Transcript CCNA SEMESTER 1 V 3.0

CCNA SEMESTER 1
V 3.0
CHAPTER 4 – Cable testing
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Students completing this chapter
should be able to:
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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.
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 Category2 6
cable.
Sine waves and square waves
A wave is energy traveling from one place to another.
Sine waves-Analog signals
Square waves-Digital signals
A = Amplitude (height or depth of
wave)
A= Amplitude (Height of Pulses)
T= Period (time to complete 1 wave cycle
F= Frequency (cycles per second) = 1/T
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Exponents and logarithms
 Exponents are used to represent very large
or very small numbers. The base of a number
raised to a positive exponent is equal to the
base multiplied by itself exponent times. For
example, 103 = 10x10x10 = 1000.
 Logarithms are similar to exponents. A
logarithm to the base of 10 of a number
equals the exponent to which 10 would have
to be raised in order to equal the number. For
example, log10 1000 = 3 because 103 = 1000.
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Decibels
Decibels are measurements of a gain or loss
in the power of a signal.
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Negative values represent losses
Positive values represent gains.
Formulas for calculating decibels
dB in form of Power (P)
dB = 10 log10 (Pfinal / Pref)
delivered power
original power
dB in form of Voltage (V)
dB = 20 log10 (Vfinal / Vref)
delivered voltage
original voltage
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Oscilloscope
 Oscilloscope is an important electronic device
used to view electrical signals such as
voltage waves and pulses.
The x-axis on the display represents time,
and the y-axis represents voltage or current.
Analyzing signals using an oscilloscope is
called time-domain analysis, because the xaxis or domain of the mathematical function
represents time.
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Spectrum analyzer
 An electronic device called a spectrum
analyzer creates graphs for frequencydomain analysis.
In frequency-domain analysis, the x-axis
represents frequency.
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Oscilloscope and Spectrum analyser
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Noise
 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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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.
 Laser noise at the transmitter or receiver of
an optical signal.
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Types of noise
 White noise
Affects all transmission frequencies equally
 Narrowband interference
Only affects small ranges of frequencies
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Bandwidth
 Analog bandwidth
Refers to the frequency range of an analog electronic
system.
The units of measurement is Hertz
 Digital bandwidth
Digital bandwidth measures how much information can
flow from one place to another in a given amount of
time.
The unit of measurement is bits per second (bps).
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Various types of cabling
 Shielded twisted-pair (STP)
STP cable is more expensive,
more difficult to install, and less
frequently used than UTP.
 Unshielded twisted pair (UTP)
UTP contains no shielding and
is more susceptible to external
noise but is the most frequently
used because it is inexpensive
and easier to install.
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Various types of cabling
 Coaxial cable
Coaxial cable is a type of
shielded cable
 Fiber optic cable
Fiber optic cable is used to
transmit data signals by
increasing and decreasing
the intensity of light to
represent binary ones and
zeros.
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Shielding material protects the data signal
from external sources of noise and from noise
generated by electrical signals within the
cable.
Shielded cable
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Coaxial cable
Shielded twisted-pair (STP)
Ushielded cable
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unshielded twisted pair (UTP).
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Fiber Optic Cable
 Optical signals are not affected by electrical
noise, and optical fiber does not need to be
grounded.
 Therefore, optical fiber is often used between
buildings and between floors within the
building.
 As costs decrease and demand for speed
increases, optical fiber may become a more
commonly used LAN media.
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Cables Comparison
Kind of
Cable
Speed and
throughput
STP
10-100Mps Medium Large
100m
Expensive
UTP
10-100Mps Medium Large
100m
Least
Expensive
Coaxial 10-100Mps Medium
500m
Inexpensive
Fiber
optic
Up to
3000m
Most
Expensive
100+ Mps
Media &
Max
connector Cable
size
Lenght
Small
Cost
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Attenuation
 Attenuation is the decrease in signal amplitude
over the length of a link.
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Factors that contribute to attenuation
 The resistance of the copper cable converts
some of the electrical energy of the signal to
heat.
 Signal energy is also lost when it leaks
through the insulation of the cable and by
impedance caused by defective connectors.
Long cable lengths and high signal frequencies
contribute to greater signal attenuation
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How to measure the attenuation
 Attenuation on a cable is measured by a
cable tester using the highest frequencies
that the cable is rated to support.
 Attenuation is expressed in decibels (dB)
using negative numbers.
 Smaller negative dB values are an indication
of better link performance.
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Impedance and impedance mismatch
 Impedance is a measurement of the
resistance of the cable to alternating current
(AC) and is measured in ohms.
 The normal, or characteristic, impedance of a
Cat5 cable is 100 ohms. If a connector is
improperly installed on Cat5, it will have a
different impedance value than the cable.
This is called an impedance discontinuity or
an impedance mismatch.
 Impedance discontinuities cause attenuation
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Crosstalk and types of crosstalk
Crosstalk is another from of noice.
Involves the transmission of signals from one
wire to a nearby wire
Poorly terminated network cabling is a cause
of crosstalk
Types of crosstalk are
Near-end Crosstalk (NEXT)
 Far-end Crosstalk (FEXT)
 Power Sum Near-end Crosstalk (PSNEXT)
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Near-end Crosstalk (NEXT)
 Near-end Crosstalk (NEXT) is 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)
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.
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Power Sum Near-end Crosstalk
(PSNEXT)
Power Sum NEXT (PSNEXT) measures the cumulative
effect of NEXT from all wire pairs in the cable.
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How to minimize noise
 Twisting one pair of wires in a cable also
helps to reduce crosstalk of data or noise
signals from an adjacent wire pair
 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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Untwisting of wire pairs must be kept to an absolute minimum
to reduce crosstalk of data or noise signals from an adjacent
wire pair and to ensure reliable LAN communications.
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RJ-45 PINOUTS
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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The ten primary test parameters that must be verified for
a cable link to meet TIA/EIA standards are:
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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
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Wire Map test
 The wire map test insures that no open or short
circuits exist on the cable.
Good wiremap
Short circuit.Two
wires are
connected to each
other
Open circuit. The
wire does not
attach properly
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Wire Map test
 The wire map test also verifies that all eight wires
are connected to the correct pins on both ends of the
cable.
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Insertion loss
 The combination of the effects of signal
attenuation and impedance discontinuities on
a communications link is called insertion loss.
Insertion loss is measured in decibels at the
far end of the cable.
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Crosstalk
 Crosstalk is meassure in four separate tests
 A cable tester measures NEXT by applying a
test signal to one cable pair.
 The equal-level far-end crosstalk (ELFEXT)
test measures FEXT
 Power sum equal-level far-end crosstalk
(PSELFEXT) is a compine effect of ELFEXT
from all wire pairs
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Return loss
 Return loss is a measure in decibels of
reflections that are caused by the impedance
discontinuities at all locations along the link.
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Propagation delay
 Propagation delay is a simple measurement
of how long it takes for a signal to travel along
the cable being tested. The delay in a wire
pair depends on its length, twist rate, and
electrical properties. Delays are measured in
hundredths of nanoseconds. One
nanosecond is one-billionth of a second, or
0.000000001 second. The TIA/EIA-568-B
standard sets a limit for propagation delay for
the various categories of UTP.
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TDR Test
 The TDR test is used not only to determine
length, but also to identify the distance to
wiring faults such as shorts and opens. When
the pulse encounters an open, short, or poor
connection, all or part of the pulse energy is
reflected back to the tester.
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Delay skew
 The propagation delay difference between
pairs is called delay skew.
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Category 6 and category 5 cable
Category 6 cable must meet more rigorous
frequency testing standards than Category 5
cable.
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Discontinuity
A pulse is a deliberate, fixed disturbances of
predictable duration used to…
•Measure propagation delay (delay skew) to
determine the value of the data being transmitted
•Find discontinuities (reflections, jitter)
•Measure transmission length
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Testing optical fiber
A fiber test instrument checks whether the optical link loss
budget has been exceeded. If the fiber fails the test, the
cable test instrument should indicate where the optical
discontinuities occur along the length of the cable link.
Usually, the problem is one or more improperly attached
connectors
Calibrated Light Source
and Power Meter
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Testing Cat5 and Cat6 cable
A quality cable tester similar to the Fluke DSP-4000
series or Fluke OMNIScanner2 can perform all the
test measurements required for Cat 5, Cat 5e, and
Cat 6 cable certifications of both permanent links
and channel links.
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Fluke DSP-LIA013
Channel/Traffic Adapter for
Cat 5e
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