Cable Testing

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Cable Testing
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Introduction

• 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.

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Student Should understand the concepts of

• 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.

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Waves..

• A wave is energy traveling from one place to
  another.
• It may be defined as any disturbance generated in
  any material, such as
• 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.

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Waves
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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.

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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.

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Signals

• Signals in a network can be represented in two
  different ways.
• Analog Signals
• Digital Signals

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Analog Signals
Length

• 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

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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

• Bits are represented by either no voltage (0) or
  3 to 6 Volts (1).
• A Signal Reference Ground attached close to a
  computers digital circuits establishes the
  baseline for no voltage.
• Bits must arrive at the destination undistorted
  in order to be properly interpreted.
• What six things can distort a bit?

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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

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Viewing Signals in Time and Frequency
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Viewing Signals in Time and Frequency
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Analog and Digital Signals in Time and Frequency
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Bandwidth
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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
• NEXT A Thermal Noise Impulse/Reference Ground
  Noise EMI/RFI NEXT B

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Signaling over Copper and Fiber
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Signaling over Copper and Fiber
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Signaling over Copper and Fiber
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Signaling over Copper and Fiber
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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
• Not exceeding a mediums distance requirement
  (100 meters for
• Cat 5 cable)
• By using repeaters that amplify the signal

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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
• Fluorescent lighting (EMI)
• Electrical motors (EMI)
• Radio systems (RFI)

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EMI/RFI Noise Example

• Source computer sends out a digital signal.

• Along the path, the signal encounters EMI noise.

• The digital signal and EMI combine to distort the
  signal.

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EMI/RFI Noise

• 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.

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Canceling EMI/RFI Noise

• 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.

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Canceling EMI/RFI Noise

• 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

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Sources of Noise on Copper Media
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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.
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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.
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Timing Problems

• Dispersionsimilar to attenuation is the
  broadening of a signal as it travels down the
  media.
• Jittercaused by unsynchronized clocking signals
  between source and destination. This means bits
  will arrive later or earlier than expected.
• Latencyis the delay of a network signal caused
  by
• Time it takes a bit to travel to its destination
• Devices the bit travels through

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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 shared-medium 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
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Cable Testing Standards
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Cable Testing Standards
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Crosstalk
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Time-based Parameters
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Testing Optical Fiber
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Testing Optical Fiber
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New Standard

• 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.