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Overview
Copper cable is used in almost every LAN. Many different types of
copper cable are available, with each type having advantages and
disadvantages. Proper selection of cabling is key to efficient network
operation. Because copper carries information using electrical current,
it is important to understand some basics of electricity when planning
and installing a network.
Optical fiber is the most frequently used medium for the longer, high
bandwidth, point-to-point transmissions required on LAN backbones
and on WANs. Using optical media, light is used to transmit data
through thin glass or plastic fiber. Electrical signals cause a fiber-optic
transmitter to generate the light signals sent down the fiber. The
receiving host receives the light signals and converts them to
electrical signals at the far end of the fiber. However, there is no
electricity in the fiber-optic cable itself. In fact, the glass used in
fiber-optic cable is a very good electrical insulator.
Physical connectivity allowed an increase in productivity by allowing
the sharing of printers, servers, and software. Traditional networked
systems require that the workstation remains stationary permitting
moves only within the limits of the media and office area.
The introduction of wireless technology removes these restraints and
brings true portability to the computing world. Currently, wireless
technology does not provide the high-speed transfers, security, or
uptime reliability of cabled networks. However, flexibility of wireless
has justified the trade off.
Administrators often consider wireless when installing a new network
or when upgrading an existing network. A simple wireless network
could be working just a few minutes after the workstations are turned
on. Connectivity to the Internet is provided through a wired
connection, router, cable or DSL modem and a wireless access point
that acts as a hub for the wireless nodes. In a residential or small
office environment these devices may be combined into a single unit.
Students completing this module should be able to:
 Discuss the electrical properties of matter.
 Define voltage, resistance, impedance, current, and circuits.
 Describe the specifications and performances of different types of
cable.
 Describe coaxial cable and its advantages and disadvantages over
other types of cable.
 Describe shielded twisted-pair (STP) cable and its uses.
 Describe unshielded twisted-pair cable (UTP) and its uses.
 Discuss the characteristics of straight-through, crossover, and
rollover cables and where each is used.
 Explain the basics of fiber-optic cable.
 Describe how fibers can guide light for long distances.
 Describe multimode and single-mode fiber.
 Describe how fiber is installed.
 Describe the type of connectors and equipment used with
fiber-optic cable.
 Explain how fiber is tested to ensure that it will function properly.
 Discuss safety issues dealing with fiber-optics.
Module 3: Networking Media
3.1 Copper Media
3.1.1 Atoms and electrons
All matter is composed of atoms. The Periodic Table of Elements
lists all known types of atoms and their properties. The atom is
comprised of:
 Electrons – Particles with a negative charge that orbit the nucleus
 Nucleus – The center part of the atom, composed of protons and
neutrons
 Protons – Particles with a positive charge
 Neutrons – Particles with no charge (neutral)
To help explain the electrical properties of elements/materials, locate
helium (He) on the periodic table. Helium has an atomic number of
2, which means that helium has 2 protons and 2 electrons. It has an
atomic weight of 4. By subtracting the atomic number (2) from the
atomic weight (4), it is learned that helium also has 2 neutrons.
The Danish physicist, Niels Bohr, developed a simplified model to
illustrate the atom. This illustration shows the model for a helium
atom. If the protons and neutrons of an atom were the size of an adult
(#5) soccer ball in the middle of a soccer field, the only thing smaller
than the ball would be the electrons. The electrons would be the size
of cherries and would be orbiting near the outer-most seats of the
stadium. In other words, the overall volume of this atom, including the
electron path, would be about the size of the stadium. The nucleus of
the atom where the protons and neutrons exist would be the size of
the soccer ball.
One of the laws of nature, called Coulomb's Electric Force Law,
states that opposite charges react to each other with a force that
causes them to be attracted to each other. Like charges react to each
other with a force that causes them to repel each other. In the case of
opposite and like charges, the force increases as the charges move
closer to each other. The force is inversely proportional to the square
of the separation distance. When particles get extremely close
together, nuclear force overrides the repulsive electrical force and
keeps the nucleus together. That is why a nucleus does not fly apart.
Examine Bohr's model of the helium atom. If Coulomb's law is true,
and if Bohr's model describes helium atoms as stable, then there
must be other laws of nature at work. How can they both be true?
 Coulomb's Law – Opposite charges attract and like charges repel.
 Bohr’s model – Protons are positive charges and electrons are
negative charges. There is more than 1 proton in the nucleus.
Electrons stay in orbit, even though the protons attract the electrons.
The electrons have just enough velocity to keep orbiting and not be
pulled into the nucleus, just like the moon around the Earth.
Protons do not fly apart from each other because of a nuclear force
that is associated with neutrons. The nuclear force is an incredibly
strong force that acts as a kind of glue to hold the protons together.
The protons and neutrons are bound together by a very powerful
force. However, the electrons are bound to their orbit around the
nucleus by a weaker force. Electrons in certain atoms, such as
metals, can be pulled free from the atom and made to flow. This sea
of electrons, loosely bound to the atoms, is what makes electricity
possible. Electricity is a free flow of electrons.
Loosened electrons that stay in one place, without moving, and with a
negative charge, are called static electricity. If these static electrons
have an opportunity to jump to a conductor, this can lead to
electrostatic discharge (ESD). A discussion on conductors follows
later in this chapter.
ESD, though usually harmless to people, can create serious
problems for sensitive electronic equipment. A static discharge can
randomly damage computer chips, data, or both. The logical circuitry
of computer chips is extremely sensitive to electrostatic discharge.
Use caution when working inside a computer, router, and so on.
Atoms, or groups of atoms called molecules, can be referred to as
materials. Materials are classified as belonging to one of three groups
depending on how easily electricity, or free electrons, flows through
them.
The basis for all electronic devices is the knowledge of how insulators,
conductors and semiconductors control the flow of electrons and
work together in various combinations.
Periodic Table of Elements
Helium Atom
Forces within the Atom
Static Electricity: Loose Electrons at Reset
3.1.2 Voltage
Voltage is sometimes referred to as electromotive force (EMF). EMF
is related to an electrical force, or pressure, that occurs when
electrons and protons are separated. The force that is created
pushes toward the opposite charge and away from the like charge.
This process occurs in a battery, where chemical action causes
electrons to be freed from the negative terminal of the battery. The
electrons then travel to the opposite, or positive, terminal through an
EXTERNAL circuit. The electrons do not travel through the battery
itself. Remember that the flow of electricity is really the flow of
electrons. Voltage can also be created in three other ways. The first
is by friction, or static electricity. The second way is by magnetism, or
electric generator. The last way that voltage can be created is by light,
or solar cell.
Voltage is related to the electrical fields emanating from the charges
associated with particles such as protons, electrons, etc. Voltage is
represented by the letter V, and sometimes by the letter E, for
electromotive force. The unit of measurement for voltage is volt (V).
Volt is defined as the amount of work, per unit charge, needed to
separate the charges.
Voltage
3.1.3 Resistance and impedance
The materials through which current flows offer varying amounts of
opposition, or resistance to the movement of the electrons. The
materials that offer very little, or no, resistance, are called conductors.
Those materials that do not allow the current to flow, or severely
restrict its flow, are called insulators. The amount of resistance
depends on the chemical composition of the materials.
All materials that conduct electricity have a measure of resistance to
the flow of electrons through them. These materials also have other
effects called capacitance and inductance associated with the flow of
electrons. The three characteristics comprise impedance, which is
similar to and includes resistance.
The term attenuation is important when learning about networks.
Attenuation refers to the resistance to the flow of electrons and why a
signal becomes degraded as it travels along the conduit.
The letter R represents resistance. The unit of measurement for
resistance is the ohm (). The symbol comes from the Greek letter ,
omega.
Electrical insulators, or insulators, are materials that allow electrons
to flow through them with great difficulty, or not at all. Examples of
electrical insulators include plastic, glass, air, dry wood, paper, rubber,
and helium gas. These materials have very stable chemical
structures, with orbiting electrons tightly bound within the atoms.
Electrical conductors, usually just called conductors, are materials
that allow electrons to flow through them with great ease. They flow
easily because the outermost electrons are bound very loosely to the
nucleus, and are easily freed. At room temperature, these materials
have a large number of free electrons that can provide conduction.
The introduction of voltage causes the free electrons to move,
resulting in a current flow.
The periodic table categorizes some groups of atoms by listing them
in the form of columns. The atoms in each column belong to
particular chemical families. Although they may have different
numbers of protons, neutrons, and electrons, their outermost
electrons have similar orbits and behave similarly when interacting
with other atoms and molecules. The best conductors are metals,
such as copper (Cu), silver (Ag), and gold (Au), because they have
electrons that are easily freed. Other conductors include solder, a
mixture of lead (Pb) and tin (Sn), and water with ions. An ion is an
atom that has more electrons, or fewer electrons, than the number of
protons in the nucleus of the atom. The human body is made of
approximately 70% water with ions, which means that the human
body is a conductor.
Semiconductors are materials where the amount of electricity they
conduct can be precisely controlled. These materials are listed
together in one column of the periodic chart. Examples include
carbon (C), germanium (Ge), and the alloy, gallium arsenide (GaAs).
The most important semiconductor which makes the best
microscopic-sized electronic circuits is silicon (Si).
Silicon is very common and can be found in sand, glass, and many
types of rocks. The region around San Jose, California is known as
Silicon Valley because the computer industry, which depends on
silicon microchips, started in that area.
Resistance and Impedance
Insulators, Conductors, Semiconductors
3.1.4 Current
Electrical current is the flow of charges created when electrons
move. In electrical circuits, the current is caused by a flow of free
electrons. When voltage, or electrical pressure, is applied and there is
a path for the current, electrons move from the negative terminal
along the path to the positive terminal. The negative terminal repels
the electrons and the positive terminal attracts the electrons. The
letter “I” represents current. The unit of measurement for current is
Ampere (Amp). Amp is defined as the number of charges per second
that pass by a point along a path.
If amperage or current can be thought of as the amount or volume of
electron traffic that is flowing, then voltage can be thought of as the
speed of the electron traffic. The combination of amperage and
voltage equals wattage. Electrical devices such as light bulbs, motors
and computer power supplies are rated in terms of watts. A watt is
how much power a device consumes or produces.
It is the current or amperage in an electrical circuit that really does the
work. As an example, static electricity has very high voltage, so much
that it can jump a gap of an inch or more. However, it has very low
amperage and as a result can create a shock but not permanent
injury. The starter motor in an automobile operates at a relatively low
12 volts but requires very high amperage to generate enough energy
to turn over the engine. Lightning has very high voltage and high
amperage and can do severe damage or injury.
Current
3.1.5 Circuits
Current flows in closed loops called circuits. These circuits must be
composed of conducting materials, and must have sources of voltage.
Voltage causes current to flow, while resistance and impedance
oppose it. Current consists of electrons flowing away from negative
terminals and towards positive terminals. Knowing these facts allows
people to control a flow of current.
Electricity will naturally flow to the earth if there is a path. Current also
flows along the path of least resistance. If a human body provides the
path of least resistance, the current will flow through it. When an
electric appliance has a plug with three prongs, one of the three
prongs serves as the ground, or zero volts. The ground provides a
conducting path for the electrons to flow to the earth because the
resistance traveling through the body would be greater than the
resistance flowing directly to the ground.
Ground typically means the zero volts level, when making electrical
measurements. Voltage is created by the separation of charges,
which means that voltage measurements must be made between two
points.
A water analogy helps to explain concepts of electricity. The higher
the water and the greater the pressure, the more the water will flow.
The water current also depends on the size of the space it must flow
through. Similarly, the higher the voltage and the greater the electrical
pressure, the more current will be produced. The electric current then
encounters resistance that, like the water tap, reduces the flow. If the
electric current is in an AC circuit, then the amount of current will
depend on how much impedance is present. If the electric current is
in a DC circuit, then the amount of current will depend on how much
resistance is present. The pump is like a battery. It provides pressure
to keep the flow moving.
The relationship among voltage, resistance, and current is voltage (V)
= current (I) multiplied by resistance (R). In other words, V=I*R. This
is Ohm’s law, named after the scientist who explored these issues.
Two ways in which current flows are Alternating Current (AC) and
Direct Current (DC). Alternating current (AC) and voltages vary over
time by changing their polarity, or direction. AC flows in one direction,
then reverses its direction and flows in the other direction, and then
repeats the process. AC voltage is positive at one terminal, and
negative at the other. Then the AC voltage reverses its polarity, so
that the positive terminal becomes negative, and the negative
terminal becomes positive. This process repeats itself continuously.
DC always flows in the same direction, and DC voltages always have
the same polarity. One terminal is always positive, and the other is
always negative. They do not change or reverse.
An oscilloscope is an electronic device used to measure electrical
signals relative to time. An oscilloscope graphs the electrical waves,
pulses, and patterns. An oscilloscope has an x-axis that represents
time, and a y-axis that represents voltage. There are usually two
y-axis voltage inputs so that two waves can be observed and
measured at the same time.
Power lines carry electricity in the form of AC because it can be
delivered efficiently over large distances. DC can be found in
flashlight batteries, car batteries, and as power for the microchips on
the motherboard of a computer, where it only needs to go a short
distance.
Electrons flow in closed circuits, or complete loops. Figure shows a
simple circuit. The chemical processes in the battery cause charges
to build up. This provides a voltage, or electrical pressure, that
enables electrons to flow through various devices. The lines
represent a conductor, which is usually copper wire. Think of a switch
as two ends of a single wire that can be opened or broken to prevent
electrons from flowing. When the two ends are closed, fixed, or
shorted, electrons are allowed to flow. Finally, a light bulb provides
resistance to the flow of electrons, causing the electrons to release
energy in the form of light. The circuits involved in networking use a
much more complex version of this very simple circuit.
For AC and DC electrical systems, the flow of electrons is always
from a negatively charged source to a positively charged source.
However, for the controlled flow of electrons to occur, a complete
circuit is required. Remember, electrical current follows the path of
least resistance. Figure shows part of the electrical circuit that
brings power to a home or office.
Water Analogy for Electricity
Oscilloscope
Series Circuit: Flashlight
Grounding of Network Equipment
3.1.6 Cable specifications
Cables have different specifications and expectations pertaining to
performance:
 What speeds for data transmission can be achieved using a
particular type of cable? The speed of bit transmission through the
cable is extremely important. The speed of transmission is affected
by the kind of conduit used.
 What kind of transmission is being considered? Will the
transmissions be digital or will they be analog-based? Digital or
baseband transmission and analog-based or broadband
transmission are the two choices.
 How far can a signal travel through a particular type of cable before
attenuation of that signal becomes a concern? In other words, will
the signal become so degraded that the recipient device might not
be able to accurately receive and interpret the signal by the time
the signal reaches that device? The distance the signal travels
through the cable directly affects attenuation of the signal.
Degradation of the signal is directly related to the distance the
signal travels and the type of cable used.
Some examples of Ethernet specifications which relate to cable type
include:
 10BASE-T
 10BASE5
 10BASE2
10BASE-T refers to the speed of transmission at 10 Mbps. The type
of transmission is baseband, or digitally interpreted. The T stands for
twisted pair.
10BASE5 refers to the speed of transmission at 10 Mbps. The type of
transmission is baseband, or digitally interpreted. The 5 represents
the capability of the cable to allow the signal to travel for
approximately 500 meters before attenuation could disrupt the ability
of the receiver to appropriately interpret the signal being received.
10BASE5 is often referred to as Thicknet. Thicknet is actually a type
of network, while 10BASE5 is the cabling used in that network.
10BASE2 refers to the speed of transmission at 10 Mbps. The type of
transmission is baseband, or digitally interpreted. The 2, in 10BASE2,
represents the capability of the cable to allow the signal to travel for
approximately 200 meters, before attenuation could disrupt the ability
of the receiver to appropriately interpret the signal being received.
10BASE2 is often referred to as Thinnet. Thinnet is actually a type of
network, while 10BASE2 is the cabling used in that network.
Cable Specifications
3.1.7 Coaxial cable
Coaxial cable consists of a hollow outer cylindrical conductor that
surrounds a single inner wire made of two conducting elements. One
of these elements, located in the center of the cable, is a copper
conductor. Surrounding the copper conductor is a layer of flexible
insulation. Over this insulating material is a woven copper braid or
metallic foil that acts as the second wire in the circuit and as a shield
for the inner conductor. This second layer, or shield reduces the
amount of outside electro-magnetic interference. Covering this shield
is the cable jacket.
For LANs, coaxial cable offers several advantages. It can be run
longer distances than shielded twisted pair, STP, and unshielded
twisted pair, UTP, cable without the need for repeaters. Repeaters
regenerate the signals in a network so that they can cover greater
distances. Coaxial cable is less expensive than fiber-optic cable, and
the technology is well known. It has been used for many years for
many types of data communication, including cable television.
When working with cable, it is important to consider its size. As the
thickness of the cable increases, so does the difficulty in working with
it. Remember that cable must be pulled through existing conduits and
troughs that are limited in size. Coaxial cable comes in a variety of
sizes. The largest diameter was specified for use as Ethernet
backbone cable, because it has a greater transmission length and
noise rejection characteristics. This type of coaxial cable is frequently
referred to as thicknet. As its nickname suggests, this type of cable
can be too rigid to install easily in some situations. Generally, the
more difficult the network media is to install, the more expensive it is
to install. Coaxial cable is more expensive to install than twisted-pair
cable. Thicknet cable is almost never used anymore, except for
special purpose installations.
In the past, ‘thinnet’ coaxial cable with an outside diameter of only
0.35 cm was used in Ethernet networks. It was especially useful for
cable installations that required the cable to make many twists and
turns. Since thinnet was easier to install, it was also cheaper to install.
This led some people to refer to it as cheapernet. The outer copper or
metallic braid in coaxial cable comprises half the electric circuit and
special care must be taken to ensure a solid electrical connection at
both ends resulting in proper grounding. Poor shield connection is
one of the biggest sources of connection problems in the installation
of coaxial cable. Connection problems result in electrical noise that
interferes with signal transmittal on the networking media. For this
reason thinnet is no longer commonly used nor supported by latest
standards (100 Mbps and higher) for Ethernet networks.
Coaxial Cable
3.1.8 STP cable
Shielded twisted-pair cable (STP) combines the techniques of
shielding, cancellation, and twisting of wires. Each pair of wires is
wrapped in metallic foil. The four pairs of wires are wrapped in an
overall metallic braid or foil. It is usually 150-Ohm cable. As specified
for use in Ethernet network installations, STP reduces electrical noise
within the cable such as pair to pair coupling and crosstalk. STP also
reduces electronic noise from outside the cable, for example
electromagnetic interference (EMI) and radio frequency interference
(RFI). Shielded twisted-pair cable shares many of the advantages
and disadvantages of unshielded twisted-pair cable (UTP). STP
affords greater protection from all types of external interference, but is
more expensive and difficult to install than UTP.
A new hybrid of UTP with traditional STP is Screened UTP (ScTP),
also known as Foil Twisted Pair (FTP). ScTP is essentially UTP
wrapped in a metallic foil shield, or screen. It is usually 100-Ohm or
120-Ohm cable.
The metallic shielding materials in STP and ScTP need to be
grounded at both ends. If improperly grounded or if there are any
discontinuities in the entire length of the shielding material, STP and
ScTP become susceptible to major noise problems. They are
susceptible because they allow the shield to act like an antenna
picking up unwanted signals. However, this effect works both ways.
Not only does the shield prevent incoming electromagnetic waves
from causing noise on data wires, but it also minimizes the outgoing
radiated electromagnetic waves. These waves could cause noise in
other devices. STP and ScTP cable cannot be run as far as other
networking media, such as coaxial cable or optical fiber, without the
signal being repeated. More insulation and shielding combine to
considerably increase the size, weight, and cost of the cable. The
shielding materials make terminations more difficult and susceptible
to poor workmanship. However, STP and ScTP still have a role,
especially in Europe.
Shielded Twisted-Pair Cable
ScTP (Screened Twisted Pair)
3.1.9 UTP cable
Unshielded twisted-pair cable (UTP) is a four-pair wire medium
used in a variety of networks. Each of the 8 individual copper wires in
the UTP cable is covered by insulating material. In addition, each pair
of wires is twisted around each other. This type of cable relies solely
on the cancellation effect produced by the twisted wire pairs, to limit
signal degradation caused by EMI and RFI. To further reduce
crosstalk between the pairs in UTP cable, the number of twists in the
wire pairs varies. Like STP cable, UTP cable must follow precise
specifications as to how many twists or braids are permitted per foot
of cable.
TIA/EIA-568-A contains specifications governing cable performance.
It calls for running two cables, one for voice and one for data, to each
outlet. Of the two cables, the one for voice must be four-pair UTP.
CAT 5 is the one most frequently recommended and implemented in
installations today.
Unshielded twisted-pair cable has many advantages. It is easy to
install and is less expensive than other types of networking media. In
fact, UTP costs less per meter than any other type of LAN cabling.
However, the real advantage is the size. Since it has such a small
external diameter, UTP does not fill up wiring ducts as rapidly as
other types of cable. This can be an extremely important factor to
consider, particularly when installing a network in an older building. In
addition, when UTP cable is installed using an RJ-45 connector,
potential sources of network noise are greatly reduced and a good
solid connection is practically guaranteed. There are disadvantages
in using twisted-pair cabling. UTP cable is more prone to electrical
noise and interference than other types of networking media, and the
distance between signal boosts is shorter for UTP than it is for coaxial
and fiber optic cables.
UTP was once considered slower at transmitting data than other
types of cable. This is no longer true. In fact, today, UTP is
considered the fastest copper-based media.
When communication occurs, the signal that is transmitted by the
source needs to be understood by the destination. This is true from
both a software and physical perspective. The transmitted signal
needs to be properly received by the circuit connection designed to
receive signals. The transmit pin of the source needs to ultimately
connect to the receiving pin of the destination. The following are the
types of cable connections used between internetwork devices.
In Figure 3 , a LAN switch is connected to a computer. The cable that
connects from the switch port to the computer NIC port is called a
straight-through cable.
In Figure 5 , two switches are connected together. The cable that
connects from one switch port to another switch port is called a
crossover cable.
In Figure 7 , the cable that connects the RJ-45 adapter on the com
port of the computer to the console port of the router or switch is
called a rollover cable.
The cables are defined by the type of connections, or pinouts, from
one end to the other end of the cable. See images two, four, and six.
A technician can compare both ends of the same cable by placing
them next to each other, provided the cable has not yet been placed
in a wall. The technician observes the colors of the two RJ-45
connections by placing both ends with the clip placed into the hand
and the top of both ends of the cable pointing away from the
technician. A straight through cable should have both ends with
identical color patterns. While comparing the ends of a cross-over
cable, the color of pins #1 and #2 will appear on the other end at pins
#3 and #6, and vice-versa. This occurs because the transmit and
receive pins are in different locations. On a rollover cable, the color
combination from left to right on one end should be exactly opposite
to the color combination on the other end.
Unshielded Twisted-Pair Cable
UTP Cabling
Connecting Different Devices
Straight-through Cable Pinout
Connecting Similar Devices
Crossover Cable
Connecting to a Console Port
Rollover Cable
3.2 Optical Media
3.2.1 The electromagnetic spectrum
The light used in optical fiber networks is one type of
electromagnetic energy. When an electric charge moves back and
forth, or accelerates, a type of energy called electromagnetic energy
is produced. This energy in the form of waves can travel through a
vacuum, the air, and through some materials like glass. An important
property of any energy wave is the wavelength.
Radio, microwaves, radar, visible light, x-rays, and gamma rays seem
to be very different things. However, they are all types of
electromagnetic energy. If all the types of electromagnetic waves are
arranged in order from the longest wavelength down to the shortest
wavelength, a continuum called the electromagnetic spectrum is
created.
The wavelength of an electromagnetic wave is determined by how
frequently the electric charge that generates the wave moves back
and forth. If the charge moves back and forth slowly, the wavelength
it generates is a long wavelength. Visualize the movement of the
electric charge as like that of a stick in a pool of water. If the stick is
moved back and forth slowly, it will generate ripples in the water with
a long wavelength between the tops of the ripples. If the stick is
moved back and forth more rapidly, the ripples will have a shorter
wavelength.
Because electromagnetic waves are all generated in the same way,
they share many of the same properties. They all travel at a rate of
300,000 kilometers per second (186,283 miles per second) through a
vacuum.
Human eyes were designed to only sense electromagnetic energy
with wavelengths between 700 nanometers and 400 nanometers
(nm). A nanometer is one billionth of a meter (0.000000001 meter) in
length. Electromagnetic energy with wavelengths between 700 and
400 nm is called visible light. The longer wavelengths of light that are
around 700 nm are seen as the color red. The shortest wavelengths
that are around 400 nm appear as the color violet. This part of the
electromagnetic spectrum is seen as the colors in a rainbow.
Wavelengths that are not visible to the human eye are used to
transmit data over optical fiber. These wavelengths are slightly longer
than red light and are called infrared light. Infrared light is used in TV
remote controls. The wavelength of the light in optical fiber is either
850 nm, 1310 nm, or 1550 nm. These wavelengths were selected
because they travel through optical fiber better than other
wavelengths.
Wavelengh
Electromagnetic Spectrum
Visible Spectrum
3.2.2 Ray model of light
When electromagnetic waves travel out from a source, they travel in
straight lines. These straight lines pointing out from the source are
called rays.
Think of light rays as narrow beams of light like those produced by
lasers. In the vacuum of empty space, light travels continuously in a
straight line at 300,000 kilometers per second. However, light travels
at different, slower speeds through other materials like air, water, and
glass. When a light ray called the incident ray, crosses the boundary
from one material to another, some of the light energy in the ray will
be reflected back. That is why you can see yourself in window glass.
The light that is reflected back is called the reflected ray.
The light energy in the incident ray that is not reflected will enter the
glass. The entering ray will be bent at an angle from its original path.
This ray is called the refracted ray. How much the incident light ray is
bent depends on the angle at which the incident ray strikes the
surface of the glass and the different rates of speed at which light
travels through the two substances.
The bending of light rays at the boundary of two substances is the
reason why light rays are able to travel through an optical fiber even if
the fiber curves in a circle.
The optical density of the glass determines how much the rays of light
in the glass bends. Optical density refers to how much a light ray
slows down when it passes through a substance. The greater the
optical density of a material, the more it slows light down from its
speed in a vacuum. The ratio of the speed of light in a material to the
speed of light in a vacuum is called the Index of Refraction. Therefore,
the measure of the optical density of a material is the index of
refraction of that material. A material with a large index of refraction is
more optically dense and slows down more light than a material with
a smaller index of refraction.
For a substance like glass, the Index of Refraction, or the optical
density, can be made larger by adding chemicals to the glass. Making
the glass very pure can make the index of refraction smaller. The
next lessons will provide further information about reflection and
refraction, and their relation to the design and function of optical fiber.
The Ray Model of Light
Index of Refraction
3.2.3 Reflection
When a ray of light (the incident ray) strikes the shiny surface of a
flat piece of glass, some of the light energy in the ray is reflected.
The angle between the incident ray and a line perpendicular to the
surface of the glass at the point where the incident ray strikes the
glass is called the angle of incidence. The perpendicular line is called
the normal. It is not a light ray but a tool to allow the measurement of
angles. The angle between the reflected ray and the normal is called
the angle of reflection. The Law of Reflection states that the angle of
reflection of a light ray is equal to the angle of incidence. In other
words, the angle at which a light ray strikes a reflective surface
determines the angle that the ray will reflect off the surface.
Reflection
Reflection
3.2.4 Refraction
When a light strikes the interface between two transparent materials,
the light divides into two parts. Part of the light ray is reflected back
into the first substance, with the angle of reflection equaling the angle
of incidence. The remaining energy in the light ray crosses the
interface and enters into the second substance.
If the incident ray strikes the glass surface at an exact 90-degree
angle, the ray goes straight into the glass. The ray is not bent.
However, if the incident ray is not at an exact 90-degree angle to the
surface, then the transmitted ray that enters the glass is bent. The
bending of the entering ray is called refraction. How much the ray is
refracted depends on the index of refraction of the two transparent
materials. If the light ray travels from a substance whose index of
refraction is smaller, into a substance where the index of refraction is
larger, the refracted ray is bent towards the normal. If the light ray
travels from a substance where the index of refraction is larger into a
substance where the index of refraction is smaller, the refracted ray is
bent away from the normal.
Consider a light ray moving at an angle other than 90 degrees
through the boundary between glass and a diamond. The glass has
an index of refraction of about 1.523. The diamond has an index of
refraction of about 2.419. Therefore, the ray that continues into the
diamond will be bent towards the normal. When that light ray crosses
the boundary between the diamond and the air at some angle other
than 90 degrees, it will be bent away from the normal. The reason for
this is that air has a lower index of refraction, about 1.000 than the
index of refraction of the diamond.
Refractioin
Refraction
3.2.5 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. The refraction would cause the loss of part of the
light energy of the ray. A design must be achieved for the fiber that
will make the outside surface of the fiber act like a mirror to the light
ray moving through the fiber. If any light ray that tries to move out
through the side of the fiber were reflected back into the fiber at an
angle that sends it towards the far end of the fiber, this would be a
good “pipe” or “wave guide” for the light waves.
The laws of reflection and refraction illustrate how to design a fiber
that guides the light waves through the fiber with a minimum energy
loss. The following two conditions must be met for the light rays in a
fiber to be reflected back into the fiber without any loss due to
refraction:
 The core of the optical fiber has to have a larger index of refraction
(n) 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, the entire incident light in the
fiber is reflected back inside the fiber. This is called total internal
reflection, which is the foundation upon which optical fiber is
constructed. Total internal reflection causes the light rays in the fiber
to bounce off the core-cladding boundary and continue its journey
towards the far end of the fiber. The light will follow a zigzag path
through the core of the fiber.
A fiber that meets the first condition can be easily created. In addition,
the angle of incidence of the light rays that enter the core can be
controlled. Restricting the following 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 will be completely reflected.
 Modes – The paths which a light ray can follow when traveling
down a fiber.
By controlling both conditions, the fiber run will have total internal
reflection. This gives a light wave guide that can be used for data
communications.
Total Internal Reflection
Total Internal Reflection
Numerical Aperture
Critical Angle
3.2.6 Multimode fiber
The part of an optical fiber through which light rays travel is called
the core of the fiber. Light rays can only enter the core if their angle
is inside the numerical aperture of the fiber. Likewise, once the rays
have entered the core of the fiber, there are a limited number of
optical paths that a light ray can follow through the fiber. These
optical paths are called modes. If the diameter of the core of the fiber
is large enough so that there are many paths that light can take
through the fiber, the fiber is called “multimode” fiber. Single-mode
fiber has a much smaller core that only allows light rays to travel
along one mode inside the fiber.
Every fiber-optic cable used for networking consists of two glass
fibers encased in separate sheaths. One fiber carries transmitted
data from device A to device B. The second fiber carries data from
device B to device A. The fibers are similar to two one-way streets
going in opposite directions. This provides a full-duplex
communication link. Just as copper twisted-pair uses separate wire
pairs to transmit and receive, fiber-optic circuits use one fiber strand
to transmit and one to receive. Typically, these two fiber cables will
be in a single outer jacket until they reach the point at which
connectors are attached.
Until the connectors are attached, there is no need for twisting or
shielding, because no light escapes when it is inside a fiber. This
means there are no crosstalk issues with fiber. It is very common to
see multiple fiber pairs encased in the same cable. This allows a
single cable to be run between data closets, floors, or buildings. One
cable can contain 2 to 48 or more separate fibers. With copper, one
UTP cable would have to be pulled for each circuit. Fiber can carry
many more bits per second and carry them farther than copper can.
Usually, five parts make up each fiber-optic cable. The parts are the
core, the cladding, a buffer, a strength material, and an outer jacket.
The core is the light transmission element at the center of the optical
fiber. All the light signals travel through the core. A core is typically
glass made from a combination of silicon dioxide (silica) and other
elements. Multimode uses a type of glass, called graded index glass
for its core. This glass has a lower index of refraction towards the
outer edge of the core. Therefore, the outer area of the core is less
optically dense than the center and light can go faster in the outer
part of the core. This design is used because a light ray following a
mode that goes straight down the center of the core does not have as
far to travel 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.
Surrounding the core is the cladding. Cladding is also made of silica
but with a lower index of refraction than the core. Light rays traveling
through the fiber core reflect off this core-to-cladding interface as they
move through the fiber by total internal reflection. Standard
multimode fiber-optic cable is the most common type of fiber-optic
cable used in LANs. A standard multimode fiber-optic cable uses an
optical fiber with either a 62.5 or a 50-micron core and a 125-micron
diameter cladding. This is commonly designated as 62.5/125 or
50/125 micron optical fiber. A micron is one millionth of a meter (1µ).
Surrounding the cladding is a buffer material that is usually plastic.
The buffer material helps shield the core and cladding from damage.
There are two basic cable designs. They are the loose-tube and the
tight-buffered cable designs. Most of the fiber used in LANs is
tight-buffered multimode cable. Tight-buffered cables have the
buffering material that surrounds the cladding in direct contact with
the cladding. The most 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.
The strength 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 is the outer jacket. The outer jacket surrounds the
cable to protect the fiber against abrasion, solvents, and other
contaminants. The color of the outer jacket of multimode fiber is
usually orange, but occasionally another color.
Infrared Light Emitting Diodes (LEDs) or Vertical Cavity Surface
Emitting Lasers (VCSELs) are two types of light source usually used
with multimode fiber. Use one or the other. 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 (6,560 ft).
Fiber Optic
Single-Mode Versus Multimode
Multimode and Single-Mode
Duplex Fiber
Fiber-Optic Cable Connector
Cross-Section Showing the Layers
Dispersion
Optical Cable Design
3.2.7 Single-mode fiber
Single-mode fiber consists of the same parts as multimode. The
outer jacket of single-mode fiber is usually yellow. The major
difference between multimode and single-mode fiber is that
single-mode allows only one mode of light to propagate through the
smaller, fiber-optic core. The single-mode core is eight to ten microns
in diameter. Nine-micron cores are the most common. A 9/125
marking on the jacket of the single-mode fiber indicates that the core
fiber has a diameter of 9 microns and the surrounding cladding is 125
microns in diameter.
An 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. This greatly increases both the speed and the distance that
data can be transmitted.
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 carrying 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 inter-building connectivity.
Warning: The laser light used with single-mode has a longer
wavelength than can be seen. The laser is so strong that it can
seriously damage eyes. Never look at the near end of a fiber that is
connected to a device at the far end. Never look into the transmit port
on a NIC, switch, or router. Remember to keep protective covers over
the ends of fiber and inserted into the fiber-optic ports of switches and
routers. Be very careful.
Figure compares the relative sizes of the core and cladding for both
types of fiber optic in different sectional views. The much smaller and
more refined fiber core in single-mode fiber is the reason single-mode
has a higher bandwidth and cable run distance than multimode fiber.
However, it entails more manufacturing costs.
Single-mode Fiber
Single-Mode and Multimode Fiber
3.2.8 Other optical components
Most of the data sent over a LAN is in the form of electrical signals.
However, optical fiber links use light to send data. Something is
needed to convert the electricity to light and at the other end of the
fiber convert the light back to electricity. This means that a transmitter
and a receiver are required.
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. There
are two types of light sources used to encode and transmit the data
through the cable:
 A light emitting diode (LED) producing infrared light with
wavelengths of either 850nm 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
 Light amplification by stimulated emission radiation (LASER) a light
source producing a thin beam of intense infrared light usually with
wavelengths of 1310nm or 1550 nm. Lasers are used with
single-mode fiber over the longer distances involved in WANs or
campus backbones. Extra care should be exercised to prevent eye
injury
Each of these light sources can be lighted and darkened very quickly
to send data (1s and 0s) at a high number of bits per second.
At the other end of the optical fiber from the transmitter is the receiver.
The receiver 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 original 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 850, 1310, or
1550 nm of light that are generated by the transmitter at the far end of
the fiber. When 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 generates the voltage
changes that represent the data 1s and 0s on a copper cable.
Connectors are attached to the fiber ends so that the fibers can be
connected to the ports on the transmitter and receiver. The type of
connector most commonly used with multimode fiber is the
Subscriber Connector (SC connector). On single-mode fiber, the
Straight Tip (ST) connector is frequently used.
In addition to the transmitters, receivers, connectors, and fibers that
are always required on an optical network, repeaters and fiber patch
panels are often seen.
Repeaters are optical amplifiers that receive attenuating light pulses
traveling long distances and restore them to their original shapes,
strengths, and timings. The restored signals can then be sent on
along the journey to the receiver at the far end of the fiber.
Fiber patch panels similar to the patch panels used with copper cable.
These panels increase the flexibility of an optical network by allowing
quick changes to the connection of devices like switches or routers
with various available fiber runs, or cable links.
Transmission Devices
ST and SC Connectors
Fiber-Optic Connectors
Fiber-Optic Patch Pannels
Fiber-Optic Patch Pannels
3.2.9 Signals and noise in optical fibers
Fiber-optic cable is not affected by the sources of external noise that
cause problems on copper media because external light cannot enter
the fiber except at the transmitter end. A buffer and an outer jacket
that stops light from entering or leaving the cable cover the cladding.
Furthermore, the transmission of light on one fiber in a cable does not
generate interference that disturbs transmission on any other fiber.
This means that fiber does not have the problem with crosstalk that
copper media does. In fact, the quality of fiber-optic links is so good
that the recent standards for gigabit and ten gigabit Ethernet specify
transmission distances that far exceed the traditional two-kilometer
reach of the original Ethernet. Fiber-optic transmission allows the
Ethernet protocol to be 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.
Signals and Noise in Optical Fibers
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.
Scattering
Bending
Fiber End Face Finishes
Fiber End Face Polishing Techniques
Splicing
Calibrated Light Sources and Light Meter
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 range.
802.11a covers WLAN devices operating in the 5 GHZ transmission
band. Using the 5 GHZ range disallows interoperability of 802.11b
devices as they operate within 2.4 GHZ. 802.11a is capable of
supplying data throughput of 54 Mbps and with proprietary
technology known as "rate doubling" has achieved 108 Mbps. In
production networks, a more standard rating is 20-26 Mbps.
802.11g provides the same throughout as 802.11a but with
backwards compatibility for 802.11b devices using Othogonal
Frequency Division Multiplexing (OFDM) modulation technology.
Cisco has developed an access point that permits 802.11b and
802.11a devices to coexist on the same WLAN. The access point
supplies ‘gateway’ services allowing these otherwise incompatible
devices to communicate.
Wireless LAN Standards
3.3.2 Wireless devices and topologies
A wireless network may consist of as few as two devices. - The
nodes could simply be desktop workstations or notebook computers.
Equipped with wireless NICs, an ‘ad hoc’ network could be
established which compares to a peer-to-peer wired network. Both
devices act as servers and clients in this environment. Although it
does provide connectivity, security is at a minimum along with
throughput. Another problem with this type of network is compatibility.
Many times NICs from different manufacturers are not compatible.
To solve the problem of compatibility, an access point (AP) is
commonly installed to act as a central hub for the WLAN
"infrastructure mode". The AP is hard wired to the cabled LAN to
provide Internet access and connectivity to the wired network. APs
are equipped with antennae and provide wireless connectivity over a
specified area referred to as a cell. Depending on the structural
composition of the location in which the AP is installed and the size
and gain of the antennae, the size of the cell could greatly vary. Most
commonly, the range will be from 91.44 to 152.4 meters (300 to 500
feet). To service larger areas, multiple access points may be installed
with a degree of overlap. The overlap permits "roaming" between
cells. This is very similar to the services provided by cellular phone
companies. Overlap, on multiple AP networks, is critical to allow for
movement of devices within the WLAN. Although not addressed in
the IEEE standards, a 20-30% overlap is desirable. This rate of
overlap will permit roaming between cells, allowing for the disconnect
and reconnect activity to occur seamlessly without service
interruption.
When a client is activated within the WLAN, it will start "listening" for
a compatible device with which to "associate". This is referred to as
"scanning" and may be active or passive.
Active scanning causes a probe request to be sent from the wireless
node seeking to join the network. The probe request will contain the
Service Set Identifier (SSID) of the network it wishes to join. When an
AP with the same SSID is found, the AP will issue a probe response.
The authentication and association steps are completed.
Passive scanning nodes listen for beacon management frames
(beacons), which are transmitted by the AP (infrastructure mode) or
peer nodes (ad hoc). When a node receives a beacon that contains
the SSID of the network it is trying to join, an attempt is made to join
the network. Passive scanning is a continuous process and nodes
may associate or disassociate with APs as signal strength changes.
Internal Wireless NIC for Desktop Server
PCMCIA NIC for Laptop
External USB Wireless NIC
Access Point
Wireless LAN
Roaming
3.3.3 How wireless LANs communicate
After establishing connectivity to the WLAN, a node will pass frames
in the same manner as on any other 802.x network. WLANs do not
use a standard 802.3 frame. Therefore, using the term wireless
Ethernet is misleading. There are three types of frames: control,
management, and data. Only the data frame type is similar to 802.3
frames. The payload of wireless and 802.3 frames is 1500 bytes;
however, an Ether frame may not exceed 1518 bytes whereas a
wireless frame could be as large as 2346 bytes. Usually the WLAN
frame size will be limited to 1518 bytes as it is most commonly
connected to a wired Ethernet network.
Since radio frequency (RF) is a shared medium, collisions can occur
just as they do on wired shared medium. The major difference is that
there is no method by which the source node is able to detect that a
collision occurred. For that reason WLANs use Carrier Sense Multiple
Access/Collision Avoidance (CSMA/CA). This is somewhat like
Ethernet CSMA/CD.
When a source node sends a frame, the receiving node returns a
positive acknowledgment (ACK). This can cause consumption of 50%
of the available bandwidth. This overhead when combined with the
collision avoidance protocol overhead reduces the actual data
throughput to a maximum of 5.0 to 5.5 Mbps on an 802.11b wireless
LAN rated at 11 Mbps.
Performance of the network will also be affected by signal strength
and degradation in signal quality due to distance or interference. As
the signal becomes weaker, Adaptive Rate Selection (ARS) may be
invoked. The transmitting unit will drop the data rate from 11 Mbps to
5.5 Mbps, from 5.5 Mbps to 2 Mbps or 2 Mbps to 1 Mbps.
IEEE 802.3 Frame Types
Adaptive Rate Selection
3.3.4 Authentication and association
WLAN authentication occurs at Layer 2. It is the process of
authenticating the device not the user. This is a critical point to
remember when considering WLAN security, troubleshooting and
overall management.
Authentication may be a null process, as in the case of a new AP and
NIC with default configurations in place. The client will send an
authentication request frame to the AP and the frame will be
accepted or rejected by the AP. The client is notified of the response
via an authentication response frame. The AP may also be
configured to hand off the authentication task to an authentication
server, which would perform a more thorough credentialing process.
Association, performed after authentication, is the state that permits a
client to use the services of the AP to transfer data.
Authentication and Association types
Unauthenticated and unassociated
The node is disconnected from the network and not associated to an
access point.
Authenticated and unassociated
The node has been authenticated on the network but has not yet
associated with the access point.
Authenticated and associated
The node is connected to the network and able to transmit and
receive data through the access point.
Methods of authentication
IEEE 802.11 lists two types of authentication processes.
The first authentication process is the open system. This is an open
connectivity standard in which only the SSID must match. This may
be used in a secure or non-secure environment although the ability of
low level network ‘sniffers’ to discover the SSID of the WLAN is high.
The second process is the shared key. This process requires the use
of Wireless Equivalency Protocol (WEP) encryption. WEP is a fairly
simple algorithm using 64 and 128 bit keys. The AP is configured with
an encrypted key and nodes attempting to access the network
through the AP must have a matching key. Statically assigned WEP
keys provide a higher level of security than the open system but are
definitely not hack proof.
The problem of unauthorized entry into WLANs is being addressed by
a number of new security solution technologies.
Authentication and Association Types
3.3.5 The radio wave and microwave spectrums
Computers send data signals electronically. Radio transmitters
convert these electrical signals to radio waves. Changing electric
currents in the antenna of a transmitter generates the radio waves.
These radio waves radiate out in straight lines from the antenna.
However, radio waves attenuate as they move out from the
transmitting antenna. In a WLAN, a radio signal measured at a
distance of just 10 meters (30 feet) from the transmitting antenna
would be only 1/100th of its original strength. Like light, radio waves
can be absorbed by some materials and reflected by others. When
passing from one material, like air, into another material, like a plaster
wall, radio waves are refracted. Radio waves are also scattered and
absorbed by water droplets in the air.
These qualities of radio waves are important to remember when a
WLAN is being planned for a building or for a campus. The process of
evaluating a location for the installation of a WLAN is called making a
Site Survey.
Because radio signals weaken as they travel away from the
transmitter, the receiver must also be equipped with an antenna.
When radio waves hit the antenna of a receiver, weak electric
currents are generated in that antenna. These electric currents,
caused by the received radio waves, are equal to the currents that
originally generated the radio waves in the antenna of the transmitter.
The receiver amplifies the strength of these weak electrical signals.
In a transmitter, the electrical (data) signals from a computer or a
LAN are not sent directly into the antenna of the transmitter. Rather,
these data signals are used to alter a second, strong signal called the
carrier signal.
The process of altering the carrier signal that will enter the antenna of
the transmitter is called modulation. There are three basic ways in
which a radio carrier signal can be modulated. For example,
Amplitude Modulated (AM) radio stations modulate the height
(amplitude) of the carrier signal. Frequency Modulated (FM) radio
stations modulate the frequency of the carrier signal as determined
by the electrical signal from the microphone. In WLANs, a third type
of modulation called phase modulation is used to superimpose the
data signal onto the carrier signal that is broadcast by the transmitter.
In this type of modulation, the data bits in the electrical signal change
the phase of the carrier signal.
A receiver demodulates the carrier signal that arrives from its antenna.
The receiver interprets the phase changes of the carrier signal and
reconstructs from it the original electrical data signal.
Radio Wave
Radio Wave
Modulation
3.3.6 Signals and noise on a WLAN
On a wired Ethernet network, it is usually a simple process to
diagnose the cause of interference. When using RF technology many
kinds of interference must be taken into consideration.
Narrowband is the opposite of spread spectrum technology. As the
name implies narrowband does not affect the entire frequency
spectrum of the wireless signal. One solution to a narrowband
interference problem could be simply changing the channel that the
AP is using. Actually diagnosing the cause of narrowband
interference can be a costly and time-consuming experience. To
identify the source requires a spectrum analyzer and even a low cost
model is relatively expensive.
All band interference affects the entire spectrum range. Bluetooth™
technologies hops across the entire 2.4 GHz many times per second
and can cause significant interference on an 802.11b network. It is
not uncommon to see signs in facilities that use wireless networks
requesting that all Bluetooth™ devices be shut down before entering.
In homes and offices, a device that is often overlooked as causing
interference is the standard microwave oven. Leakage from a
microwave of as little as one watt into the RF spectrum can cause
major network disruption. Wireless phones operating in the 2.4GHZ
spectrum can also cause network disorder.
Generally the RF signal will not be affected by even the most extreme
weather conditions. However, fog or very high moisture conditions
can and do affect wireless networks. Lightning can also charge the
atmosphere and alter the path of a transmitted signal.
The first and most obvious source of a signal problem is the
transmitting station and antenna type. A higher output station will
transmit the signal further and a parabolic dish antenna that
concentrates the signal will increase the transmission range.
In a SOHO environment most access points will utilize twin
omnidirectional antennae that transmit the signal in all directions
thereby reducing the range of communication.
Omindirecational Antenna
3.3.7 Wireless security
As previously discussed in this chapter, wireless security can be
difficult to achieve. Where wireless networks exist there is little
security. This has been a problem from the earliest days of WLANs.
Currently, many administrators are weak in implementing effective
security practices.
A number of new security solutions and protocols, such as Virtual
Private Networking (VPN) and Extensible Authentication Protocol
(EAP) are emerging. With EAP, the access point does not provide
authentication to the client, but passes the duties to a more
sophisticated device, possibly a dedicated server, designed for that
purpose. Using an integrated server VPN technology creates a tunnel
on top of an existing protocol such as IP. This is a Layer 3 connection
as opposed to the Layer 2 connection between the AP and the
sending node.
 EAP-MD5 Challenge – Extensible Authentication Protocol is the
earliest authentication type, which is very similar to CHAP
password protection on a wired network.
 LEAP (Cisco) – Lightweight Extensible Authentication Protocol is
the type primarily used on Cisco WLAN access points. LEAP
provides security during credential exchange, encrypts using
dynamic WEP keys, and supports mutual authentication.
 User authentication – Allows only authorized users to connect,
send and receive data over the wireless network.
 Encryption – Provides encryption services further protecting the
data from intruders.
 Data authentication – Ensures the integrity of the data,
authenticating source and destination devices.
VPN technology effectively closes the wireless network since an
unrestricted WLAN will automatically forward traffic between nodes
that appear to be on the same wireless network. WLANs often extend
outside the perimeter of the home or office in which they are installed
and without security intruders may infiltrate the network with little
effort. Conversely it takes minimal effort on the part of the network
administrator to provide low-level security to the WLAN.
Wireless Security
Summary
An understanding of the following key points should have been
achieved:
 All matter is composed of atoms, and the three main parts of an
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atom are: protons, neutrons, and electrons. The protons and
neutrons are located in the center part of the atom (nucleus)
Electrostatic discharge (ESD) can create serious problems for
sensitive electronic equipment
Attenuation refers to the resistance to the flow of electrons and why
a signal becomes degraded as it travels
Currents flow in closed loops called circuits, which must be
composed of conducting materials and must have sources of
voltage
A multimeter is used to measure voltage, current, resistance, and
other electrical quantities expressed in numeric form
Three types of copper cables used in networking are:
straight-through, crossover, and rollover
Coaxial cable consists of a hollow outer cylindrical conductor that
surrounds a single inner wire conductor
UTP cable is a four-pair wire medium used in a variety of networks
STP cable combines the techniques of shielding, cancellation, and
twisting of wires
Optical fiber is a very good transmission medium when it is
properly installed, tested, and maintained
Light energy, a type of electromagnetic energy wave, is used to
transmit large amounts of data securely over relatively long
distances
The light signal carried by a fiber is produced by a transmitter that
converts an electrical signal into a light signal
The light that arrives at the far end of the cable is converted back
to the original electrical signal by the receiver
Fibers are used in pairs to provide full duplex communications
Light rays obey the laws of reflection and refraction as they travel
through a glass fiber, which allows fibers with the property of total
internal reflection to be manufactured
Total internal reflection makes light signals stay inside the fiber,
even if the fiber is not straight
Attenuation of a light signal becomes a problem over long cables
especially if sections of cable are connected at patch panels or
spliced
Cable and connectors must be properly installed and thoroughly
tested with high quality optical test equipment before their use
Cable links must be tested periodically with high quality optical test
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instruments to check whether the link has deteriorated in any way
Care must always be taken to protect eyes when intense light
sources like lasers are used
Understanding the regulations and standards that apply to wireless
technology will ensure that deployed networks will be interoperable
and in compliance
Compatibility problems with NICs are solved by installing an access
point (AP) to act as a central hub for the WLAN
Three types of frames are used in wireless communication: control,
management, and data
WLANs use Carrier Sense Multiple Access/Collision Avoidance
(CSMA/CA)
WLAN authentication is a process that authenticates the device,
not the user
Module 3: Summary