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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 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 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 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 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. Decibels Answers to examples in 4.1.4 • -30 decibels. A loss • 1.7 microwatts • -113.9 decibels. A major loss. 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 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) … Building a Square Wave Measuring Devices Spectrum Analyzer. Gives a bar graph representing all frequencies and amplitudes present in a waveform. 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. 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. 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 Types of Signal Degradation 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! Attenuation Attenuation means loss of signal amplitude If a signal gets too small, it can not be decoded at the receiving end 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 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. Noise – Where are the 1’s and 0’s? 5 volts 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. 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). 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. Noise Pickup External • • • • Impulse Radio - EMI RFI Line to ground Motor Crosstalk - NEXT Twisted Pairs minimize noise pickup Crosstalk and other evils 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. Wiring Errors (Fluke Tester) Or Crossover Cable 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. 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. 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. End of Chapter 4