that can be used for data communications, as shown in
Figure 3-21.
Figure 3-21 Light Wave Path
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Optical Media 141
Fiber-Optic Cables
Fiber-optic cable is a networking medium that uses modulated light for data transmis-
sions through thin strands of glass. Signals that represent data bits are converted into
beams of light. It is important to recognize that while electricity is required to generate
and interpret the fiber-optic signals at the end devices, no electricity is in the cable itself
as there is with copper media. In fact, fiber-optic cable components are very good insu-
lators. Many characteristics of fiber-optic media are superior to copper.
Every fiber-optic cable used for networking consists of two glass fibers encased in sep-
arate sheaths. One fiber carries transmitted data from device A to device B; the second
fiber carries data from device B to device A. A fiber for data goes in each direction,
similar to two one-way streets going in opposite directions. This arrangement provides
a full-duplex communication link. Just as copper twisted-pair uses separate wire pairs
to transmit (Tx) and receive (Rx), fiber-optic circuits use one fiber strand to transmit
and one to receive, as illustrated in Figure 3-22. Typically, these two fiber cables are in
a single outer jacket until they reach the point at which connectors are attached.
Figure 3-22 Duplex Fiber
At this point, the two fiber cables are separated. No need for twisting or shielding

exists because no light escapes when it is inside a fiber, which means no crosstalk issues
exist with fiber. It is common to see multiple fiber pairs encased in the same cable. This
arrangement allows a single cable to be run between data closets, floors, or buildings.
One cable can contain 2, 4, 8, 12, 24, 48, or more separate fibers. With copper, one
UTP cable has to be pulled for each circuit. Fiber can carry many more bits per second
and carry them farther than copper can.
As illustrated in Figure 3-23 and Figure 3-24, five parts typically make up each fiber-
optic cable:
■ The core
■ The cladding
■ A buffer
■ A strengthening material
■ An outer jacket
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142 Chapter 3: Networking Media
Figure 3-23 Five Elements of a Fiber-Optic Cable
Figure 3-24 Cross Section Showing the Five Elements of a Fiber-Optic Cable
The core is the light transmission element at the center of the optical fiber, and all the
light signals travel through the core. This core is typically glass made from a combina-
tion of silica (silicon dioxide) and other elements. Surrounding the core is the cladding,
also made of silica but with a lower index of refraction than the core. Light rays travel-
ing through the fiber core reflect off this core-to-cladding interface where the core and
cladding meet, which keeps light in the core as it travels down the fiber.
Surrounding the cladding is a buffer material, usually plastic, that helps shield the core
and cladding from damage.
The strengthening material surrounds the buffer, preventing the fiber cable from being
stretched when installers pull it. The material used is often Kevlar, the same material
used to produce bulletproof vests. The final element, the outer jacket, surrounds the
cable to protect the fiber against abrasion, solvents, and other contaminants. This outer
jacket composition can vary depending on the cable usage.

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Optical Media 143
The part of an optical fiber through which light rays travel is called the core of the fiber.
Light rays cannot enter the core of an optical fiber at all angles. The rays can enter the
core only if their angle is inside the fiber’s numerical aperture. Likewise, once the rays
have entered the fiber’s core, a limited number of optical paths exist that a light ray can
follow through the fiber. These optical paths are called modes. If the diameter of a fiber’s
core is large enough so that many paths exist that light can take as it passes through
the fiber, the fiber is called multimode fiber. Single-mode fiber has a much smaller core
that allows light rays to travel along only one path (one mode) inside the fiber. Figure 3-25
illustrates the differences between multimode and single-mode fibers.
Figure 3-25 Single-Mode Versus Multimode
Table 3-3 compares the features of single-mode and multimode fiber.
The next two sections cover the two basic types of optical fiber, multimode and single-
mode, in more detail.
Table 3-3 Features of Single-Mode and Multimode Fiber
Single-Mode Fiber Multimode Fiber
Core Features Small core (10 microns
or less)
Larger core than single-mode
cable (50 or 62.5 microns or
greater)
Dispersion
Characteristics
Less dispersion Allows greater dispersion and,
therefore, loss of signal
Distance
Characteristics
Suited for long-distance
applications (up to 3 kilo-

meters [9,842 feet])
Used for long-distance application,
but shorter than single-mode (up
to 2 kilometers [6,560 feet])
Light Source Uses lasers as the light source
often within campus back-
bones for distances of several
thousand meters
Uses LEDs as the light source,
often within LANs or distances
of a couple hundred meters
within a campus network
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144 Chapter 3: Networking Media
Multimode Fiber
Multimode fiber allows multiple modes (paths) of light to propagate through the fiber-
optic core, as compared to single-mode fiber, which allows only one mode. Multiple
modes of light propagating through fiber might travel different distances, depending
on their entry angles. This angle causes them to arrive at the destination (receiving
end of the cable) at slightly different times—a phenomenon called modal dispersion.
Multimode uses a type of glass, called graded index glass, which has a lower index of
refraction towards the outer edge of the core. This glass causes the light to slow down
when passing through the center of the core and accelerate when passing through the
outer areas of the core, ensuring that all modes of light reach the end at approximately
the same time. This design is used because a light ray following a mode that goes straight
down the center of the core does not have to go as far as a ray following a mode that
bounces around in the fiber. All rays should arrive at the end of the fiber together. Then,
the receiver at the end of the fiber receives a strong flash of light rather than a long,
dim pulse.
A standard multimode fiber-optic cable (the most common type of fiber-optic cable used

in LANs) uses an optical fiber with either a 62.5- or a 50-micron core and a 125-micron
diameter cladding. This cable is commonly designated as 62.5/125 or 50/125 micron
optical fiber. A micron is one millionth of a meter. 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.
Infrared light emitting diodes (LEDs) or vertical cavity surface emitting lasers (VCSELs)
are usually the light source used with multimode fiber. LEDs are a little cheaper to
build and require somewhat less safety concerns than lasers. However, LEDs cannot
transmit light over cable as far as the lasers. Multimode fiber (62.5/125) can carry data
distances of up to 2000 meters (6560 feet). Multimode fiber is mainly used in LAN
applications including backbone cabling.
Lab Activity Optical Fiber Purchase
In this lab, you are introduced to the variety and prices of network cabling
and related components in the market. This lab looks specifically at fiber-optic
patch cables and bulk fiber cable.
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Optical Media 145
Single-Mode Fiber
Single-mode fiber uses only one mode of light to propagate through the fiber-optic
core. In single-mode fiber-optic cabling, the core is much smaller than in multimode.
The single-mode core is 8 to 10 microns in diameter. Nine micron cores are the most
common. A 9/125 marking on a single-mode fiber’s jacket indicates that the core fiber
has a diameter of 9 microns and the surrounding cladding is 125 microns in diameter.
The size of the core in single-mode fiber leaves very little room for light to bounce
around. Furthermore, a very focused infrared laser is used as the light source in single-
mode fiber. The ray of light it generates enters the core at a 90-degree angle. As a result
the data carrying light ray pulses in single-mode fiber are essentially transmitted in a
straight line right down the middle of the core, as shown in Figure 3-26. This greatly
increases both the speed and the distance that data can be transmitted.
Figure 3-26 Single-Mode Fiber

Because of its design, single-mode fiber is capable of higher rates of data transmission
(bandwidth) and greater cable run distances than multimode fiber. Single-mode fiber
can carry LAN data up to 3000 meters. Multimode is only capable of up to 2000 meters.
Lasers and single-mode fibers are more expensive than LEDs and multimode fiber.
Because of these characteristics, single-mode fiber is often used for interbuilding con-
nectivity or WANs (for example, telephone company network connections).
Figure 3-27 compares the relative sizes of the core and cladding for both types of fiber-
optic cable in different sectional views. The much smaller and more refined fiber core
in single-mode fiber, although it entails more manufacturing costs, is the reason single-
mode has a higher bandwidth and cable run distance than multimode fiber.
Figure 3-27 Single-Mode and Multimode Fiber
WARNING
Be aware that the
laser light used with
single-mode has a
longer wavelength
than can be seen, and
it is so strong that it
can seriously damage
eyes. Never look at
the end of a fiber that
is connected to a device
at its far end. Never
look into the transmit
port on a NIC, switch,
or router. There is
nothing that can be
seen anyway. Remem-
ber to keep protective
covers over the ends

of fiber and inserted
into the fiber-optic
ports of switches
and routers. Be very
careful!
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146 Chapter 3: Networking Media
The following summarizes the features of fiber-optic cables:
■ Speed and throughput—More than 1 Gbps
■ Average cost per node—Expensive
■ Media and connector size—Small
■ Maximum cable length—More than 10 kilometers (km) for single mode; up to
2 km for multimode
Cable Designs
As illustrated in Figure 3-28, two basic cable designs exist:
■ Loose-tube
■ Tight-buffered
Figure 3-28 Loose-Tube Versus Tight-Buffer Construction
Tight-buffered cables have the buffering material that surrounds the cladding in direct
contact with the cladding. The main practical difference between the two designs is the
applications for which they are used. Loose-tube cable is primarily used for outside-
building installations, while tight-buffered cable is used inside buildings. Most of the
fiber used in LANs is tight-buffered multimode cable.
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Optical Media 147
Other Optical Networking Components
Most of the data sent over a LAN is in the form of electrical signals. But, optical fiber
links use light to send data. So, something is needed to convert the electricity to light
and, at the other end of the fiber, convert the light back to electricity. This situation
means that two devices are always required: a transmitter and a receiver.

In addition to transmitters and receivers, this section also discusses different types of
fiber-optic cable connectors as well as devices used in optical networking.
Transmitters
The transmitter receives data to be transmitted from switches and routers. This data
is in the form of electrical signals. The transmitter converts the electronic signals into
their equivalent light pulses. Two types of light sources encode and transmit the data
through the cable. These two sources are
■ Light emitting diodes (LEDs)—An LED produces infrared light with wavelengths
of either 850 or 1310 nm. These are used with multimode fiber in LANs. Lenses
are used to focus the infrared light on the end of the fiber. LEDs have fewer safety
concerns.
■ Light amplification by stimulated emission radiation (Laser)—Laser is a light
source producing a thin beam of intense infrared light usually with wavelengths
of 1310 or 1550 nm. Lasers are used with single-mode fiber over the longer dis-
tances involved in WANs or campus backbones. Exercise extra care to prevent
eye injury.
Each light source can be lighted and darkened quickly to send data 1s and 0s at a high
number of bits per second.
Receiver
At the other end of the optical fiber from the transmitter is the receiver. It functions
something like the photoelectric cell in a solar-powered calculator. When light strikes
the receiver, it produces electricity. The first job of the receiver is to detect a light pulse
that arrives from the fiber. Then the receiver converts the light pulse back into the orig-
inal electrical signal that first entered the transmitter at the far end of the fiber. Now
the signal is again in the form of voltage changes. The signal is ready to be sent over
copper wire into any receiving electronic device such as a computer, switch, or router.
The semiconductor devices that are usually used as receivers with fiber-optic links are
called p-intrinsic-n diodes (PIN photodiodes).
PIN photodiodes are manufactured to be sensitive to the particular wavelength of light
(850, 1310, or 1550 nm) generated by the transmitter at the far end of the fiber. When

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148 Chapter 3: Networking Media
struck by a pulse of light at the proper wavelength, the PIN photodiode quickly produces
an electric current of the proper voltage for the network. It instantly stops producing the
voltage when no light strikes the PIN photodiode. This process generates the voltage
changes that represent the data 1s and 0s on a copper cable.
Connectors
Connectors are attached to the fiber ends so that the fibers can be connected to the ports
on the transmitter and receiver. The most common type of connector used with multi-
mode fiber is the subscriber connector (SC ), as shown in Figure 3-29. On single-mode
fiber, the straight tip (ST) connector, as shown in Figure 3-30, is frequently used. With
SC and ST, there is one connector for each fiber. Newer connectors combine the send
and receive fibers into one modular connector, comparable in size to an RJ-45, to save
space.
Figure 3-29 SC Connector
Figure 3-30 ST Connector
Optical Amplifiers and Fiber Patch Panels
In addition to the transmitters, receivers, connectors, and fibers that are always required
on an optical network, you can sometimes see several other devices on an optical fiber
network.
Repeaters are optical amplifiers that receive attenuating light pulses traveling long
distances and restore them to their original shapes, strengths, and timings. Then the
restored signals can be sent on along the journey to the receiver at the far end of
the fiber.
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