CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 16 potx

Measuring Electricity 119
Voltage
Because electrons and protons have opposite charges, they are attracted to each other
with a force similar to the attractive force of the north and south poles of two magnets.
When the charges are separated, this separation creates an attractive force or pressure
field between the charges. This force is voltage. The force that is created pulls toward
the opposite charge and pushes away from the like charge. This process occurs in a
battery, where chemical action causes electrons to be freed from the battery’s negative
terminal and to travel to the opposite, or positive, terminal through an external circuit—
not through the battery itself. The separation of charges results in voltage. Voltage can
also be created by friction (static electricity), by magnetism (electric generator), or by
solar energy.
Voltage is represented by the letter V. The unit of measurement for voltage is the volt,
and it is also represented with the letter V (for example, 12 V = 12 volts).
Two kinds of voltage exist:
■ Direct-current (DC) voltage—A battery is an example of a DC voltage source.
The movement of electrons in a DC circuit is always in the same direction, from
negative to positive.
■ Alternating-current (AC) voltage—In an AC circuit, the positive and negative ter-
minals of the AC voltage source regularly change to negative and positive and
back again, as shown in Figure 3-3. This change makes the direction of electron
movement change, or alternate, with respect to time.
Figure 3-3 Alternating Current
Lab Activity Safe Handling and Use of a Multimeter
In this lab, you learn how to use or handle a multimeter correctly.
Lab Activity Voltage Measurements
In this lab, you demonstrate the ability to measure voltage with the multimeter.
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120 Chapter 3: Networking Media
Current
Electrical current is the flow of charges that is created when electrons move. When

voltage (electrical pressure) is applied and a path for the current exists, electrons move
from the negative terminal (which repels them), along the path, to the positive terminal
(which attracts them).
Current is represented by the letter I. The unit of measurement for current is the ampere,
and it is represented by the letter A, or by the abbreviation amp. An amp is defined as
the number of charges per second that pass by a point along a path. It can be thought
of as the amount of electron traffic that is flowing through a circuit; the more electrons
that pass by any given point in a circuit, the higher the current.
Current that results from DC voltage always flows in the same direction, from negative
to positive. Current that results from AC voltage flows in one direction, then changes
direction, and then alternates back to the original direction, and so on.
Wattage
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 (quantity of electrons past a given point) and voltage (pres-
sure or speed of electrons) equals wattage or electrical power. A watt (W) is the basic
unit of electrical power or work done by electricity. Wattage equals voltage times amper-
age (W = V × I). Electrical devices such as light bulbs, motors, and computer power
supplies are rated in terms of watts, which is how much power they consume or pro-
duce. 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 injure someone. 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 cause severe
damage or injury.
Resistance and Impedance
Conductors exchange electrons very easily, so it does not take much voltage to cause
electrons to move through them. Conversely, the electrons in insulators are bound to
their orbits much more tightly, so they oppose the movement of electrons. Resistance

is the property of a material that resists electron movement. Conductors have low
resistance, and insulators have high resistance.
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Measuring Electricity 121
Resistance is represented by the letter R. The unit of measurement for resistance is the
ohm, and it is represented by the Greek letter, omega (Ω), because omega sounds like ohm.
The term resistance is generally used when referring to DC circuits. The resistance to
the movement of electrons in an AC circuit is called impedance. Impedance is repre-
sented by the letter Z. Like resistance, its unit of measurement is the ohm, represented
by Ω.
Circuits
Electrons move best through conductive materials. Although air in a dry climate can
be conductive, as noticed through shocks of static electricity, electrons cannot jump
across air from a battery to an unconnected, nearby piece of copper wire. Current, or
electron movement, occurs only in circuits that form complete loops. These circuits are
known as closed circuits.
Figure 3-4 shows a simple circuit, typical of a lantern-style flashlight. The switch is like
two ends of a single wire that can be opened (or broken) and then closed (or shorted)
to prevent or allow current.
Figure 3-4 Serial Circuit—Flashlight
Lab Activity Resistance Measurements
In this lab, you demonstrate the ability to measure resistance and continuity
with the multimeter.
6V
Lantern
+
6V
Lantern
+
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122 Chapter 3: Networking Media
The top of Figure 3-4 illustrates the flashlight with its switch turned off. The chemical
processes in the battery cause charges to be separated, which provides voltage. How-
ever, because is no complete path for electron movement exists, there is no current,
and the bulb will not be lit.
As shown in the bottom of Figure 3-4, the switch is turned on, and a complete path of
conductive wire for current exists. The bulb provides resistance to the flow of electrons,
causing the current to release energy in the form of light.
The circuits involved in networking use the same concepts as this very simple circuit,
but networking circuits are much more complex. When you are learning a new concept,
it is often helpful to relate the concept to a familiar example. The circuit discussed pre-
viously can be compared to a water circuit, as illustrated in Figure 3-5. The pressure
that causes water flow comes from the weight of the water in the tank. The tap can be
compared with the switch in the previous example. When the tap is turned off, it blocks
water from moving. When the tap is turned on, it allows water to move and also pro-
vides resistance to the flow of water, because a small tap will allow a lesser flow than a
large tap. Finally, the pipe provides a closed path for the flow of water to cycle back
into the tank.
Figure 3-5 Water Circuit Analogy for Flowing Electrons
Lab Activity Communications Circuits
In this lab, you build series circuits and explore their basic properties.
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Copper Media 123
Copper Media
Copper is the most common medium for signal wiring. Copper wires are the components
of a cable that carry the signals from the source computer to the destination computer.
Copper has several important properties that make it well suited for electronic cabling:
■ Conductivity—Copper is perhaps best known for its ability to conduct electric
current. Copper is also an excellent conductor of heat. This property makes it
useful in cooking utensils, radiators, and refrigerators.

■ Corrosion resistance—Copper does not rust and is fairly resistant to corrosion;
the copper corrodes as copper oxide at a somewhat slower pace than other metals.
■ Ductility—Copper possesses great ductility, the ability to be drawn into thin wires
without breaking. For example, copper rod that is 1 centimeter (cm) in diameter
can be heated, rolled, and drawn into a wire that is thinner than a human hair.
■ Malleability—Pure copper is highly malleable (easy to shape). It does not crack
when hammered, stamped, forged, or spun into unusual shapes. Copper can be
worked (shaped) when it is hot or cold.
■ Strength—Cold-rolled copper has a tensile strength 3500 to 4900 kilograms
per square centimeter. Copper keeps its strength and toughness up to about 400°
Fahrenheit (F) (204° Celsius [C]).
This section focuses on two types of copper cable used for networks:
■ Twisted-pair—Twisted-pair cables are composed of one or more pairs of copper
wires. Most data and voice networks use twisted-pair cabling.
■ Coaxial—Coaxial cable has one center conductor of either solid or stranded
copper wire. Coaxial cable, once the choice for local-area network (LAN)
cabling, is now used primarily for video connections, high-speed connections
such as T3 (or E3) lines, and cable television.
American Wire Gauge System
The diameter of cable wires or conductors is commonly measured using the American
wire gauge (AWG) system. AWG is a U.S. standard for measuring the diameter of pri-
marily copper and aluminum cable. Typical residential wiring is AWG 12 or 14. The
conductor or wire size used in the UTP in most telephone local loops (from the central
office to a home or residence) is between 19 and 26 AWG. Most newer telephone wire
is from 22 to 26 gauge with 24 gauge being the most common. The lower the gauge
number the thicker the wire. Thicker wire has less resistance and can carry more cur-
rent resulting in a better signal over longer distances. A wire with an AWG size of 24
would be 1/24th of an inch in diameter.
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124 Chapter 3: Networking Media

Twisted-Pair Cable
Twisted-pair cable is a type of cabling that is used for telephone communications and
most modern Ethernet networks. A pair of wires forms a circuit that can transmit data.
The pairs are twisted to provide protection against crosstalk, the noise generated by
adjacent pairs.
The wire pairs are twisted for two reasons. First, when a wire is carrying a current,
that current creates a magnetic field around the wire. This field can interfere with sig-
nals on nearby wires. To combat this, pairs of wires carry signals in opposite directions,
so that the two magnetic fields also occur in opposite directions and cancel each other
out. This process is known as cancellation. Twisting the pairs holds the two wires closer
together and helps to ensure effective cancellation within the cable.
Second, network data is sent using two wires in a twisted pair. One copy of the data is
sent on each wire, and the two copies are mirror images of each other. These signals
are called differential signals. If the two wires are twisted together, noise seen on one
wire is also be seen on the other wire. When the data is received, one copy is inverted,
and the two signals are then compared. In this manner the receiver can filter out noise
because the noise signals cancel each other.
Two basic types of twisted-pair cable exist: shielded twisted-pair (STP) and unshielded
twisted-pair (UTP). The following sections discuss UTP and STP cable in more detail.
Shielded Twisted-Pair Cable
Shielded twisted-pair (STP) cable contains four pairs of thin, copper wires covered in
color-coded plastic insulation that are twisted together. Each pair is wrapped in metallic
foil, and then the four pairs are collectively wrapped in another layer of metallic braid
or foil. This layer is wrapped with a plastic outer jacket. Figure 3-6 illustrates an
example of STP.
Figure 3-6 Shielded Twisted-Pair Cable
Outer
Jacket
Overall
Shield

Pair
Shields
Twisted Pair
RJ-45 Connector
Color-Coded
Plastic
Insulation
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Copper Media 125
Screened twisted-pair (ScTP), also known as foil twisted-pair (FTP), is a variation of
STP. ScTP is essentially STP with just one layer of foil shielding around the set of all
four-wire pairs, as shown in Figure 3-7. The shielding in both STP and ScTP reduces
unwanted electrical noise. This noise reduction provides a major advantage of STP
over unshielded cable.
Figure 3-7 Screened Twisted-Pair Cable
However, shielded cable is more difficult to install than unshielded cable because the
metallic shielding needs to be grounded. If improperly installed, STP and ScTP become
very susceptible to noise problems because an ungrounded shield acts like an antenna,
picking up unwanted signals. STP and ScTP cable cannot be run as far as coaxial and
fiber-optic cable without the use of repeaters. The insulation and shielding consider-
ably increase the size, weight, and cost of the cable. Despite these disadvantages,
shielded copper cable is still used as networking media today, especially in Europe.
The following summarizes the features of STP cable:
■ Speed and throughput—10 to 100 Mbps
■ Average cost per node—Moderately expensive
■ Media and connector size—Medium to large
■ Maximum cable length—100 meters (m) (short)
Unshielded Twisted-Pair Cable
Unshielded twisted-pair (UTP) cable is a common networking media. It consists of
four pairs of thin, copper wires covered in color-coded plastic insulation that are

twisted together, as shown in Figure 3-8. The wire pairs are then covered with a plastic
outer jacket. The connector used on a UTP cable is called a registered jack 45 (RJ-45)
connector, as shown in Figure 3-9.
Outer
Jacket
Overall
Shield
Twisted Pair
RJ-45 Connector
Color-Coded
Plastic
Insulation
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126 Chapter 3: Networking Media
Figure 3-8 Unshielded Twisted-Pair Cable
Figure 3-9 RJ-45 Connector
UTP cable has many advantages. It has a small diameter and does not require ground-
ing, so it is the easiest type of cable to install. Its size provides an additional advantage
because more UTP cable can fit in a given area than other copper media. It is also the
least expensive type of networking media, and the connector is the easiest to build. It
supports the same data speeds as other copper media.
The primary disadvantage to UTP is that it is more susceptible to electrical noise and
interference than any other type of networking media. Because it has no shielding, it
relies solely on the cancellation and differential signals to reduce the effects of noise.
The other main disadvantage is that its maximum run length is less than that allowed
for coaxial and fiber-optic cables.
Twisted Pair
RJ-45 Connector
Color-Coded
Plastic

Insulation
Outer
Jacket
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Copper Media 127
Although UTP was once considered to be slower at transmitting data than other types
of cable, this is no longer true. In fact, UTP is considered the fastest copper-based
medium today. The following summarizes the features of UTP cable:
■ Speed and throughput—10 to 1000 Mbps
■ Average cost per node—Least expensive
■ Media and connector size—Small
■ Maximum cable length—100 m (short)
Commonly used types of UTP cabling are as follows:
■ Category 1 (CAT 1)—Used for telephone communications. Not suitable for
transmitting data.
■ Category 2 (CAT 2)—Capable of transmitting data at speeds up to 4 Mbps.
■ Category 3 (CAT 3)—Used in 10BASET Ethernet networks. Can transmit data at
speeds up to 10 Mbps.
■ Category 4 (CAT 4)—Used in Token Ring networks. Can transmit data at speeds
up to 16 Mbps.
■ Category 5 (CAT 5)—Can transmit data at speeds up to 100 Mbps. Used in Fast
Ethernet networks.
■ Category 5e (CAT 5e)—Used in networks running at speeds up to 1000 Mbps
(1 Gbps). Used in Gigabit Ethernet (GigE) networks.
■ Category 6 (CAT 6)—The specification for CAT 6 is new, was released on
February 3, 2003, and is currently available for installation and use. Used in
Gigabit Ethernet (GigE) networks.
Typically, CAT 5 and higher network cable consists of four pairs of 24 AWG multi-
strand copper wires. Older cabling installations run CAT 3 for voice and CAT 5 for
data. Most new installations run a minimum of CAT 5e for voice and data. Although

CAT 5e costs a little more it is worth it in the long run.
When comparing UTP and STP, keep the following points in mind:
■ The speed of both types of cable is usually satisfactory for local-area distances.
■ These are the least-expensive media for data communication. UTP is less expen-
sive than STP.
■ Because most buildings are already wired with UTP, many transmission standards
are adapted to use it to avoid costly rewiring with an alternative cable type. You
must take care to ensure that the category level of the cable is adequate to handle
the bandwidth desired. As an example, a building wired with CAT 3 cable cannot
support Fast Ethernet, which requires at least CAT 5.
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128 Chapter 3: Networking Media
Coaxial Cable
Coaxial cable, as shown in Figure 3-10, consists of four main parts:
■ Copper conductor
■ Plastic insulation
■ Braided copper shielding
■ Outer jacket
At the center of the cable is a solid copper conductor. Surrounding that conductor is
a layer of flexible plastic insulation. A woven copper braid or metallic foil is wrapped
around the insulation. This layer acts as the second wire in the cable. It also acts as
a shield for the inner conductor and helps reduce the amount of outside interference.
Covering this shield is the outer cable jacket. The connector used on coaxial cable
is called a BNC, short for British Naval Connector or Bayonet Neill Concelman,
connector.
Figure 3-10 Coaxial Cable
Coaxial cable was a popular choice with LANs in the past. It offered several advantages.
It can be run with fewer boosts from repeaters for longer distances between network
nodes than either STP or UTP cable. Although more expensive than UTP, coaxial cable
is less expensive than fiber-optic cable. The technology is well known, because it has

been used for many years in various types of data communication. For example, coaxial
cable is commonly used in homes to deliver cable television signals and high-speed
Internet access. For cable TV, RG-59 is commonly used inside the home and has a center
conductor of 20 AWG. RG-6 is most often used from the street pedestal to the home
due to heavier shielding and a larger center conductor of 18 AWG. RG-11 cable is heavier
still with a center conductor of 14 AWG and is used to bring cable into neighborhoods.
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