35
Chapter 3 (CCNA1 – Module 3)
Networking Media
Electricity is a fact of modern life. We use it to perform a variety of tasks. It is
brought to our homes, schools, and offices by power lines that carry it in the form
of alternating current (AC). Another type of current, called direct current (DC), is
the current found in a flashlight, car battery, and on the motherboard of a
computer.
It is important to understand the difference between these two types of current
flow. Direct current flows at a constant value when circuits are turned on.
Alternating current rises and falls in current values as power companies
manufacture it.
When electricity reaches our homes, schools, and offices, it is carried to
appliances and machines via wires concealed in walls, floors, and ceilings.
Consequently, inside these buildings, AC power-line noise is all around us. If not
properly addressed, power-line noise can present problems for a network.
In fact, as you will discover the more you work with networks, AC line noise
coming from a nearby video monitor or hard disk drive can be enough to create
errors in a computer system. It does this by burying the desired signals and
preventing a computer's logic gates from detecting the leading and trailing edges
of the square signal waves. This problem can be further compounded when a
computer has a poor ground connection.
The third type of electricity is static electricity. This most damaging
uncontrollable form of electricity must be dealt with to protect sensitive electronic
equipment. Such static discharges can destroy semiconductors and data in a
seemingly random fashion as they shoot through a computer like bullets. As it can
with problems related to AC line noise, good grounding helps solve problems that
arise from electrostatic discharge.
Networking media are the various physical environments through which
transmission signals pass. For computers to communicate encoded information
with each other, networking media must physically connect them to each other.
The networking media used to connect computers varies. Several kinds of
network media can be used to connect computers:
• Coaxial cable
• Unshielded twisted-pair (UTP) cable
• Shielded twisted-pair (STP) cable
• Fiber-optic cable
36
Concept Questions
Demonstrate your knowledge of these concepts by answering the following
questions in the space provided.
1.
Each wire in a cable can act like an antenna. When this happens, the wire
actually absorbs electrical signals from other wires in the cable and from
electrical sources outside the cable. If the resulting electrical noise reaches a
high enough level, it can become difficult for network interface cards to
discriminate the noise from the data signal. When electrical noise on the
cable originates from signals on other wires in the cable, this is known as
crosstalk. How can you minimize crosstalk?
Crosstalk can be avoided by adhering to standard termination procedures, proper
installation methods, and use of quality twisted-pair cables. Ensure that termination
of the cable endpoint is completed properly. Avoid untwisting the conductive cables
and minimize unshielded cable lengths at termination endpoints.
2.
To ensure optimal performance, it is important for the network media to
carry the signal from one device to another with as little degradation as
possible. In networking, several factors can cause the signal to degrade. Some
of these factors are internal, whereas others are external. Name some of the
factors that can cause a signal to degrade and how to correct the problem.
AC line noise creates problems in our homes, schools, and offices. AC line noise
coming from a nearby video monitor or hard disk drive can be enough to create errors
in a computer system.
Electricity is carried to appliances and machines by wires concealed in walls, floors,
and ceilings. Consequently, inside these buildings, AC power-line noise is all around
us. If not properly prevented, power-line noise can cause problems for a network.
3.
Inside copper wires, factors such as opposition to the flow of electrons
(resistance), opposition to changes in voltage (capacitance), and opposition to
changes in current (inductance) can cause signals to degrade. External
sources of electrical impulses that can attack the quality of electrical signals
on the cable include lighting, electrical motors, and radio systems. These
types of interference are referred to as electromagnetic interference (EMI)
and radio frequency interference (RFI). How can you protect your network
from RFI?
You can limit EMI and RFI in a number of ways. One way is to increase the size of
the conductor wires. Another way is to improve the type of insulating material used.
However, such changes increase the size and cost of the cable faster than they
improve its quality. Therefore, it is more typical for network designers to specify a
cable of good quality, and to provide specifications for the maximum recommended
cable length between nodes. Two techniques that cable designers have used
successfully in dealing with EMI and RFI are shielding and cancellation.
37
4.
Networking media is defined as the various physical environments through
which transmission signals pass. Several types of network media can be used
to connect computers. Identify these different types of network media.
The following cable types can be used to connect computers:
⇒
Shielded twisted-pair cable
⇒
Unshielded twisted-pair cable
⇒
Coaxial cable
⇒
Fiber-optic cable
5.
Coaxial cable is a type of network media. Describe how coaxial cable is made.
Coaxial cable is made up of a single inner wire conductor that is surrounded by a
hollow cylindrical conductor.
6.
UTP cable is used in a variety of networks. How many wires make up this
type of cable?
Unshielded twisted-pair (UTP) cable is a four-pair wire medium used in a variety of
networks. Eight wires are contained within the outer insulation.
7.
STP cable combines the techniques of shielding, cancellation, and twisting of
wires. What is shielding, and why is it important?
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 normally 150-ohm cable.) STP reduces
electrical noise originating inside the cable (crosstalk) and outside the cable
(electromagnetic interference [EMI] and radio frequency interference [RFI]).
8.
Fiber-optic cable is a networking medium. How does it carry signals?
Fiber-optic cable is a networking medium capable of conducting modulated light
transmission. Fiber-optic cable does not carry electrical pulses as do other forms of
networking media that employ copper wire. Instead, signals that represent bits are
converted into pulses of light.
9.
Various criteria, such as rate of data transfer and expense, help determine
which type of media should be used. Which media is the fastest and which is
the least expensive?
Fiber optic cabling has the fastest possible transport speed; UTP Cat 5 represents the
least expensive type of media.
10.
The data link layer of the OSI model provides access to the networking
media and physical transmission across the medium. If you were going to
build a network, what media would you use and why?
Fiber-optic cabling would be a consideration for all long distances in building-tobuilding connections. A fiber-optic cabling has the capability to easily transport data
38
at speeds of 10+ Gbps. UTP cabling is generally selected for most office connectivity
where occurrences of EMI and RFI are not anticipated. In the industrial and
warehouse environments, typically shielded twisted-pair cabling represents the best
choice of media to avoid and EMI and RFI. Of course if pricing is not a concern,
fiber-optic cabling is the cabling that represents the greatest throughput with the
lowest chance of interference.
Vocabulary Exercise
Define the following terms as completely as you can. Use the online curriculum or
Chapter 3 from the Cisco Networking Academy Program CCNA 1 and 2
Companion Guide, Third Edition, for help.
Alternating current (AC) —Electrical current that reverses its direction regularly and
continuously. It is the form of electrical power found in residential and commercial
buildings.
AM (amplitude modulation) —Modulation technique whereby information is conveyed
through the amplitude of the carrier signal.
AMP—The unit of measurement for current is ampere and is the number of charges per
second that pass by a point along a path.
Angle of incidence—Angle of incidence is the angle at which the ray hits the glass
surface.
Analog transmission—Signal transmission over wires or through the air in which
information is conveyed through variation of some combination of signal amplitude,
frequency, and phase.
Attenuation—Loss of communication signal energy.
Backbone—The part of a network that acts as the primary path for traffic that is most
often sourced from, and destined for, other networks.
Bridge—Device that connects and passes packets between two network segments that
use the same communications protocol. Bridges operate at the data link layer (Layer 2) of
the OSI reference model. In general, a bridge will filter, forward, or flood an incoming
frame based on the MAC address of that frame.
Coaxial cable—Cable consisting of a hollow outer cylindrical conductor that surrounds a
single inner wire conductor. Two types of coaxial cable are currently used in LANs: 50ohm cable, which is used for digital signaling; and 75-ohm cable, which is used for
analog signal and high-speed digital signaling.
Circuits—Communications paths between two or more points.
Collision Domain—The area within the network from where the data packets originated
and collided. All shared media environments are collision domains. One wire might be
connected to another wire through patch cables, transceivers, patch panels, repeaters, and
hubs. All these Layer 1 interconnections are part of a collision domain.
39
Conductor—Any material with a low resistance to electrical current. Any material
capable of carrying an electrical current.
Crosstalk—The unwanted reception of electromagnetic signals on a wire from a nearby
wire.
Digital signal—Language of computers comprising only two states, on and off, which are
indicated by a series of voltage pulses.
Direct current (DC) —Electrical current that travels in only one direction. Direct current
is generally used in electronic circuits.
Dispersion—The broadening of light signals along the length of the fiber.
Electricity—Electrons in certain atoms can be pulled free from the atom and made to
flow. This is electricity—a free flow of electrons.
electromagnetic interference (EMI) —An electromagnetic field that has the potential to
disrupt the operation of electronic components, devices, and systems in its vicinity.
Electrons—Particles have a negative charge and orbit the nucleus.
Electronic Industries Association (EIA) —EIA is a group that specifies electrical
transmission standards. The EIA and TIA have developed numerous well-know
communication standards.
Electrostatic discharge (ESD) —A flow or spark of electricity that originates from a
static source such as a carpet and arcs across a gap to another object.
Fiber-optic cable—Physical medium capable of conducting modulated light
transmission. Compared with other transmission media, fiber-optic cable is more
expensive, but is not susceptible to electromagnetic interference, and is capable of higher
data rates. Sometimes called optical fiber.
FM (frequency modulation) —Modulation technique in which signals of different
frequencies represent different data values.
Hub—1) Generally, a term used to describe a device that serves as the center of a star
topology network. 2) Hardware or software device that contains multiple independent but
connected modules of network and internetwork equipment. Hubs can be active (where
they repeat signals sent through them) or passive (where they do not repeat, but merely
split, signals sent through them). 3) In Ethernet and IEEE 802.3, an Ethernet multiport
repeater, sometimes referred to as a concentrator.
Impedance—The total opposition to current flow (due to AC and DC voltages). The term
resistance generally is used when referring to DC voltages. Impedance is the general
term, and is the measure of how the flow of electrons is resisted, or impeded. Impedance
is represented by the letter Z. Its unit of measurement, like that for resistance, is the ohm
( ).
Institute of Electrical and Electronics Engineers (IEEE) —IEEE is professional
organization whose activities include the development of communications and network
standards. IEEE LAN standards are the predominant LAN standards.
40
Latency—1) Delay between the time a device requests access to a network and the time
it is granted permission to transmit. 2) Delay between the time when a device receives a
frame and the time that frame is forwarded out the destination port.
Linear bus topology—Has all its nodes connected directly to one link, and has no other
connections between nodes. Each host is wired to a common wire. One advantage of this
topology is that all hosts are connected to each other, and thus can communicate directly.
One disadvantage of this topology is that a break in the cable disconnects hosts from each
other.
Media—Refers to various physical environments through which transmission signals
pass. Common network media include twisted-pair, coaxial, fiber-optic cable, and the
atmosphere (through which microwave, laser, and infrared transmission occurs).
Multimeter—A tool used to measure voltage, current, and resistance. It generally has
two wires for that reason. The black wire is referred to as the ground, or reference
ground. A negative terminal on a battery also is referred to as 0 volts, or reference
ground.
Multimode—A type of fiber-optic cable that can transmit more than one light path.
Neutrons—Particles have no charge (neutral), and along with protons, form the nucleus
Noise—An unwanted electrical signal on a wire that interferes with the quality of the
signal by altering its shape.
Oscilloscope—An important and sophisticated electronic device used to study electrical
signals. Because it is possible to control electricity precisely, deliberate electrical patterns
called waves can be created. An oscilloscope graphs the electrical waves, pulses, and
patterns. It 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.
patch panel—A patch panel is an assembly of pin locations and ports that can be
mounted on a rack or wall bracket in the wiring closet. Patch panels act like switchboards
that connect workstations’ cables to each other and to the outside.
PM (phase modulation) —The phase, or beginning and ending points of a given cycle,
of the wave is varied to carry the message.
Propagation—Means travel. When a NIC puts a voltage or light pulse onto a physical
medium, that square pulse made up of waves travels along the medium (propagates).
Propagation means that a lump of energy, representing 1 bit, travels from one place to
another. The speed at which it propagates depends on the actual material used in the
medium, the geometry (structure) of the medium, and the frequency of the pulses.
Protons—Particles have a positive charge, and along with neutrons, form the nucleus.
Radio frequency interference(RFI) —RFI is the noise on wires caused by radio signals.
Reflection—Reflection is the photons of light striking a surface and leaving that surface
in an equal but opposite direction.
41
Refraction—Refraction is the change in direction of a beam of light when it enters
another medium.
Repeater—Device that regenerates and propagates electrical signals between two
network segments.
Resistance—The unit of measurement for resistance is the ohm and is represented by the
letter R.
Router—Network layer device that uses one or more metrics to determine the optimal
path along which network traffic should be forwarded. Routers forward packets from one
network to another based on network layer information. Occasionally called a gateway
(although this definition of gateway is becoming increasingly outdated).
Shielded twisted-pair (STP) —Two-pair wiring medium used in a variety of network
implementations. STP cabling has a layer of shielded insulation to reduce EMI.
Single-mode—An optical fiber has only one mode of light transmission. Contrast with
multimode.
Standard—A standard is a set of rules or procedures that are either widely used or
officially specified.
Switch—1) Network device that filters, forwards, and floods frames based on the
destination address of each frame. The switch operates at the data link layer of the OSI
model. 2) General term applied to an electronic or mechanical device that allows a
connection to be established as necessary and terminated when there is no longer a
session to support.
Telecommunications Industry Association (TIA)—A standards association that
publishes standards for telecommunications.
Thicknet—An early form of coaxial cable using 10BASE-5 for networking. Thicknet
was once desirable because it could carry signals up to 500 meters.
Thinnet—A simple, thin, coaxial network cable for the 10Base-2 system. Thinnet could
carry a signal only 185 meters, but was much easier to work with than thicknet.
Twisted cable—Relatively low-speed transmission medium consisting of two insulated
wires arranged in a regular spiral pattern. The wires can be shielded or unshielded.
Twisted pair is common in telephony applications and is increasingly common in data
networks.
Unshielded twisted-pair (UTP) —Four-pair wire medium used in a variety of networks.
UTP does not require the fixed spacing between connections that is necessary with
coaxial-type connections.
Wavelength—The length of a wave measured from any point on one wave to the
corresponding point on the next wave. The wavelength of light is usually measured in
nanometers (nm).
Focus Questions
42
1.
What are some examples of electrical insulators?
Electrical 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 stable chemical structures, with orbiting electrons tightly
bound within the atoms.
2.
What are some examples of electrical conductors?
Electrical 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 freed easily. The best conductors
are metals, such as copper (Cu), silver (Ag), and gold (Au). All these
metals are located in one column of the periodic table and have electrons
that are freed easily, making them excellent materials for carrying a
current.
3.
What are some examples of semiconductors?
Semiconductors are materials where the amount of electricity they conduct
can be controlled precisely. 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, the one that makes the best microscopic-sized electronic
circuits, is silicon (Si).
4.
What is silicon made of? Is it an insulator, conductor, or
semiconductor?
Silicon is 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. The switches, or gates, inside a microprocessor are
made up of semiconductors.
5.
Do semiconductors allow the amount of electricity to be controlled?
Semiconductors are materials where the amount of electricity they conduct
can be controlled precisely.
6.
When does voltage occur?
Voltage, sometimes referred to as Electromotive Force (EMF), is an
electrical force, or pressure, that occurs when electrons and protons are
separated. The force that is created pulls toward the opposite charge and
pushes away from the like charge.
7.
What is it called when static, or resting, electrons move and a flow of
charges is created?
43
Electrical current, or current, is the flow of charges that is created when
electrons move. In electrical circuits, current is caused by a flow of free
electrons. When voltage (electrical pressure) is applied, and there is a path
for the current, electrons move from the negative terminal (which repels
them), along the path, to the positive terminal (which attracts them).
8.
What is the difference between AC and DC?
Alternating current (AC) is one of the two ways in which current flows.
AC voltages vary with time by changing their polarity, or direction. AC
flows in one direction, and then reverses its direction and repeats the
process. AC voltage is positive at one terminal and negative at the other;
then, it reverses its polarity and the positive terminal becomes negative
and the negative terminal becomes positive. This process repeats itself
continuously.
Direct current (DC) is the other way in which current flows. 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.
9.
How do you measure impedance? What is its abbreviation?
Impedance is the total opposition to current flow (due to AC and DC
voltages). The term resistance generally is used when referring to DC
voltages. Impedance is the general term, and is the measure of how the
flow of electrons is resisted, or impeded. Impedance is represented by the
letter Z. Its unit of measurement, like that for resistance, is the ohm.
10.
What three components are necessary to make up a circuit, and how
do they allow the control of current?
Currents only flow in closed loops called circuits. These circuits must be
composed of conducting materials, and must have sources of voltage.
Voltage causes current to flow; whereas, resistance and impedance oppose
it. Knowing these facts allows people to control a flow of current.
11.
What equipment do you use to graph electrical waves, pulses, and
patterns?
An oscilloscope graphs the electrical waves, pulses, and patterns. It 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.
12.
What are some of the characteristics of an analog signal?
An analog signal has the following characteristics:
•
Waviness
44
•
A continuously varying voltage-versus-time graph
•
Typical of things in nature
•
Used widely in telecommunications for more than 100 years
The two important characteristics of a sine wave are its amplitude (A), its
height and depth, and its period (T), which is the length of time to
complete one cycle. You can calculate the frequency (f), wiggleyness, of
the wave with the formula f = 1/T.
13.
What is the basic building block of information on a data network?
The basic building block of information is 1 binary digit, known as the bit
or pulse. One bit, on an electrical medium, is the electrical signal
corresponding to binary 0 or binary 1. This can be as simple as 0 volts for
binary 0, and +5 volts for binary 1, or a more complex encoding. Signal
reference ground is an important concept relating to all networking media
that use voltages to carry messages.
14.
What are the five sources of noise that can affect a bit on a wire?
NEXT –A, thermal noise, impulse/reference ground noise, EMI/RFI,
NEXT –B
15.
At what speed do modern networks typically work?
The speed at which a network propagates depends on the actual material
used in the medium, the geometry (structure) of the medium, and the
frequency of the pulses. The time it takes the bit to travel from one end of
the medium and back is referred to as the round-trip time (RTT).
Assuming no other delays, the time it takes the bit to travel down the
medium to the far end is RTT/2 The fact that the bit takes a small amount
of time to travel along the medium does not normally cause network
problems. However, with the ever-increasing data transmission rates of
today’s networks, sometimes you must account for the amount of time it
takes the signal to travel.
16.
Compare and contrast four different Ethernet LAN devices in
increasing order of complexity (and typically cost).
Repeaters Repeaters regenerate and retime signals, thereby increasing
the distance that signals can travel and still be clearly interpreted at the
destination. Repeaters deal only with packets at the bit level; therefore,
they are Layer 1 devices. Repeaters are internetworking devices that exist
at the physical layer (Layer 1) of the OSI model.
Hubs Hubs combine connectivity with the amplifying and retiming
properties of repeaters. It is typical to see 4, 8, 12, and up to 24, ports on
45
multiport repeaters. This allows many devices to be cheaply and easily
interconnected.
Bridges Bridges are devices that connect and pass packets between two
network segments that use the same communications protocol. Bridges
operate at the data link layer (Layer 2) of the OSI reference model. In
general, a bridge will filter, forward, or flood an incoming frame based on
the MAC address of that frame.
Switches 1) Switches are network devices that filter, forward, and flood
frames based on the destination address of each frame. The switch
operates at the data link layer of the OSI model. 2) Switch is a general
term applied to an electronic or mechanical device that allows a
connection to be established as necessary and terminated when there is no
longer a session to support.
17.
What are some of the characteristics of shielded twisted-pair (STP)?
Shielded twisted-pair cable combines the techniques of shielding and
cancellation via the 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 normally 150-ohm cable.) STP reduces electrical noise
originating inside the cable (crosstalk) and outside the cable
(electromagnetic interference [EMI] and radio frequency interference
[RFI]). Shielded twisted-pair cable shares many of the advantages and
disadvantages of unshielded twisted-pair (UTP) cable. STP affords greater
protection from all types of external interference, but it is more expensive
and difficult to install than UTP.
18.
What are some of the characteristics of unshielded twisted-pair
(UTP)?
Unshielded twisted-pair cable is a four-pair wire medium used in a variety
of networks. Insulating material covers each of the eight individual copper
wires in the UTP cable. 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.
19.
What are some of the characteristics of coaxial cable?
Coaxial cable consists of a hollow outer cylindrical conductor that
surrounds a single inner wire conductor—the two conductors are separated
by insulation. In the center of the cable is a single copper wire. A layer of
flexible insulation surrounds it. Over this insulating material, a woven
copper braid or metallic foil acts as the second of two wires in the circuit.
46
It also acts as a shield for the inner conductor. This second layer, or shield,
helps reduce the amount of outside interference. The cable jacket covers
this shield.
20.
What is the difference between STP and UTP?
UTP 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. UTP uses 100 ohm of resistance; whereas, STP
generally has 150 ohms of resistance. STP, unlike UTP cabling, has a
metallic braid or foil that adds additional protection against EMI and RFI.
21.
What is the difference between STP and coaxial cable?
Both STP and coaxial cable use a metallic braid or foil for added
insulation. STP has individual shielding around each individual pair of
wires. STP can transfer data at 100 Mbps; whereas, coaxial is limited to 10
Mbps. Coax is terminated using a BNC or AUI connector; whereas, STP
cabling is usually terminated with an RJ connector.
22.
What are the advantages of fiber-optic cable?
Fiber-optic cabling allows for increased distance while also having the
capability of transferring data at a much faster rate than other types of
media. Single-mode fiber will support 10 Gbps transmission speeds. Fiberoptic cabling is more secure because data cannot be extracted from the
cable inductively. Fiber-optic cabling is also unaffected by RFI and EMI.
23.
What are the disadvantages of fiber-optic cable?
The cost of fiber cabling and fiber equipment is significantly greater than
that of twisted-pair cabling. Fiber is also very fragile and breaks easily
during installation or thereafter. Termination and installation cost is more
than double that of twisted-pair cabling.
26.
What is the medium for wireless communication?
Wireless signals are electromagnetic waves that can travel through the
vacuum of outer space and through media, such as the air in our
atmosphere. Therefore, no cable medium is necessary for wireless signals.
27.
What are TIA/EIA standards?
The TIA/EIA standards specify the minimum requirements for
multiproduct and multivendor environments. They allow for the planning
and the installation of LAN systems without dictating the use of specific
equipment giving LAN designers the freedom to create options for
improvement and expansion.
28.
What are patch panels?
47
Patch panels are convenient groupings of RJ-45 jacks. They come in 12,
24, and 48 ports, and typically are rack mounted. The front sides are RJ-45
jacks; the back sides are punch-down blocks that provide connectivity or
conducting paths. They are classified as Layer 1 components.
29.
How many conductors does an RJ-45 jack have?
An RJ-45 jack/connector is made up of eight conductors, four individual
pairs.
30.
What is a shared media environment?
A shared media environment occurs when multiple hosts have access to
the same medium. If several PCs are attached to the same physical wire or
optical fiber, or share the same airspace, for example, they all share the
same media environment. Occasionally, you might hear someone say “all
the computers are on the same wire.” It means they all share the same
media even though the “wire” might be CAT 5 UTP, which has four pairs
of wire.
31.
Where on a network do collisions occur?
If, for example, only one cable interconnects all the devices on a network, the possibility of
conflicts with more than one user sending data at the same time is high. The same is true if only
non-filtering devices, such as repeaters, connect segments of a network. Ethernet allows only one
data packet to access the cable at any one time. If more than one node attempts to transmit at the
same time, a collision occurs, and the data from each device suffers damage
32.
What happens to the signal in a collision?
When a collision occurs, the data packets that are involved are destroyed,
bit by bit. In a collision, the signals interfere, or collide, with each other
and create a third and invalid state.
33.
How do you recognize a collision domain?
If you connect several computers to a single medium that has no other
networking devices attached, you have a shared-access situation and you
have a single collision domain. Depending on the particular technology
used, this situation limits the number of computers that can use that
portion of the medium, also called a segment.
34.
How do repeaters extend collision domains?
Repeaters regenerate and retime bits, but they cannot filter the flow of
traffic that passes through them. Signals that arrive at one port of a
repeater are sent out on all other ports. The network on both sides of the
repeater is one large collision domain.
35.
How do hubs extend collision domains?
48
The extended collision domain introduced by a hub results in diminished
network performance. The degradation in performance depends on the
degree of usage of the network by the computers on that network. The hub
operates just like the repeater with the addition of numerous nodes, thus
increasing the collision domain.
36.
Do repeaters filter network traffic?
Both repeaters and hubs are Layer 1 devices; therefore, they perform no
filtering of network traffic.
37.
What is the four-repeater rule?
The four-repeater rule in Ethernet states that no more than four repeaters
or repeating hubs can be installed between any two computers on the
network. If followed, the rules guarantee that if a collision occurs, every
node in the collision domain knows it has occurred. This is crucial to the
successful operation of the network protocol
38.
How can the segmentation of collision domains be achieved?
You can reduce the size of collision domains by using intelligent
networking devices that break up the domains. Examples of this type of
networking device are bridges, switches, and routers.
49
CCNA Exam Review Questions
The following questions help you review for the CCNA exam. Answers appear in
Appendix B, "CCNA Exam Review Questions Answer Key."
1.
Which of the following correctly describes the type of signal that the network
media carries?
A.
Coaxial cable carries pulses of light.
B.
UTP cable carries impedance signals.
C.
STP cable carries electrical impulses.
D.
Fiber-optic cable carries electrical impulses.
2.
Which network media carries pulses of light?
A.
Coaxial cable
B.
Fiber-optic cable
C.
UTP cable
D.
STP cable
3.
Which of the following is an external source of degradation of the signal on
cabling?
A.
EMI caused by electrical motors
B.
RFI caused by light leakage
C.
Impedance caused by radio systems
D.
RFI caused by lighting
4.
Which of the following describes cabling signal degradation by an external
source?
A.
Poor cabling shield connection
B.
RFI caused by radio systems
C.
EMI caused by twisting of wires
D.
Impedance caused by electrical motors
5.
What is the cause of crosstalk?
A.
Cable wires that are too large in diameter
B.
Too much noise in a cable's data signal
C.
Electrical motors and lighting
D.
Electrical signals from other wires in a cable
6.
How does crosstalk occur?
A.
Two wires are placed in close proximity to each other.
B.
Network interface cards fail to discriminate the noise from the data signal.
C.
Electrical noise originates from signals on other wires in the cable.
D.
Wires in a cable absorb electrical impulses from sources outside the cable.
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7.
What is a cost-effective way to limit cable signal degradation?
A.
Specify the maximum cable length between nodes.
B.
Increase the size of the conductors in the cabling.
C.
Improve the type of insulating material.
D.
Use a braid or foil covering on wires as a shield.
8.
How can cable signal degradation be limited in a cost-effective way?
A.
Improve the type of insulating material.
B.
Place same-circuit wires close to each other.
C.
Use a braid or foil covering on wires as a shield.
D.
Increase the diameter of the conductor in the cabling.
9.
What is cancellation in networking media?
A.
The magnetic fields of same-circuit wires cancel each other.
B.
External magnetic fields cancel the fields inside network cabling.
C.
Wires in the same circuit cancel each other's electrical current flow.
D.
Twisting wire pairs cancels the electrical impedance in the wires.
10.
Which of the following describes cancellation in cabling?
A.
Wires in the same circuit cancel each other's electrical current flow.
B.
Twisting wire pairs provides self-shielding within the network media.
C.
The magnetic fields of wires on different electrical circuits cancel each
other.
D.
External magnetic fields cancel the fields inside network cabling.
11.
Which of the following describes impedance in networking media?
A.
Impedance involves resistance and reactance to current caused by signal
degradation.
B.
Electrical components in the NICs create impedance on the networking
media.
C.
Signal degradation causes impedance.
D.
Networking media impedance needs to match the network interface card
electrical components.
12.
When can impedance degrade the signal in networking media?
A.
When resistance opposes reactance
B.
When cable impedance does not match network interface card electrical
components
C.
When networking media is not properly shielded from EMI/RFI
interference
D.
When cancellation techniques are not employed
13.
Which of the following best describes attenuation?
A.
The termination of a message
B.
The interception of a message
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C.
D.
The weakening of a message
The ignoring of a message
14.
Which of the following best describes how data is transmitted on a network?
A.
As hexadecimal code
B.
As ASCII text
C.
As 1s and 0s
D.
As voltage pulses
15.
Which of the following best describes the states of digital signals?
A.
Alphanumeric
B.
Octets
C.
On or off
D.
Yes or no
16.
What does the binary number 1 correspond to in a digital signal?
A.
On
B.
One
C.
The letter A
D.
Off
17.
What does the binary number 0 correspond to in a digital signal?
A.
On
B.
One
C.
The letter A
D.
Off
18.
Which best describes a digital signal?
A.
A sine wave of normal shape and amplitude
B.
An electrical technique used to convey binary signals
C.
A language of computers with only two states—on and off—which are
indicated by a series of voltage pulses
D.
A transmission sent by a transceiver back to a controller to let it know the
collision circuitry is functional
19.
How do computers recognize digital signals?
A.
They receive a broadcast signal from the network.
B.
They look for ARP requests that match their IP address.
C.
They monitor the network connection for modulations.
D.
They measure and compare the signals to a reference point.
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20.
What is the signal reference ground?
A.
A neutral contact point where the computer chassis and the network
connection meet
B.
A point that devices use to measure and compare incoming digital
signals to
C.
A device that the name server uses to send messages over the network
D.
A ground that prevents users from receiving shocks when power fails
21.
What is the point that a device uses to measure and compare incoming digital
signals called?
A.
Input point
B.
Zero point
C.
Null reference setting
D.
Signal reference ground
22.
How is the signal reference ground established?
A.
By connecting the ground wire to the network wire
B.
By connecting the network wire to the jumper connector
C.
By connecting the ground plane to the computer's cabinet
D.
By connecting the computer chassis to the network cable
23.
What purpose does the computer chassis serve?
A.
It prevents electrical short circuits and electrical fires.
B.
It serves as signal reference ground and AC power-line ground.
C.
It amplifies digital signals.
D.
It reduces electromagnetic interference.
24.
What is the most likely cause of interference on a network?
A.
Improper cabling and jack choices
B.
Electromagnetic interference from radios and other electrical devices
C.
High voltage device in the vicinity
D.
Problems with the power ground
25.
What is the most likely cause of problems with the power ground?
A.
Length of the neutral and ground wires in electrical outlets
B.
Excessive stripping or untwisting of cable
C.
Equipment not located in a climate-controlled area
D.
Poor-quality cabling material used in the network
26.
What do long neutral and ground wires in electrical outlets act as?
A.
Lightning rods
B.
Amplifiers for digital signals
C.
Antenna for electrical noise
D.
Line signal dampeners
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27.
How does electrical noise affect networks?
A.
It shuts down the network.
B.
It burns out network devices, especially hubs.
C.
It reduces data transmission speed through the network because errortrapping routines are initiated.
D.
It distorts or buries digital signals to the point that they become
unrecognizable.
28.
How can the problem of electrical noise be avoided?
A.
By limiting the number and type of electrical devices near the LAN
B.
By working closely with your electrical contractor and the local power
company
C.
By making sure that all electrical devices are FCC and UL listed
D.
By installing surge suppressors on every network device
29.
How can having a single power transformer dedicated to your LAN reduce
electrical noise?
A.
You can detect and filter out fluctuations in line voltage before they reach
your LAN.
B.
You can specify the size and capacity of the transformer.
C.
You can place the transformer in a central location.
D.
You can control how and where devices such as motors or high current
devices are attached.
30.
What does installing separate breaker boxes for each office area do to electrical
noise?
A.
It reduces the chance of electrical noise.
B.
It reduces the need for surge protectors.
C.
It eliminates the need for a generator.
D.
It eliminates the need for network rewiring.
31.
What type of fiber-optic cable is required by the TIA/EIA-568B standard for
horizontal cabling?
A.
Two pairs of 100-ohm cable
B.
Two pairs of 150-ohm cable
C.
Two fibers of 62.5/125 um multimode cable
D.
Four fibers of 62.5/125 um multimode cable
32.
How can you determine which category of UTP cable a cabling belongs to?
A.
By looking at the end connectors
B.
By reading the UL marking
C.
By measuring the cable diameter
D.
By the color of the cable sheathing
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33.
Why do networks need to use an access method?
A.
To regulate access to the networking media equitably
B.
To regulate the access of data into certain parts of networking media
C.
To keep unwanted, foreign users from having access to the network
D.
To prioritize data transmissions so that important items have greater
access
34.
Which of the following best describes an access method?
A.
The method that software uses to access network file servers
B.
The method that is used to verify users as authorized for access to the
network
C.
The way that users access the network
D.
The way that network devices access the network medium
35.
Ethernet uses what access method?
A.
Token header transmission protocol.
B.
Ethernet does not use an access method.
C.
Carrier sense multiple access collision detect.
D.
Ethernet transmission carrier collision detect.
36.
Which of the following best describes a collision?
A.
The frames from two devices impact and are damaged when they meet on
the physical media.
B.
Two nodes transmit at the same time and one data packet has priority, so it
obliterates the lesser packet.
C.
Two data transmissions cross paths on the network media and corrupt each
other.
D.
A data transmission is corrupted due to an energy spike over the network
media.
37.
Which of the following best describes a backoff algorithm?
A.
A process wherein the network holds up some data so that other, more
important data can get through
B.
The retransmission delay that is enforced when a collision occurs
C.
The signal that a device on the network sends out to tell the other devices
that data is being sent
D.
A mathematical function that networking software performs to prioritize
data packets
38.
What is most important when considering the type of networking media to use in
an installation?
A.
Management's wishes
B.
Availability of networking media from local sources
C.
Applicable fire, building, and safety codes
D.
Your experience and expertise
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39.
Which grade of UTP cabling described in the TIA/EIA-568B standard is used for
running CDDI and can transmit data at speeds up to 100 Mbps?
A.
Category 2
B.
Category 3
C.
Category 4
D.
Category 5
40.
Which grade of UTP cabling described in the TIA/EIA-568B standard is the one
most frequently recommended and implemented in installations today?
A.
Category 2
B.
Category 3
C.
Category 4
D.
Category 5
41.
What is Category 5 UTP cabling suitable for?
A.
Transmitting data at speeds up to 10 Mbps
B.
Transmitting data at speeds up to 100 Mbps
C.
10BASE-T networks
D.
Token Ring networks
42.
What type of STP cable does the TIA/EIA-568B standard require for horizontal
cabling?
A.
Two pairs of 100-ohm cable
B.
Two pairs of 150-ohm cable
C.
Four pairs of 100-ohm cable
D.
Four pairs of 150-ohm cable
43.
What type of UTP cable does the TIA/EIA-568B standard require for horizontal
cabling?
A.
Two pairs of 100-ohm cable
B.
Two pairs of 150-ohm cable
C.
Four pairs of 100-ohm cable
D.
Four pairs of 150-ohm cable