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

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Background Signals and Noise Note to instructor: Have curriculum open

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

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

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Logarithms b Logarithms are a way to express differences between numbers that are orders of magnitude apart. b 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. b Log 1 = 0 because 10 0 = 1 b Log 8 = 0.9031 because 10 0..9031 = 8

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Logarithms b Run interactive activities in 4.1.3 b The logarithm of ten raised to any power is the power itself. b Log 1000 = log 10 3 = 3 b Log 1,000,000 = log 10 6 = 6

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

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Decibels b Answers to examples in 4.1.4 -30 decibels. A loss-30 decibels. A loss 1.7 microwatts1.7 microwatts -113.9 decibels. A major loss.-113.9 decibels. A major loss.

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

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

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Building a Square Wave

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

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Measuring Devices b Time Delay Reflectometer b Because the wires inside the cable are twisted, signals actually travel farther than the physical length of the cable. b 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.

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

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Ten tests for Category 5 cable b Wire map b Insertion loss b Near-end crosstalk (NEXT) b Power sum near-end crosstalk (PSNEXT) b Equal-level far-end crosstalk (ELFEXT) b Power sum equal-level far-end crosstalk (PSELFEXT) b Return loss b Propagation delay b Cable length b Delay skew

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Types of Signal Degradation

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

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

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

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

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

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Analog vs Digital Bandwidth b Analog bandwidth typically refers to the frequency range of an analog electronic system. b The units of measurement for analog bandwidth is Hertz, the same as the unit of frequency. b 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.

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Analog vs Digital Bandwidth b Digital bandwidth measures how much information can flow from one place to another in a given amount of time. b The fundamental unit of measurement for digital bandwidth is bits per second (bps). b 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).

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

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Noise Pickup b External ImpulseImpulse Radio - EMI RFIRadio - EMI RFI Line to groundLine to ground MotorMotor b Crosstalk - NEXT b Twisted Pairs minimize noise pickup

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Crosstalk and other evils

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

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Wiring Errors (Fluke Tester) Or Crossover Cable

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

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Fiber Optic Cable Testing b Just as with UTP cable, improperly installed connectors are the main cause of light reflection and signal strength loss in optical fiber. b 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. b If the fiber fails the test, the cable test instrument should indicate where the optical discontinuities occur along the length of the cable link.

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

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End of Chapter 4

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