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SUNY Ulster
Cisco Semester 1
Unit 4 – Cable Testing
K. Wick CCAI
Signals and Noise
Note to instructor: Have curriculum open
Sine Waves
 Shape
 Amplitude
 Frequency
 Wavelength
 Period = 1/f
 Objective 4.1.1 has a nice demo of
sinusoidal waves.
Square Waves
 Objective 4.1.2 does NOT show a pure
square wave.
 It shows a square wave with a DC
component. (Wave is offset from zero)
 Square wave vs Periodic Pulse vs Pulse
 Logarithms are a way to express
differences between numbers that are
orders of magnitude apart.
 If we ask “To what power do we raise ten to
be equal to a number in question”, that is
the logarithm of the number.
 Log 1 = 0 because 100 = 1
 Log 8 = 0.9031 because 100..9031 = 8
 Run interactive activities in 4.1.3
 The logarithm of ten raised to any power is
the power itself.
 Log 1000 = log 103 = 3
 Log 1,000,000 = log 106 = 6
 Cisco says, “There are two formulas for
calculating decibels”:
 dB = 10 log10 (Pfinal / Pref)
 dB = 20 log10 (Vfinal / Vreference)
 They are partially correct.
 These formulas depend on equal input and
output impedance.
 Answers to examples in 4.1.4
• -30 decibels. A loss
• 1.7 microwatts
• -113.9 decibels. A major loss.
Measuring Devices
A multimeter or digital multimeter
(DMM) measures voltage, current,
resistance, continuity and
sometimes other parameters.
An oscilloscope gives a visual
display of voltage versus time.
– A Cable meter will test a
cable for correct wiring
Fourier Analysis ????
 Cisco is insane!
 OK here is a basic:
 Every complex waveform can be made by
adding a series of sinusoids of proper
frequency and amplitude.
 A square wave is the sum of the series
 A * sin(x) + A/3 * sin(3x) + A/5 * sin(5x) …
Building a Square Wave
Measuring Devices
 Spectrum Analyzer.
 Gives a bar graph representing all
frequencies and amplitudes present in a
Measuring Devices
 Time Delay Reflectometer
 Because the wires inside the cable are
twisted, signals actually travel farther than
the physical length of the cable.
 A TDR measurement sends a pulse signal
down a wire pair and measures the amount
of time required for the pulse to return on
the same wire pair.
Measuring Devices
 Time Delay Reflectometer
 Also used to identify the distance to wiring
faults such as shorts and opens.
 This presumes that we know the propagation
speed of the specific wire type.
Ten tests for Category 5 cable
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
Types of Signal Degradation
Propagation and Delay
 Propagation means travel of a signal
 Propagation Delay is the time it takes a
signal to travel from point to point.
 It is measured in hundreths of nanoseconds.
 Skew: The difference in delay between pairs.
 Read CISCO questions carefully, watch the
exact wording!
 Attenuation means loss of signal amplitude
 If a signal gets too small, it can not be
decoded at the receiving end
Sometimes on a physical medium a signal travels
to the end of the medium and part of the signal
reflects back toward the source.
 This reflection can interfere with the original
 Reflections are especially bad with
impedance mismatches in the physical
(Caused by wrong media or bad connections)
 Nominal Z for Cat 5 cable is 100 ohms
 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.
Noise – Where are the 1’s and 0’s?
5 volts
Analog vs Digital Bandwidth
Analog bandwidth typically refers to the
frequency range of an analog electronic system.
 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
Analog vs Digital Bandwidth
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).
Analog vs Digital Bandwidth
During cable testing, analog bandwidth is used to
determine the digital bandwidth of a copper
 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 at each
frequency is calculated.
 In general, media that will support higher analog
bandwidths without high degrees of attenuation
will also support higher digital bandwidths.
Noise Pickup
 External
Radio - EMI RFI
Line to ground
 Crosstalk - NEXT
 Twisted Pairs minimize noise pickup
Crosstalk and other evils
 Near End – at near end of link.
 Far End
 Power Sum – cumulative effect of crosstalk
on all pairs in the cable.
 For all – Larger negative numbers mean
LESS crosstalk. (-30dB vs -20dB).
 Some testers leave out the minus sign.
Wiring Errors (Fluke Tester)
Or Crossover Cable
Fiber Optic Cable Testing
Remember that a fiber link consists of two
separate glass fibers functioning as independent
data pathways.
 Fiber optic cable does not suffer from crosstalk or
noise pickup.
 Attenuation does occur on fiber links, but to a
lesser extent than on copper cabling.
 Fiber links are subject to the optical equivalent of
UTP impedance discontinuities.
Fiber Optic Cable Testing
Just as with UTP cable, improperly installed
connectors are the main cause of light reflection
and signal strength loss in optical fiber.
 If attenuation weakens the light signal at the
receiver, then data errors will result. Testing fiber
optic cable primarily involves shining a light
down the fiber and measuring whether a
sufficient amount of light reaches the receiver.
 If the fiber fails the test, the cable test
instrument should indicate where the optical
discontinuities occur along the length of the
cable link.
Category 6 UTP and STP
 Cables certified as Cat 6 cable must pass
the same ten tests as Cat 5 cable.
 Cat 6 cable must pass these tests with
higher scores to be certified.
 It must be capable of carrying frequencies
up to 250 MHz (vs 100 MHz) and must have
lower levels of crosstalk and return loss.
End of Chapter 4