1. James Chen ydjames@ydu.edu.tw
Module 4: Cable Testing
James Chen
2018/11/10
Yuda college of business

2. Yuda college of business
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
2018/11/10
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Background for Studying Frequency-Based Cable Testing
2018/11/10
Yuda college of business

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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(潮汐)
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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5. 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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6. Sine waves and square waves (cont.)
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7. Sine waves and square waves (cont.)
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8. Sine waves and square waves (cont.)
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
2018/11/10
Yuda college of business

9. Exponents and logarithms
In networking, there are three important number systems:
Base 2 – binary
Base 10 – decimal
Base 16 – hexadecimal
Power and exponent
10 * = 10^2 (10 raised to the second power, exponent = 2)
10 * 10 * 10 = 10^3 (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 (縮短的)log
log (10^9) = 9
log (10^-3) = -3
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Decibels(分貝)
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.
2018/11/10
Yuda college of business

11. 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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12. Viewing signals in time and frequency (cont.)
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13. 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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14. Analog and digital signals in time and frequency (cont.)
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15. 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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18. Signaling over copper and fiber optic cabling
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19. 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.
2018/11/10
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20. 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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22. 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 (忽略)link faults.
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23. 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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Yuda college of business

24. 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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Yuda college of business

25. 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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26. 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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27. 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(侵襲)the receiver at different intervals causing signal jitter.
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29. 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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30. 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 (預算)
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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2018/11/10
Yuda college of business