Transcript CCNA 1 Chapter 4 Cable Testing and Cabling LANs and WANs

Cable Testing
Introduction
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Networking media is the backbone of network.
Inferior quality of network cabling may cause
network failures and unreliable performance.
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.
Student Should understand the concepts of:
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Sine waves and square waves.
Calculate exponents and logarithms.
Calculate decibels.
Time, frequency, and noise.
Digital and analog bandwidth.
Noise levels
Attenuation and impedance mismatch.
crosstalk, near-end crosstalk, far-end crosstalk, and power
sum near-end crosstalk.
Grounding and wire twisting
copper cable tests defined in TIA/EIA-568-B.
Category 5 and Category 6 cable.
Waves……..
A wave is energy traveling from one place
to another.
 It may be defined as any disturbance
generated in any material, such as:
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– Throwing a stone in water
– Sound (created by disturbance in air)
– Electric supply at your house (electric impulses
generated in conductor wire by providing a
pressure/ force or voltage level.
Waves
Sine Waves
They repeat the same pattern at regular
intervals.
They occur naturally and change regularly
over time.
No two adjacent points on a graph have the
same value.
Square Waves
They repeat the same pattern at regular
intervals.
They repeat the flat pattern on both the top
and bottom of the wave.
They do not continuously vary with time.
Signals
Signals in a network can be represented in
two different ways.
 Analog Signals
 Digital Signals
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Length
Analog Signals
Time
Is wavy
Has a continuously varying voltage versus
time graph
Is typical of things in nature
Has been widely used in Telecommunications
for over 100 years
Digital Signal
Digital signals have a square wave with
instant transitions from low to high voltage
states (0 to 1).
 If voltage present then 1.
 If no voltage then 0
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Networks Use Digital Signaling
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Bits are represented by either no voltage (0) or
+3 to +6 Volts (1).
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A Signal Reference Ground attached close to a
computer’s digital circuits establishes the
baseline for no voltage.
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Bits must arrive at the destination undistorted in
order to be properly interpreted.
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What six things can distort a bit?
Decibels
The decibel (dB) is a measurement unit important in describing
networking signals. The decibel is related to the exponents and
logarithms.
•Pfinal is the delivered power measured in Watts
•Pfinal is the delivered power measured in Watts
•Vfinal is the delivered voltage measured in Volts
•Vreference is the original voltage measured in Volts
Viewing Signals in Time and
Frequency
Viewing Signals in Time and
Frequency
Analog and Digital Signals
in Time and Frequency
Bandwidth
Noise in Time and Frequency
Noise is unwanted additions to the signal
Noise is unavoidable
Too much noise can corrupt a bit turning a binary 1
into a binary 0, or a 0 into a 1, thus destroying the
message.
There are five kinds of noise:
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NEXT A; Thermal Noise; Impulse/Reference Ground Noise;
EMI/RFI; & NEXT B
Signaling over Copper and Fiber
Signaling over Copper and Fiber
Signaling over Copper and Fiber
Signaling over Copper and Fiber
Attenuation and Insertion Loss on
Copper Media
Attenuation is the loss of
signal strength.
The signal degrades or
losses amplitude as it
travels (propagates)
along the medium
Loss of amplitude means that the receiving device can
no longer distinguish a 1 bit from a 0 bit.
Attenuation is prevented by:
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Not exceeding a medium’s distance requirement (100 meters for
Cat 5 cable)
By using repeaters that “amplify” the signal
EMI/RFI Noise
EMI (Electromagnetic Interference) and
RFI (Radio Frequency Interference) attack
the quality of electrical signals on the cable.
 Sources of EMI/RFI include:
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– Fluorescent lighting (EMI)
– Electrical motors (EMI)
– Radio systems (RFI)
EMI/RFI Noise Example
Digital Signal
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Source computer sends out
a digital signal.
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Along the path, the signal
encounters EMI noise.
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The digital signal and EMI
combine to distort the signal.
EMI
Distorted Signal
EMI/RFI Noise
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Two ways to prevent EMI/RFI Noise:
– Through shielding the wires in the cable with a
metal braid or foil. (Increases cost and diameter
of the cable)
– Through cancellation the wires are twisted
together in pairs to provide self-shielding
within the network media.
Canceling EMI/RFI Noise
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UTP Cat 5 has eight
wires twisted into four
pairs.
In each pair, one wire is
sending data and the
other is receiving.
As the electrons flow
down the wire, they
create a small, circular
magnetic field around the
wire.
Canceling EMI/RFI Noise
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Since the two wires are
close together, their
opposing magnetic fields
cancel each other.
They also cancel out
outside magnetic fields
(EMI/RFI).
Twisting of the wires
enhances cancellation
Sources of Noise on Copper
Media
Types of Crosstalk
Near-end crosstalk (NEXT)
NEXT computed as the
ratio of voltage amplitude
between the test signal and
the crosstalk signal when
measured from the same
end of the link.
Crosstalk is avoided
by using proper installation including:
Strict adherence to RJ-45 termination procedures
Using high quality twisted pair cabling
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Types of Crosstalk
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.
The noise caused by FEXT still travels back to the
source, but it is attenuated as it returns.
Types of Crosstalk
Power sum near-end crosstalk (PSNEXT)
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.
Timing Problems
Dispersion—similar to attenuation; is the
broadening of a signal as it travels down the
media.
 Jitter—caused by unsynchronized clocking
signals between source and destination. This
means bits will arrive later or earlier than
expected.
 Latency—is the delay of a network signal
caused by:
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– Time it takes a bit to travel to its destination
– Devices the bit travels through
Collisions
Collisions occur in broadcast topologies
where devices share access to the network
media.
 A collision happens when two devices
attempt to communicate on the sharedmedium at the same time.
 Collisions destroy data requiring the source
to retransmit.
 The prevention of collisions will be
discussed in more detail later in the
semester.
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Cable Testing Standards
Cable Testing Standards
Cable Testing Standards
Crosstalk
Time-based Parameters
Testing Optical Fiber
Testing Optical Fiber
New Standard
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On June 20, 2002, the Category 6 (or Cat 6)
addition to the TIA-568 standard was published.
This new standard specifies the original set of
performance parameters that need to be tested for
Ethernet cabling as well as the passing scores for
each of these tests.
A quality cable tester is the Fluke
DSP-LIA013 Channel/Traffic Adapter
for Cat5e.