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SUNY Ulster Cisco Semester 1

SUNY Ulster Cisco Semester 1. Unit 4 – Cable Testing K. Wick CCAI. Background. Signals and Noise Note to instructor: Have curriculum open. Sine Waves. Shape Amplitude Frequency Wavelength Period = 1/f Objective 4.1.1 has a nice demo of sinusoidal waves. Square Waves.

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SUNY Ulster Cisco Semester 1

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  1. SUNY UlsterCisco Semester 1 Unit 4 – Cable Testing K. Wick CCAI

  2. Background Signals and Noise Note to instructor: Have curriculum open

  3. Sine Waves • Shape • Amplitude • Frequency • Wavelength • Period = 1/f • Objective 4.1.1 has a nice demo of sinusoidal waves.

  4. Square Waves • Objective 4.1.2 does NOT show a pure square wave. • It shows a square wave with a DC component. (Wave is offset from zero) • Square wave vs Periodic Pulse vs Pulse

  5. Logarithms • Logarithms are a way to express differences between numbers that are orders of magnitude apart. • If we ask “To what power do we raise ten to be equal to a number in question”, that is the logarithm of the number. • Log 1 = 0 because 100 = 1 • Log 8 = 0.9031 because 100..9031 = 8

  6. Logarithms • Run interactive activities in 4.1.3 • The logarithm of ten raised to any power is the power itself. • Log 1000 = log 103 = 3 • Log 1,000,000 = log 106 = 6

  7. Decibels • Cisco says, “There are two formulas for calculating decibels”: • dB = 10 log10 (Pfinal / Pref) • dB = 20 log10 (Vfinal / Vreference) • They are partially correct. • These formulas depend on equal input and output impedance.

  8. Decibels • Answers to examples in 4.1.4 • -30 decibels. A loss • 1.7 microwatts • -113.9 decibels. A major loss.

  9. Measuring Devices • A multimeter or digital multimeter (DMM) measures voltage, current, resistance, continuity and sometimes other parameters. • An oscilloscope gives a visual display of voltage versus time. • A Cable meter will test a cable for correct wiring

  10. Fourier Analysis ???? • Cisco is insane! • OK here is a basic: • Every complex waveform can be made by adding a series of sinusoids of proper frequency and amplitude. • A square wave is the sum of the series • A * sin(x) + A/3 * sin(3x) + A/5 * sin(5x) …

  11. Building a Square Wave

  12. Measuring Devices • Spectrum Analyzer. • Gives a bar graph representing all frequencies and amplitudes present in a waveform.

  13. Measuring Devices • Time Delay Reflectometer • Because the wires inside the cable are twisted, signals actually travel farther than the physical length of the cable. • A TDR measurement 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.

  14. Measuring Devices • Time Delay Reflectometer • Also used to identify the distance to wiring faults such as shorts and opens. • This presumes that we know the propagation speed of the specific wire type.

  15. Ten tests for Category 5 cable • 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

  16. Types of Signal Degradation

  17. Propagation and Delay • Propagation means travel of a signal • Propagation Delay is the time it takes a signal to travel from point to point. • It is measured in hundreths of nanoseconds. • Skew: The difference in delay between pairs. • Read CISCO questions carefully, watch the exact wording!

  18. Attenuation • Attenuation means loss of signal amplitude • If a signal gets too small, it can not be decoded at the receiving end

  19. Reflection • Sometimes on a physical medium a signal travels to the end of the medium and part of the signal reflects back toward the source. • This reflection can interfere with the original signal. • Reflections are especially bad with impedance mismatches in the physical layer. • (Caused by wrong media or bad connections) • Nominal Z for Cat 5 cable is 100 ohms

  20. Attenuation (signal deterioration) and noise (signal interference) cause problems in networks because the data is not recognizable when it is received. • Proper attachment of cable connectors and proper cable installation are important. If standards are followed in these areas, attenuation and noise levels are minimized.

  21. Noise – Where are the 1’s and 0’s? 5 volts

  22. Analog vs Digital Bandwidth • Analog bandwidth typically refers to the frequency range of an analog electronic system. • The units of measurement for analog bandwidth is Hertz, the same as the unit of frequency. • Examples of analog bandwidth values are 3 kHz for telephony, 20 kHz for audible signals, 5 kHz for AM radio stations, and 200 MHz for FM radio stations.

  23. Analog vs Digital 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).

  24. Analog vs Digital Bandwidth • During cable testing, analog bandwidth is used to determine the digital bandwidth of a copper cable. • Analog frequencies are transmitted from one end and received on the opposite end. • The two signals are then compared, and the amount of attenuation of the signal at each frequency is calculated. • In general, media that will support higher analog bandwidths without high degrees of attenuation will also support higher digital bandwidths.

  25. Noise Pickup • External • Impulse • Radio - EMI RFI • Line to ground • Motor • Crosstalk - NEXT • Twisted Pairs minimize noise pickup

  26. Crosstalk and other evils

  27. Crosstalk • Near End – at near end of link. • Far End • Power Sum – cumulative effect of crosstalk on all pairs in the cable. • For all – Larger negative numbers mean LESS crosstalk. (-30dB vs -20dB). • Some testers leave out the minus sign.

  28. Wiring Errors (Fluke Tester) Or Crossover Cable

  29. Fiber Optic Cable Testing • Remember that a fiber link consists of two separate glass fibers functioning as independent data pathways. • Fiber optic cable does not suffer from crosstalk or noise pickup. • Attenuation does occur on fiber links, but to a lesser extent than on copper cabling. • Fiber links are subject to the optical equivalent of UTP impedance discontinuities.

  30. Fiber Optic Cable Testing • Just as with UTP cable, improperly installed connectors are the main cause of light reflection and signal strength loss in optical fiber. • If attenuation weakens the light signal at the receiver, then data errors will result. Testing fiber optic cable primarily involves shining a light down the fiber and measuring whether a sufficient amount of light reaches the receiver. • If the fiber fails the test, the cable test instrument should indicate where the optical discontinuities occur along the length of the cable link.

  31. Category 6 UTP and STP • Cables certified as Cat 6 cable must pass the same ten tests as Cat 5 cable. • Cat 6 cable must pass these tests with higher scores to be certified. • It must be capable of carrying frequencies up to 250 MHz (vs 100 MHz) and must have lower levels of crosstalk and return loss.

  32. End of Chapter 4

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