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Mod 4 – Cable Testing
CCNA 1
Overview
Students completing this module should be able to:
• Differentiate between sine waves and square waves.
• Define and calculate exponents and logarithms.
• Define and calculate decibels.
• Define basic terminology related to time, frequency, and noise.
• Differentiate between digital bandwidth and analog bandwidth.
• Compare and contrast noise levels on various types of cabling.
• Define and describe the affects of attenuation and impedance
mismatch.
• Define crosstalk, near-end crosstalk, far-end crosstalk, and power sum
near-end crosstalk.
• Describe how crosstalk and twisted pairs help reduce noise.
• Describe the ten copper cable tests defined in TIA/EIA-568-B.
• Describe the difference between Category 5 and Category 6 cable.
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Background for Studying FrequencyBased Cable Testing
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Differentiate between sine waves and square waves.
Define and calculate exponents and logarithms.
Define and calculate decibels.
Define basic terminology related to time, frequency, and noise.
Differentiate between digital bandwidth and analog bandwidth.
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Amplitude and Frequency
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Analog Signal
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Other information
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For the next several slides we will explain analog signals
from information from the following sources:
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Digital and Analog Bandwidth
Bandwidth = The width or carrying capacity of a communications circuit.
Digital bandwidth = the number of bits per second (bps) the circuit can
carry
• used in digital communications such as T-1 or DDS
• measure in bps
• T-1 -> 1.544 Mbps
Analog bandwidth = the range of frequencies the circuit can carry
• used in analog communications such as voice (telephones)
• measured in Hertz (Hz), cycles per second
• voice-grade telephone lines have a 3,100 Hz bandwidth
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Digital and Analog Bandwidth
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Available at http://www.thinkgeek.com
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Digital and Analog Bandwidth
DTE
DCE
digital
analog
PSTN
Dial-up network
Modulation
DTE
DCE
digital
analog
PSTN
Dial-up network
Digital Signals
Demodulation
• digital signal = a signal whose state consists of discrete elements such
GOLDMAN:
asDATACOMM
high or low, on or off
FIG.02-14
Analog Signals
• analog signal = a signal which is “analogous” to sound waves
• telephone lines are designed to carry analog signals
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Sound Waves
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Analog Signals, Modulation and Modem Standards
• A perfect or steady tone makes a wave with consistent height
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(amplitude) and pitch (frequency) which looks like a sine wave. (Figure
4-15)
A cycle or one complete cycle of the wave
The frequency (the number of cycles) of the wave is measured in Hertz
Hertz (Hz) = the number of cycles per second
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Transmission Terminology (whatis.com)
Broadband transmission =
• In general, broadband refers to telecommunication in which a wide band of
frequencies is available to transmit information.
• Because a wide band of frequencies is available, information can be multiplexed
and sent on many different frequencies or channels within the band
concurrently, allowing more information to be transmitted in a given amount of
time (much as more lanes on a highway allow more cars to travel on it at the
same time).
Baseband transmission
1) Describing a telecommunication system in which information is carried in digital
(or analog) form on a single unmultiplexed signal channel on the transmission
medium. This usage pertains to a baseband network such as Ethernet and token
ring local area networks.
Narrowband transmission
• Generally, narrowband describes telecommunication that carries voice
information in a narrow band of frequencies.
• More specifically, the term has been used to describe a specific frequency range
set aside by the U.S. Fcc for mobile or radio services, including paging systems,
from 50 cps to 64 Kbps.
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Carrier Signal
Carrier Signal or Analog Wave = An electronic signal
used to modulate data in broadband transmission,
usually a sine wave.
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Carrier Signal
Three parts of any analog wave are:
1. amplitude - the height of the wave
2. frequency - the pitch of the wave
3. phase - the shift or position of the wave
These are the three parts we can modulate or change the carrier signal
or wave!
Modulate = Change
More in a moment.
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Telephone Lines, Modems, and PSTN
• Voice grade telephone lines and equipment are designed to transmit
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tones between 300 and 3,400 Hertz
bandwidth = 3,100 Hz or 3.1 KHz
“most” of our human voice falls into this range
Economics dictated the size of this bandwidth
(Keyboard example)
The “maximum” number of cycles (highest frequency) of an analog signal
over voice grade telephone lines is 3,400 Hz (cycles per second)
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Telephone Lines, Modems, and PSTN
DTE
DCE
digital
analog
PSTN
Dial-up network
Modulation
DTE
DCE
digital
analog
PSTN
Dial-up network
Demodulation
Modem
• MOdulator/DEModulator
• converts analog signals to digital and digital signals to analog
• used for transmitting digital information between computers over voiceGOLDMAN: DATACOMM
FIG.02-14
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grade telephone lines
Computers use transmission interface standards such as RS-232-C
using positive and negative voltages which form square waves, whereas
the PSTN is designed to carry analog signals (sine waves)
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Modulation
modulation =
1. the process of varying the characteristic of an electrical carrier wave
(analog, sine wave) as the information on that wave varies
Three types of modulation
1. amplitude modulation
2. frequency modulation
3. phase modulation
2. the process of converting digital signals to analog
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Amplitude Modulation
Amplitude Modulation (AM)
• a modulation technique to vary the height the electrical signal (the
sine wave or carrier wave with modems) to transmit ones and zeroes,
while the frequency of the wave remains constant
• different amplitudes for 0’s and 1’s
• a.k.a. amplitude shift keying, ASK
• Figure 4-22
• frequency for each bit remains constant
• volume = amplitude
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Amplitude Modulation
Different amplitudes for 0’s and 1’s, while the frequency of the wave
remains constant
Full duplex
• different amplitudes and frequencies are used for different directions
Disadvantage
• Voice-grade telephone lines are susceptible to distortions which affect
amplitudes, as volume fades, the amplitude lowers
• Amplitude modulation only effective for low speed transmissions
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Frequency Modulation
Frequency Modulation
• a modulation technique to vary the frequency of the sine wave (or
carrier wave) to transmit ones and zeroes, while the amplitude
remains constant
• different frequencies for 0’s and 1’s
• a.k.a. frequency shift keying, FSK
• Figure 4-23
• two separate frequencies for ones and zeroes
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Frequency Modulation
Full Duplex
• requires a minimum of four frequencies, two frequencies for each
direction
• i.e. CCITT V.21 for 300 baud modems:
Originating
Sending
Modem
Modem
1270 Hz
1
2225 Hz
1070 Hz
0
2025 Hz
• loss of amplitude will not cause errors in transmission
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Frequency Modulation
Conceptually:
If voice-grade telephone lines can transmit a “maximum” of 3,400 Hz
(cycles per second), between 300 Hz and 3,400 Hz,
AND
If one cycle = 1 bit,
Then a maximum of 3,400 bits per second can be transmitted over voice
grade telephone lines? (Hold that thought!)
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Phase Modulation
Phase Modulation (PM)
• a modulation technique to vary the phase of the sine wave (or carrier
wave) to transmit ones and zeroes, while the amplitude and the
frequency remains constant
• sine waves repeat themselves indefinitely
• shifting the wave breaks the wave abruptly and starts it again a few
degrees forward or backward
• A different phase shift, 0 to 360 degrees, is used to transmit one or
more bits
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Phase Modulation
A different phase shift, 0 to 360 degrees, is used to transmit one
or more bits
Full Duplex
• requires a minimum of two frequencies, one frequency for each
direction
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Bits per second vs. Baud and High-speed
modems
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So far, discussed transmission of one bit at a time,
via high or low amplitude, high or low frequency,
phase shift or no phase shift
older modems sent only one bit per signal change,
bps = baud
baud rate = the number of these signal changes per
second
What if we could transmit more than one bit with
each signal change (baud), amplitude, frequency of
phase shift?
Remember, voice-grade phone lines limit
transmission to 3,400 Hz or 3,400 bps with 1 cycle
per bit
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Dibit Modulation
Dibit Amplitude
modulation
Dibit Modulation
• 2 bits per baud, per cycle
• Two bits or dibit modulation:
00, 01, 10, 11
Using Amplitude Modulation
• use four different amplitudes (wave heights)
Using Frequency Modulation
• use four different frequencies
Using Phase Modulation
• use four different phases
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Summary of Modulations
•Amplitude Modulation
(AM)
•Frequency Modulation
(FM)
•Phase Shift Keying
(PSK)
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Back to Cisco Curriculum….
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Digital Signals
• 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
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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, and
frequency.
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Exponents and logarithms (Not testable)
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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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Exponents and logarithms (Not testable)
• One way to work with the very large and very small numbers that occur
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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.
To take the “log” of a number use a calculator or the flash activity.
For example, log (109) equals 9, log (10-3) = -3.
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Decibels
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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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Decibels
There are two formulas for calculating decibels:
dB = 10 log10 (Pfinal / Pref)
dB = 20 log10 (Vfinal / Vreference)
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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Decibels (Not testable)
• The first formula describes decibels in terms of power (P), and the
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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.
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Decibels (Not testable)
• Enter values for dB and Pref to discover the correct power.
• This formula could be used to see how much power is left in a radio
wave after it has traveled over a distance through different materials,
and through various stages of electronic systems such as a radio.
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Viewing signals in time and frequency
• An oscilloscope is an important electronic device used to view
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electrical signals such as voltage waves and pulses.
The x-axis on the display represents time, and the y-axis represents
voltage or current.
There are usually two y-axis inputs, so two waves can be observed
and measured at the same time.
Analyzing signals using an oscilloscope is called time-domain
analysis, because the x-axis or domain of the mathematical function
represents time.
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Viewing signals in time and frequency
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Engineers also use frequency-domain analysis to study
signals.
In frequency-domain analysis, the x-axis represents
frequency.
An electronic device called a spectrum analyzer creates
graphs for frequency-domain analysis.
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Analog and digital signals in time and
frequency
• To understand the complexities of networking signals and cable
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testing, examine how analog signals vary with time and with frequency.
Imagine the combination of several sine waves.
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Noise in time and frequency
• Noise is an important concept in communications systems, including
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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.
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. Laser noise at the transmitter
or receiver of an optical signal
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Noise in time and frequency
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
• Electromagnetic interference (EMI), which is noise from
nearby sources such as motors and lights
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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.
Example of analog bandwidth values are 3 kHz for
telephony
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Bandwidth
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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).
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Signals and Noise
• Compare and contrast noise levels on various types of cabling.
• Define and describe the affects of attenuation and impedance
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mismatch.
Define crosstalk, near-end crosstalk, far-end crosstalk, and power sum
near-end crosstalk.
Describe how crosstalk and twisted pairs help reduce noise.
Describe the ten copper cable tests defined in TIA/EIA-568-B.
Describe the difference between Category 5 and Category 6 cable.
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Signaling over copper and fiber optic
cabling
• In order for the LAN to operate properly, the receiving device must be
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able to accurately interpret the binary ones and zeros transmitted as
voltage levels.
Since current Ethernet technology supports data rates of billions of bits
per second, each bit must be recognized, even though duration of the
bit is very small.
The voltage level cannot be amplified at the receiver, nor can the bit
duration be extended in order to recognize the data.
This means that as much of the original signal strength must be
retained, as the signal moves through the cable and passes through
the connectors.
In anticipation of ever-faster Ethernet protocols, new cable installations
should be made with the best available cable, connectors, and
interconnect devices such as punch-down blocks and patch panels.
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Attenuation and insertion loss on copper
media
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Attenuation is the decrease in signal amplitude over the
length of a link.
Long cable lengths and high signal frequencies contribute
to greater signal attenuation.
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Sources of noise on copper media
• Crosstalk involves the transmission of signals from one wire to a
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nearby wire.
When voltages change on a wire, electromagnetic energy is generated.
This energy radiates outward from the transmitting wire like a radio
signal from a transmitter.
Adjacent wires in the cable act like antennas, receiving the transmitted
energy, which interferes with data on those wires.
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Sources of noise on copper media
• Twisted-pair cable is designed to take advantage of the effects of
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crosstalk in order to minimize noise.
In twisted-pair cable, a pair of wires is used to transmit one signal.
The wire pair is twisted so that each wire experiences similar crosstalk.
Because a noise signal on one wire will appear identically on the other
wire, this noise be easily detected and filtered at the receiver.
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Types of crosstalk
There are three distinct types of crosstalk:
• Near-end Crosstalk (NEXT)
• Far-end Crosstalk (FEXT)
• Power Sum Near-end Crosstalk (PSNEXT)
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Near-end Crosstalk (NEXT)
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Near-end crosstalk (NEXT) is computed as the ratio of
voltage amplitude between the test signal and the crosstalk
signal when measured from the same end of the link.
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Far-end Crosstalk (FEXT)
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Due to attenuation, crosstalk occurring further away from
the transmitter creates less noise on a cable than NEXT.
This is called far-end crosstalk, or FEXT.
The noise caused by FEXT still travels back to the source,
but it is attenuated as it returns.
Thus, FEXT is not as significant a problem as NEXT.
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Power Sum Near-end Crosstalk (PSNEXT)
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Power Sum NEXT (PSNEXT) measures the cumulative
effect of NEXT from all wire pairs in the cable.
PSNEXT is computed for each wire pair based on the
NEXT effects of the other three pairs.
The combined effect of crosstalk from multiple
simultaneous transmission sources can be very detrimental
to the signal.
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Cable testing standards
The ten primary test parameters that must be verified for a
cable link to meet TIA/EIA standards are:
• Wire map
• Insertion loss
• Near-end crosstalk (NEXT)
• Power sum near-end crosstalk (PSNEXT)
• Equal-level far-end crosstalk (ELFEXT)
• Power sum equal-level far-end crosstalk (PSELFEXT)
• Return loss
• Propagation delay
• Cable length
• Delay skew
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Cable testing standards
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The Ethernet standard specifies that each of the pins on an
RJ-45 connector have a particular purpose.
A NIC transmits signals on pins 1 and 2, and it receives
signals on pins 3 and 6.
The wires in UTP cable must be connected to the proper
pins at each end of a cable.
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Cable testing
standards
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The wire map test insures that no open or short circuits
exist on the cable.
An open circuit occurs if the wire does not attach properly
at the connector.
A short circuit occurs if two wires are connected to each
other.
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Cable testing standards
• The wire map test also verifies that all eight wires are connected to the
correct pins on both ends of the cable.
• There are several different wiring faults that the wire map test can
detect.
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Other test parameters
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Return loss is a measure in decibels of reflections that are
caused by the impedance discontinuities at all locations
along the link.
Recall that the main impact of return loss is not on loss of
signal strength.
The significant problem is that signal echoes caused by the
reflections from the impedance discontinuities will strike
the receiver at different intervals causing signal jitter.
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Time-based parameters
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Testers measure the length of the wire based on the
electrical delay as measured by a Time Domain
Reflectometry (TDR) test, not by the physical length of the
cable jacket.
Since the wires inside the cable are twisted, signals
actually travel farther than the physical length of the cable.
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Testing optical fiber
• Fiber links are subject to the optical equivalent of UTP impedance
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discontinuities.
When light encounters an optical discontinuity, some of the light signal
is reflected back in the opposite direction with only a fraction of the
original light signal continuing down the fiber towards the receiver.
This results in a reduced amount of light energy arriving at the receiver,
making signal recognition difficult.
Just as with UTP cable, improperly installed connectors are the main
cause of light reflection and signal strength loss in optical fiber.
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Testing optical fiber
• Absence of electrical signals.
• There are no crosstalk problems on fiber optic cable.
• External electromagnetic interference or noise has no affect on fiber
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cabling.
Attenuation does occur on fiber links, but to a lesser extent than on
copper cabling.
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A new standard
• On June 20, 2002, the Category 6 (or Cat 6) addition to the TIA-568
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standard was published.
The official title of the standard is ANSI/TIA/EIA-568-B.2-1.
Although the Cat 6 tests are essentially the same as those specified by
the Cat 5 standard, Cat 6 cable must pass the tests with higher scores
to be certified.
Cat6 cable must be capable of carrying frequencies up to 250 MHz and
must have lower levels of crosstalk and return loss.
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Summary
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