Презентация на тему: " Copyright 2003 www.ciscopress.com CCNA 1 Chapter 4, Part 1 Cable Testing By Your Name." — Транскрипт:
Copyright CCNA 1 Chapter 4, Part 1 Cable Testing By Your Name
Copyright Objectives Describe background for studying frequency- based cable testing Describe signals and noise
Copyright Waves A wave is energy traveling from one place to another. Networking professionals are specifically interested in voltage waves on copper media, light waves in optical fiber, and alternating electric and magnetic fields called electromagnetic waves.
Copyright Sine Waves and Square Waves Sine waves, or sinusoids, are graphs of mathematical functions that repeat the same pattern at regular intervals. Square wave graphs do not continuously vary with time. The values remain the same for some time, and then suddenly change.
Copyright Exponents and Logarithms In networking, there are three important number systems: –Base 2 – Binary –Base 10 – Decimal –Base 16 – Hexadecimal Decimal numbers have 10 different placeholders, the numbers 0 through 9. Hexadecimal numbers have 16 different placeholders, the numbers 0 through 9 and the letters A through F. A logarithm is essentially the opposite of an exponent. A logarithm equals the exponent that a given base would have to be raised to in order to generate a certain value.
Copyright Decibels There are two formulas for calculating decibels: –dB = 10 log 10 (P final / P ref ) –dB = 20 log 10 (V final / V ref ) 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.
Copyright Viewing Signals in Time and Frequency Data symbolizing characters, words, pictures, video, or music can be represented electrically by voltage patterns on wires and in electronic devices. The data represented by these voltage patterns can be converted to light waves or radio waves, and then back to voltage waves.
Copyright Analog and Digital Signals in Time and Frequency
Copyright Noise in Time and Frequency There are many possible sources of noise: –Nearby cables that 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 –Laser noise at the transmitter or receiver of an optical signal
Copyright Bandwidth Analog bandwidth typically refers to the frequency range of an analog electronic system. Digital bandwidth measures how much information can flow from one place to another in a given amount of time. –1 kbps = 1000 bps –1 Mbps = 1,000,000 bps = 1000 kbps –1 Gbps = 1,000,000,000 bps = 1,000 Mbps
Copyright Signaling over Copper and Fiber On copper cable, data signals are represented by voltage levels that represent binary 1s and 0s –Shielded (coaxial cable) and unshielded –There are two types of twisted-pair cable: shielded twisted pair (STP) and unshielded twisted pair (UTP). Fiber-optic cable is used to transmit data signals by increasing and decreasing the intensity of light to represent binary 1s and 0s.
Copyright Attenuation and Insertion Loss on Copper Media Attenuation is the decrease in signal amplitude over the length of a link. Impedance discontinuities caused by defective or improperly installed connectors also contributes to attenuation. Impedance is a measurement of the resistance of the cable to alternating current (AC), and is measured in ohms.
Copyright Noise on Copper Media Crosstalk involves the transmission of signals from one wire to a nearby wire. It can also be caused by signals on separate, nearby cables. When crosstalk is caused by a signal on another cable, it is called alien crosstalk. Crosstalk is more destructive at higher transmission frequencies. Cable testing instruments measure crosstalk by applying a test signal to one wire pair. The cable tester then measures the amplitude of the unwanted crosstalk signals induced on the other wire pairs in the cable.
Copyright Types of Crosstalk There are three distinct types of crosstalk: –Near-end crosstalk (NEXT) –Far-end crosstalk (FEXT) –Power sum near-end crosstalk (PSNEXT)
Copyright Types of Crosstalk 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. FEXT crosstalk occurs further away from the transmitter and creates less noise on a cable than NEXT. PSNEXT is computed for each wire pair based on the NEXT effects of the other three pairs.
Copyright Cable Testing Standards TIA/EIA standards –Greatest impact on networking media standards TIA/EIA-568-A –Most widely used standards for technical performance of networking media Differentiating between connections –RJ-11, RJ-45, DB 15 UTP implementation –RF-45
Copyright Details of TIA/EIA-568-A TIA/EIA standards address the following six elements of the LAN cabling process: –Horizontal cabling –Telecommunications closets –Backbone cabling –Equipment rooms –Work areas –Entrance facilities
Copyright Time Base 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 the hundredths of nanoseconds. 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.
Copyright Testing Optical Fiber On a fiber-optic link, the acceptable amount of signal power loss that can occur without dropping below the requirements of the receiver must be calculated. A fiber test instrument checks whether the optical link loss budget has been exceeded.
Copyright New Standard On June 20, 2002, the Category 6 (or Cat 6) addition to the TIA-568 standard was published. This new standard specifies the original set of performance parameters that need to be tested for Ethernet cabling as well as the passing scores for each of these tests. A quality cable tester is the Fluke DSP-LIA013 Channel/Traffic Adapter for Cat5e.