Mod 4 Cable Testing

1
Mod 4 Cable Testing

• CCNA 1 version 3.0

2
Overview

• Students completing this module should be able
  to
• 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
  Category 6 cable.

3
Background for Studying Frequency-Based Cable
Testing

• 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.

4
Amplitude and Frequency
5
Analog Signal
6
Other information

• For the next several slides we will explain
  analog signals from information from the
  following sources

7
Digital and Analog Bandwidth

• Bandwidth The width or carrying capacity of a
  communications circuit.
• Digital bandwidth the number of bits per second
  (bps) the circuit can carry
• used in digital communications such as T-1 or DDS
• measure in bps
• T-1 -gt 1.544 Mbps
• Analog bandwidth the range of frequencies the
  circuit can carry
• used in analog communications such as voice
  (telephones)
• measured in Hertz (Hz), cycles per second
• voice-grade telephone lines have a 3,100 Hz
  bandwidth

8
Digital and Analog Bandwidth

• Available at http//www.thinkgeek.com

9
Digital and Analog Bandwidth

• Digital Signals
• digital signal a signal whose state consists of
  discrete elements such as high or low, on or off
• Analog Signals
• analog signal a signal which is analogous to
  sound waves
• telephone lines are designed to carry analog
  signals

10
Sound Waves
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Analog Signals, Modulation and Modem Standards

• A perfect or steady tone makes a wave with
  consistent height (amplitude) and pitch
  (frequency) which looks like a sine wave. (Figure
  4-15)
• A cycle or one complete cycle of the wave
• The frequency (the number of cycles) of the wave
  is measured in Hertz
• Hertz (Hz) the number of cycles per second

12
Transmission Terminology (whatis.com)

• Broadband transmission
• In general, broadband refers to telecommunication
  in which a wide band of frequencies is available
  to transmit information.
• Because a wide band of frequencies is available,
  information can be multiplexed and sent on many
  different frequencies or channels within the band
  concurrently, allowing more information to be
  transmitted in a given amount of time (much as
  more lanes on a highway allow more cars to travel
  on it at the same time).
• Baseband transmission
• 1) Describing a telecommunication system in which
  information is carried in digital (or analog)
  form on a single unmultiplexed signal channel on
  the transmission medium. This usage pertains to a
  baseband network such as Ethernet and token ring
  local area networks.
• Narrowband transmission
• Generally, narrowband describes telecommunication
  that carries voice information in a narrow band
  of frequencies.
• More specifically, the term has been used to
  describe a specific frequency range set aside by
  the U.S. Fcc for mobile or radio services,
  including paging systems, from 50 cps to 64 Kbps.

13
Carrier Signal

• Carrier Signal or Analog Wave An electronic
  signal used to modulate data in broadband
  transmission, usually a sine wave.

14
Carrier Signal

• Three parts of any analog wave are
• 1. amplitude - the height of the wave
• 2. frequency - the pitch of the wave
• 3. phase - the shift or position of the wave
• These are the three parts we can modulate or
  change the carrier signal or wave!
• Modulate Change
• More in a moment.

15
Telephone Lines, Modems, and PSTN

• Voice grade telephone lines and equipment are
  designed to transmit tones between 300 and 3,400
  Hertz
• bandwidth 3,100 Hz or 3.1 KHz
• most of our human voice falls into this range
• Economics dictated the size of this bandwidth
• (Keyboard example)
• The maximum number of cycles (highest
  frequency) of an analog signal over voice grade
  telephone lines is 3,400 Hz (cycles per second)

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Telephone Lines, Modems, and PSTN

• Modem
• MOdulator/DEModulator
• converts analog signals to digital and digital
  signals to analog
• used for transmitting digital information between
  computers over voice-grade telephone lines
• Computers use transmission interface standards
  such as RS-232-C using positive and negative
  voltages which form square waves, whereas the
  PSTN is designed to carry analog signals (sine
  waves)

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Modulation

• modulation
• 1. the process of varying the characteristic of
  an electrical carrier wave (analog, sine wave) as
  the information on that wave varies
• Three types of modulation
• 1. amplitude modulation
• 2. frequency modulation
• 3. phase modulation
• 2. the process of converting digital signals to
  analog

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Amplitude Modulation

• Amplitude Modulation (AM)
• a modulation technique to vary the height the
  electrical signal (the sine wave or carrier wave
  with modems) to transmit ones and zeroes, while
  the frequency of the wave remains constant
• different amplitudes for 0s and 1s
• a.k.a. amplitude shift keying, ASK
• Figure 4-22
• frequency for each bit remains constant
• volume amplitude

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Amplitude Modulation

• Different amplitudes for 0s and 1s, while the
  frequency of the wave remains constant
• Full duplex
• different amplitudes and frequencies are used for
  different directions
• Disadvantage
• Voice-grade telephone lines are susceptible to
  distortions which affect amplitudes, as volume
  fades, the amplitude lowers
• Amplitude modulation only effective for low speed
  transmissions

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Frequency Modulation

• Frequency Modulation
• a modulation technique to vary the frequency of
  the sine wave (or carrier wave) to transmit ones
  and zeroes, while the amplitude remains constant
• different frequencies for 0s and 1s
• a.k.a. frequency shift keying, FSK
• Figure 4-23
• two separate frequencies for ones and zeroes

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Frequency Modulation

• Full Duplex
• requires a minimum of four frequencies, two
  frequencies for each direction
• i.e. CCITT V.21 for 300 baud modems
• Originating Sending
• Modem Modem
• 1270 Hz 1 2225 Hz
• 1070 Hz 0 2025 Hz
• loss of amplitude will not cause errors in
  transmission

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Frequency Modulation

• Conceptually
• If voice-grade telephone lines can transmit a
  maximum of 3,400 Hz (cycles per second),
  between 300 Hz and 3,400 Hz,
• AND
• If one cycle 1 bit,
• Then a maximum of 3,400 bits per second can be
  transmitted over voice grade telephone lines?
  (Hold that thought!)

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Phase Modulation

• Phase Modulation (PM)
• a modulation technique to vary the phase of the
  sine wave (or carrier wave) to transmit ones and
  zeroes, while the amplitude and the frequency
  remains constant
• sine waves repeat themselves indefinitely
• shifting the wave breaks the wave abruptly and
  starts it again a few degrees forward or backward
• A different phase shift, 0 to 360 degrees, is
  used to transmit one or more bits

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Phase Modulation

• A different phase shift, 0 to 360 degrees, is
  used to transmit one or more bits
• Full Duplex
• requires a minimum of two frequencies, one
  frequency for each direction

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Bits per second vs. Baud and High-speed modems

• So far, discussed transmission of one bit at a
  time, via high or low amplitude, high or low
  frequency, phase shift or no phase shift
• older modems sent only one bit per signal change,
  bps baud
• baud rate the number of these signal changes
  per second
• What if we could transmit more than one bit with
  each signal change (baud), amplitude, frequency
  of phase shift?
• Remember, voice-grade phone lines limit
  transmission to 3,400 Hz or 3,400 bps with 1
  cycle per bit

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Dibit Modulation
Dibit Amplitude modulation

• Dibit Modulation
• 2 bits per baud, per cycle
• Two bits or dibit modulation
• 00, 01, 10, 11
• Using Amplitude Modulation
• use four different amplitudes (wave heights)
• Using Frequency Modulation
• use four different frequencies
• Using Phase Modulation
• use four different phases

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Summary of Modulations

• Amplitude Modulation (AM)
• Frequency Modulation (FM)
• Phase Shift Keying (PSK)

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Back to Cisco Curriculum.
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Digital Signals

• Square waves, like sine waves, are periodic.
• However, square wave graphs do not continuously
  vary with time.
• The wave holds one value for some time, and then
  suddenly changes to a different value.
• This value is held for some time, and then
  quickly changes back to the original value.
• Square waves represent digital signals, or
  pulses. Like all waves, square waves can be
  described in terms of amplitude, period, and
  frequency.

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Exponents and logarithms (Not testable)

• Numbers with exponents are used to easily
  represent very large or very small numbers.
• It is much easier and less error-prone to
  represent one billion numerically as 109 than as
  1000000000.
• Many calculations involved in cable testing
  involve numbers that are very large, so exponents
  are the preferred format.
• Exponents can be explored in the flash activity.

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Exponents and logarithms (Not testable)

• One way to work with the very large and very
  small numbers that occur in networking is to
  transform the numbers according to the rule, or
  mathematical function, known as the logarithm.
• Logarithms are referenced to the base of the
  number system being used.
• For example, base 10 logarithms are often
  abbreviated log.
• To take the log of a number use a calculator or
  the flash activity.
• For example, log (109) equals 9, log (10-3) -3.
  

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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 described in prior sections.
• There are two formulas for calculating decibels
  
• dB 10 log10 (Pfinal / Pref)
• dB 20 log10 (Vfinal / Vreference)

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Decibels

• There are two formulas for calculating decibels
  
• dB 10 log10 (Pfinal / Pref)
• dB 20 log10 (Vfinal / Vreference)
• The variables represent the following values
• dB measures the loss or gain of the power of a
  wave.
• Decibels are usually negative numbers
  representing a loss in power as the wave travels,
  but can also be positive values representing a
  gain in power if the signal is amplified
• log10 implies that the number in parenthesis will
  be transformed using the base 10 logarithm rule
• Pfinal is the delivered power measured in Watts
• Pref is the original power measured in Watts
• Vfinal is the delivered voltage measured in Volts
  
• Vreference is the original voltage measured in
  Volts

34
Decibels (Not testable)

• The first formula describes decibels in terms of
  power (P), and the second in terms of voltage
  (V).
• Typically, 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.

35
Decibels (Not testable)

• Enter values for dB and Pref to discover the
  correct power.
• This formula could be used to see how much power
  is left in a radio wave after it has traveled
  over a distance through different materials, and
  through various stages of electronic systems such
  as a radio.

36
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 on the display represents time, and
  the y-axis represents voltage or current.
• There are usually two y-axis inputs, so two waves
  can be observed and measured at the same time.
• Analyzing signals using an oscilloscope is called
  time-domain analysis, because the x-axis or
  domain of the mathematical function represents
  time.

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Viewing signals in time and frequency

• Engineers also use frequency-domain analysis to
  study signals.
• In frequency-domain analysis, the x-axis
  represents frequency.
• An electronic device called a spectrum analyzer
  creates graphs for frequency-domain analysis.

38
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.
• Imagine the combination of several sine waves.

39
Noise in time and frequency

• Noise is an important concept in communications
  systems, including LANS.
• While 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.
• All communications systems have some amount of
  noise.
• Even though noise cannot be eliminated, its
  effects can be minimized if the sources of the
  noise are understood. Laser noise at the
  transmitter or receiver of an optical signal

40
Noise in time and frequency

• There are many 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

41
Bandwidth

• Analog bandwidth typically refers to the
  frequency range of an analog electronic system.
• Analog bandwidth could be used to describe the
  range of frequencies transmitted by a radio
  station or an electronic amplifier.
• The units of measurement for analog bandwidth is
  Hertz, the same as the unit of frequency.
• Example of analog bandwidth values are 3 kHz for
  telephony

42
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).

43
Signals and Noise

• 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
  Category 6 cable.

44
Signaling over copper and fiber optic cabling

• In order for the LAN to operate properly, the
  receiving device must be able to accurately
  interpret the binary ones and zeros transmitted
  as voltage levels.
• Since current Ethernet technology supports data
  rates of billions of bits per second, each bit
  must be recognized, even though duration of the
  bit is very small.
• The voltage level cannot be amplified at the
  receiver, nor can the bit duration be extended in
  order to recognize the data.
• This means that as much of the original signal
  strength must be retained, as the signal moves
  through the cable and passes through the
  connectors.
• In anticipation of ever-faster Ethernet
  protocols, new cable installations should be made
  with the best available cable, connectors, and
  interconnect devices such as punch-down blocks
  and patch panels. 

45
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.

46
Sources of noise on copper media

• Crosstalk involves the transmission of signals
  from one wire to a nearby wire.
• When voltages change on a wire, electromagnetic
  energy is generated.
• This energy radiates outward from the
  transmitting wire like a radio signal from a
  transmitter.
• Adjacent wires in the cable act like antennas,
  receiving the transmitted energy, which
  interferes with data on those wires.

47
Sources of noise on copper media

• Twisted-pair cable is designed to take advantage
  of the effects of crosstalk in order to minimize
  noise.
• 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. 

48
Types of crosstalk

• There are three distinct types of crosstalk
• 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 computed as the
  ratio of voltage amplitude between the test
  signal and the crosstalk signal when measured
  from the same end of the link.

50
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.
• Thus, FEXT is not as significant a problem as
  NEXT.

51
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.

52
Cable testing standards

• The ten primary test parameters that must be
  verified for a cable link to meet 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

53
Cable testing standards

• 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.

54
Cable testing standards

• 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

• The wire map test also verifies that all eight
  wires are connected to the correct pins on both
  ends of the cable.
• There are several different wiring faults that
  the wire map test can detect.

56
Other test parameters

• Return loss 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.

57
Time-based parameters

• Testers measure the length of the wire based on
  the electrical delay as measured by a Time Domain
  Reflectometry (TDR) test, not by the physical
  length of the cable jacket.
• Since the wires inside the cable are twisted,
  signals actually travel farther than the physical
  length of the cable.

58
Testing optical fiber

• Fiber links are subject to the optical equivalent
  of UTP impedance discontinuities.
• When light encounters an optical discontinuity,
  some of the light signal is reflected back in the
  opposite direction with 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.
• Just as with UTP cable, improperly installed
  connectors are the main cause of light reflection
  and signal strength loss in optical fiber.

59
Testing optical fiber

• Absence of electrical signals.
• There are no crosstalk problems on fiber optic
  cable.
• External electromagnetic interference or noise
  has no affect on fiber cabling.
• Attenuation does occur on fiber links, but to a
  lesser extent than on copper cabling.

60
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.
• Although the Cat 6 tests are essentially the same
  as those specified by the Cat 5 standard, 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.

61
Summary