Module%204:%20Cable%20Testing

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Module 4 Cable Testing

• James Chen
• ydjames_at_ydu.edu.tw

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Outline

• Background for Studying Frequency-Based Cable
  Testing
• Waves
• Sine waves and square waves
• Exponents and logarithms
• Decibels
• Viewing signals in time and frequency
• Analog and digital signals in time and frequency
• Noise in time and frequency
• Bandwidth
• Signals and Noise
• Signaling over copper and fiber optic cabling
• Attenuation and insertion loss on copper media
• Sources of noise on copper media
• Types of crosstalk
• Cable testing standards
• Other test parameters
• Time-based parameters
• Testing optical fiber
• A new standard

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• Background for Studying Frequency-Based Cable
  Testing

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Waves

• A wave is energy traveling from one place to
  another.
• A bucket of water that is completely still.
• no waves, no disturbances
• The ocean always has some sort of detectable
  waves.
• wind and tide(??)
• measured in meters.
• How frequently the waves reach the shore ?
• period
• It is the amount of time between each wave,
  measured in seconds.
• frequency
• It is the number of waves that reach the shore
  each second, measured in Hertz.
• One Hertz is equal to one wave per second, or one
  cycle per second. 
• Electromagnetic waves
• voltage waves on copper media
• light waves in optical fiber
• Pulse
• The disturbance is caused in a fixed or
  predictable duration.

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Sine waves and square waves

• Sine waves, or sinusoids
• periodical
• repeat the same pattern at regular intervals
• continuously varying with time
• analog waves

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Sine waves and square waves (cont.)
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Sine waves and square waves (cont.)
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Sine waves and square waves (cont.)

• Square waves
• periodical
• do not continuously vary with time
• The wave holds one value for some time, and then
  suddenly changes to a different value.
• digital signals, or pulses

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Exponents and logarithms

• In networking, there are three important number
  systems
• Base 2 binary
• Base 10 decimal
• Base 16 hexadecimal
• Power and exponent
• 10 10 102 (10 raised to the second
  power, exponent 2)
• 10 10 10 103 (10 raised to the third
  power, exponent 3)
• Numbers with exponents are used to easily
  represent very large or very small numbers.
• Logarithm
• base 10 logarithms are often abbreviated (???)log
• log (109) 9
• log (10-3) -3

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Decibels(??)

• The first formula describes decibels in terms of
  power (P), and the second in terms of voltage
  (V).
• dB measures the loss or gain of the power of a
  wave.
• negative values a loss in power as the wave
  travels
• positive values a gain in power if the signal
  is amplified
• Light waves on optical fiber and radio waves in
  the air are measured using the power formula.
• Electromagnetic waves on copper cables are
  measured using the voltage formula.

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Viewing signals in time and frequency

• An oscilloscope is an important electronic device
  used to view electrical signals such as voltage
  waves and pulses.
• The x-axis represents time.
• The y-axis represents voltage or current.
• Time-domain analysis
• Spectrum analyzer
• The x-axis represents frequency.
• Frequency-domain analysis
• Electromagnetic signals use different frequencies
  for transmission so that different signals do not
  interfere with each other.
• Frequency modulation (FM) radio signals use
  frequencies that are different from television or
  satellite signals. When listeners change the
  station on a radio, they are changing the
  frequency that the radio is receiving

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Viewing signals in time and frequency (cont.)
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Analog and digital signals in time and frequency

• To understand the complexities of networking
  signals and cable testing, examine how analog
  signals vary with time and with frequency.
• First, consider a single-frequency electrical
  sine wave, whose frequency can be detected by the
  human ear. If this signal is transmitted to a
  speaker, a tone can be heard. How would a
  spectrum analyzer display this pure tone? 
• Next, imagine the combination of several sine
  waves. The resulting wave is more complex than a
  pure sine wave. Several tones would be heard. How
  would a spectrum analyzer display this? The graph
  of several tones shows several individual lines
  corresponding to the frequency of each tone.
• Finally, imagine a complex signal, like a voice
  or a musical instrument. What would its spectrum
  analyzer graph look like? If many different tones
  are present, a continuous spectrum of individual
  tones would be represented.

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Analog and digital signals in time and frequency
(cont.)
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Noise in time and frequency

• Noise is an important concept in communications
  systems
• undesirable signals
• Noise can originate from natural and
  technological sources.
• Noise is added to the data signals in
  communications systems.
• There are many possible sources of noise
• Nearby cables which carry data signals.
• Radio frequency interference (RFI)
• Noise is from other signals being transmitted
  nearby.
• Electromagnetic interference (EMI)
• Noise is from nearby sources such as motors and
  lights
• Laser noise at the transmitter or receiver of an
  optical signal
• White noise
• Noise that affects all transmission frequencies
  equally.
• Narrowband interference
• Noise that only affects small ranges of
  frequencies.

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Bandwidth

• Analog bandwidth
• Analog bandwidth could be used to describe the
  range of frequencies transmitted by a radio
  station or an electronic amplifier.
• measurement unit is Hertz
• Digital bandwidth
• Digital bandwidth measures how much information
  can flow from one place to another in a given
  amount of time.
• measurement unit is bits per second (bps).
• During cable testing, analog bandwidth is used to
  determine the digital bandwidth of a copper
  cable.
• Analog Digital
• Media that will support higher analog bandwidths
  without high degrees of attenuation will also
  support higher digital bandwidths.

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• Signals and Noise

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Signaling over copper and fiber optic cabling
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Attenuation and insertion loss on copper media

• Attenuation is the decrease in signal amplitude
  over the length of a link.
• Long cable lengths and high signal frequencies
  contribute to greater signal attenuation.
• Attenuation is expressed in decibels (dB) using
  negative numbers.
• Smaller negative dB values are an indication of
  better link performance.
• There are several factors that contribute to
  attenuation.
• the resistance of the copper cable
• the insulation of the cable
• impedance of a Cat5 cable is 100 ohms.
• Impedance discontinuity or an impedance mismatch.
  
• a portion of a transmitted signal will be
  reflected back to the transmitting device, much
  like an echo.
• jitter and results in data errors.
• Insertion loss
• The combination of the effects of signal
  attenuation and impedance discontinuities on a
  communications link.

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Sources of noise on copper media

• Noise is any electrical energy on the
  transmission cable that makes it difficult for a
  receiver to interpret the data sent from the
  transmitter.
• TIA/EIA-568-B certification of a cable now
  requires testing for a variety of types of noise.
  
• Crosstalk involves the transmission of signals
  from one wire to a nearby wire.
• Twisted-pair cable is designed to take advantage
  of the effects of crosstalk in order to minimize
  noise.
• Higher categories of UTP require more twists on
  each wire pair in the cable to minimize crosstalk
  at high transmission frequencies.

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Types of crosstalk

• There are three distinct types of crosstalk
• Near-end Crosstalk (NEXT)
• Far-end Crosstalk (FEXT)
• Power Sum Near-end Crosstalk (PSNEXT)
• Near-end crosstalk (NEXT)
• It is computed as the ratio of voltage amplitude
  between the test signal and the crosstalk signal
  when measured from the same end of the link.
• Negative value of decibels (dB).
• Low negative numbers indicate more noise, just as
  low negative temperatures indicate more heat
  (closed to zero).
• By tradition, cable testers do not show the minus
  sign indicating the negative NEXT values.
• A NEXT reading of 30 dB (which actually indicates
  -30 dB) indicates less NEXT noise and a better
  cable than does a NEXT reading of 10 dB.

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Types of crosstalk (cont.)

• NEXT needs to be measured from each pair to each
  other pair in a UTP link, and from both ends of
  the link.
• To shorten test times, some cable test
  instruments allow the user to test the NEXT
  performance of a link by using larger frequency
  step sizes than specified by the TIA/EIA
  standard. The resulting measurements may not
  comply with TIA/EIA-568-B, and may overlook
  (??)link faults.

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Types of crosstalk (cont.)

• Far-end crosstalk (FEXT)
• Due to attenuation, crosstalk occurring further
  away from the transmitter creates less noise on a
  cable than NEXT.
• The noise caused by FEXT still travels back to
  the source, but it is attenuated as it returns.
  Thus, FEXT is not as significant a problem as
  NEXT.

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Types of crosstalk (cont.)

• Power Sum NEXT (PSNEXT)
• It measures the cumulative effect of NEXT from
  other 3 pairs in the cable.
• TIA/EIA-568-B certification now requires this
  PSNEXT test.
• Some Ethernet standards such as 10BASE-T and
  100BASE-TX receive data from only one wire pair
  in each direction.
• For newer technologies such as 1000BASE-T that
  receive data simultaneously from multiple pairs
  in the same direction, power sum measurements are
  very important tests.

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Cable testing standards

• The TIA/EIA-568-B standard specifies ten tests
  that a copper cable must pass if it will be used
  for modern, high-speed Ethernet LANs.
• TIA/EIA standards are
• 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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Cable testing standards (cont.)

• 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 wire map test insures that no open or short
  circuits exist on the cable.
• An open circuit occurs if the wire does not
  attach properly at the connector.
• A short circuit occurs if two wires are connected
  to each other.

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Cable testing standards (cont.)

• The wiring faults (TIA/EIA-568-B)

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Other test parameters

• 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.
• Equal-level far-end crosstalk (ELFEXT)
• Pair-to-pair ELFEXT is expressed in dB as the
  difference between the measured FEXT and the
  insertion loss of the wire pair whose signal is
  disturbed by the FEXT.
• ELFEXT is an important measurement in Ethernet
  networks using 1000BASE-T technologies.
• Power sum equal-level far-end crosstalk
  (PSELFEXT)
• It is the combined effect of ELFEXT from all wire
  pairs.
• Return loss
• It is a measure in decibels of reflections that
  are caused by the impedance discontinuities at
  all locations along the link.
• Recall that the main impact of return loss is not
  on loss of signal strength.
• The significant problem is that signal echoes
  caused by the reflections from the impedance
  discontinuities will strike(??)the receiver at
  different intervals causing signal jitter.

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Time-based parameters

• 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.
  
• TIA/EIA-568-B-1 specifies that the physical
  length of the link shall be calculated using the
  wire pair with the shortest electrical delay.
• Time Domain Reflectometry (TDR) test
• Since the wires inside the cable are twisted,
  signals actually travel farther than the physical
  length of the cable.
• It 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.
• 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.
• This can calculate the approximate distance to
  the wiring fault.

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Time-based parameters (cont.)

• Delay skew
• The propagation delays of different wire pairs in
  a single cable can differ slightly because of
  differences in the number of twists and
  electrical properties of each wire pair.
• The delay difference between pairs is called
  delay skew.
• Delay skew is a critical parameter for high-speed
  networks in which data is simultaneously
  transmitted over multiple wire pairs, such as
  1000BASE-T Ethernet.
• If the delay skew between the pairs is too great,
  the bits arrive at different times and the data
  cannot be properly reassembled.
• All cable links in a LAN must pass all of the
  tests in the TIA/EIA-568-B standard.
• These tests ensure that the cable links will
  function reliably at high speeds and frequencies.
  
• High quality cable test instruments should be
  correctly used to ensure that the tests are
  accurate.

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Testing optical fiber

• A fiber link consists of two separate glass
  fibers.
• There are no crosstalk problems on fiber optic
  cable.
• External electromagnetic interference or noise
  has no affect on fiber cabling.
• Optical discontinuity
• Some of the light signal is reflected back in the
  opposite direction.
• Only a fraction of the original light signal
  continuing down the fiber towards the receiver.
• This results in a reduced amount of light energy
  arriving at the receiver, making signal
  recognition difficult.
• Improperly installed connectors are the main
  cause of light reflection and signal strength
  loss in optical fiber.
• The strength of the light signal that arrives at
  the receiver is important.
• If attenuation weakens the light signal at the
  receiver, then data errors will result.
• Optical link loss budget (??)
• The acceptable amount of signal power loss that
  can occur without dropping below the requirements
  of the receiver.
• If the fiber fails the test, The problem usually
  is one or more improperly attached connectors.

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A new standard

• On June 20, 2002, the Category 6 (or Cat 6)
  addition to the TIA-568 standard was published.
• The official title of the standard is
  ANSI/TIA/EIA-568-B.2-1.
• Cat 6 cable must pass the tests with higher
  scores to be certified.
• Cat6 cable must be capable of carrying
  frequencies up to 250 MHz and must have lower
  levels of crosstalk and return loss.
• 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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• END