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Transcript
SUNY Ulster
Cisco Semester 1
Unit 4 – Cable Testing
K. Wick CCAI
Background
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
 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
Logarithms
 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
Decibels
 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.
Decibels
 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
waveform.
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

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

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
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
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
 Attenuation means loss of signal amplitude
 If a signal gets too small, it can not be
decoded at the receiving end
Reflection
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
signal.

 Reflections are especially bad with
impedance mismatches in the physical
layer.
(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
stations.

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
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 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
•
•
•
•
Impulse
Radio - EMI RFI
Line to ground
Motor
 Crosstalk - NEXT
 Twisted Pairs minimize noise pickup
Crosstalk and other evils
Crosstalk
 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