Bài giảng mạng máy tính căn bản chương 4 phan vĩnh thuần

Chương 4
KIỂM TRA CÁP (Cable testing)
OverView
• Networking media is literally and
physically the backbone of a network.
Inferior quality of network cabling
results in network failures and
unreliable performance.
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• Copper, optical
fiber, and
wireless
networking media all require testing to
determine the quality. These tests involve
certain
electrical
and
mathematical
concepts and terms, such as signal, wave,
frequency, and noise. Understanding this
vocabulary is helpful when learning about
networking, cabling, and cable testing.
• The goal of the first lesson in this module is
to provide some basic definitions so that
the cable testing concepts presented in the

second lesson will be better understood.
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• The second lesson of this module
describes the issues relating to the
testing of media used for physical
layer connectivity in local-area
networks (LANs). In order for the
LAN to function properly, the
physical layer medium must meet the
industry standard specifications.
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• Attenuation (signal deterioration) and
noise (signal interference) cause
problems in networks because the
data is not recognizable when it is
received. Proper attachment of cable
connectors
and

proper
cable
installation are important. If standards
are
followed
in
these
areas,
attenuation and noise levels are
minimized.
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4.1. Cơ sở nghiên cứu kiểm tra cáp dựa vào tần số
(Background for Studying Frequency-Based
Cable Testing)
4.1.1. Sóng (Wave)
• Networking
professionals
are
specifically
interested in voltage waves on copper media, light
waves in optical fiber, and alternating electric and
magnetic fields called electromagnetic waves. The
amplitude of an electrical signal still represents
height, but it is measured in volts instead of

meters. The period is the amount of time to
complete one cycle, measured in seconds. The
frequency is the number of complete cycles per
second, measured in Hertz.
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4.1.2. Sóng sin và sóng vuông (Sine waves and
square waves)
• Sine waves, or sinusoids, are graphs of
mathematical functions. Sine waves have
certain characteristics. Sine waves are periodic,
which means that they repeat the same pattern
at regular intervals. Sine waves are continuously
varying, which means that no two adjacent
points on the graph have the same value.
• Sine waves are graphical representations of
many natural occurrences that change regularly
over time. Since sine waves are continuously
varying, they are examples of analog waves.
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• Square waves, like sine waves, are
periodic. However, square wave graphs
do not continuously vary with time.
The wave holds one value for some
time, and then suddenly changes to a
different value. This value is held for
some time, and then quickly changes
back to the original value. Square
waves represent digital signals, or
pulses. Like all waves, square waves
can be described in terms of
amplitude,
period,
andNetwork
frequency
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4.1.3. Lũy thừa và logarith (Exponents and
logarithms)
• In networking, there are three important
number systems:

– Base 2 – binary
– Base 10 – decimal
– Base 16 – hexadecimal
• Exponent of the base of a number system
also refers to the value of each digit. The least
significant digit has a value of base0, or one.
The next digit has a value of base1. This is
equal to 2 for binary numbers, 10 for decimal
numbers, and 16 for hexadecimal numbers.
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• Numbers with exponents are used to
easily represent very large or very small
numbers. It is much easier and less
error-prone to represent one billion

numerically as 109 than as 1000000000.
Many calculations involved in cable
testing involve numbers that are very
large, so exponents are the preferred
format. Exponents can be explored in
the flash activity.
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• One way to work with the very large and
very small numbers that occur in
networking is to transform the numbers
according to the rule, or mathematical
function, known as the logarithm.
Logarithms are referenced to the base of
the number system being used. For
example, base 10 logarithms are often
abbreviated log. While the study of
logarithms is beyond the scope of this
course,
the
terminology
is
used
commonly in calculating decibels, a way
of measuring signals on copper, optical,

and
wireless
media.
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4.1.4. Decibels
• The decibel (dB) is a measurement
unit
important
in
describing
networking signals. The decibel is
related
to
the
exponents
and
logarithms described in prior sections.
There are two formulas for calculating
decibels:
dB = 10 log10 (Pfinal / Pref)
dB = 20 log10 (Vfinal
/ Vreference)
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• The variables represent the following values:
– dB measures the loss or gain of the power of a
wave. Decibels are usually negative numbers
representing a loss in power as the wave travels, but
can also be positive values representing a gain in
power if the signal is amplified
– log10 implies that the number in parenthesis will be
transformed using the base 10 logarithm rule
– Pfinal is the delivered power measured in Watts
– Pref is the original power measured in Watts
– Vfinal is the delivered voltage measured in Volts
– Vreference is the original voltage measured in Volts

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• The first formula describes decibels
in terms of power (P), and the second
in terms of voltage (V). Typically, light
waves on optical fiber and radio
waves in the air are measured using
the power formula. Electromagnetic

waves on copper cables are
measured using the voltage formula.
These formulas have several things
in common.
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4.1.5. Tạp âm trong miền tần số và miền
thời gian (Noise in time and frequency)
• Noise is an important concept in
communications systems, including
LANS. While noise usually refers to
undesirable sounds, noise related to
communications refers to undesirable
signals. Noise can originate from natural
and technological sources, and is added
to the data signals in communications
systems.
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• All communications systems have
some amount of noise. Even though
noise cannot be eliminated, its effects

can be minimized if the sources of the
noise are understood. There are many
possible sources of noise:
– Nearby cables which carry data signals
– Radio frequency interference (RFI), which
is noise from other signals being
transmitted nearby
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– Electromagnetic interference (EMI),
which is noise from nearby sources
such as motors and lights
– Laser noise at the transmitter or receiver
of an optical signal

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• Noise that affects all transmission frequencies
equally is called white noise. Noise that only
affects small ranges of frequencies is called

narrowband interference. When detected on a
radio receiver, white noise would interfere with all
radio stations. Narrowband interference would
affect only a few stations whose frequencies are
close together. When detected on a LAN, white
noise would affect all data transmissions, but
narrowband interference might disrupt only
certain signals. If the band of frequencies affected
by the narrowband interference included all
frequencies transmitted on the LAN, then the
performance of the entire LAN would be
compromised
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4.1.6. Băng thông (Bandwidth)
• Bandwidth is an extremely important
concept in communications systems.
Two ways of considering bandwidth
that are important for the study of
LANs are analog bandwidth and digital

bandwidth.

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• Analog bandwidth typically refers to the
frequency range of an analog electronic
system. Analog bandwidth could be used
to describe the range of frequencies
transmitted by a radio station or an
electronic amplifier. The units of
measurement for analog bandwidth is
Hertz, the same as the unit of frequency.
Examples of analog bandwidth values are
3 kHz for telephony, 20 kHz for audible
signals, 5 kHz for AM radio stations, and
200 MHz for FM radio stations.
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• Digital bandwidth measures how much
information can flow from one place to

another in a given amount of time. The
fundamental unit of measurement for
digital bandwidth is bits per second
(bps). Since LANs are capable of
speeds of millions of bits per second,
measurement is expressed in kilobits
per second (Kbps) or megabits per
second (Mbps). Physical media, current
technologies, and the laws of physics
limit bandwidth. Computer Network
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• During cable testing, analog bandwidth
is used to determine the digital
bandwidth of a copper cable. Analog
frequencies are transmitted from one
end and received on the opposite end.
The two signals are then compared, and
the amount of attenuation of the signal is
calculated. In general, media that will
support higher analog bandwidths
without high degrees of attenuation will
also
support
higher
digital
bandwidths
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4.2. Tín hiệu và tạp âm (Signals and Noise)
4.2.1. Phát tín hiệu qua dây đồng và cáp quang
(Signaling over copper and fiber optic cabling)
• On copper cable, data signals are represented
by voltage levels that represent binary ones and
zeros. The voltage levels are measured with
respect to a reference level of zero volts at both
the transmitter and the receiver. This reference
level is called the signal ground. It is important
that both transmitting and receiving devices
refer to the same zero volt reference point. When
they do, they are said to be properly grounded.
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