KIỂM TRA cáp (cable testing) (MẠNG máy TÍNH cơ bản SLIDE)

Chương KIỂM TRA CÁP (Cable testing) OverView • Networking media is literally and physically the backbone of a network Inferior quality of network cabling results in network failures and unreliable performance Computer Network • Copper, optical fiber, and wireless networking media all require testing to determine the quality These tests involve certain electrical and mathematical concepts and terms, such as signal, wave, frequency, and noise Understanding this vocabulary is helpful when learning about networking, cabling, and cable testing.  • The goal of the first lesson in this module is to provide some basic definitions so that the cable testing concepts presented in the second lesson will be better understood Computer Network • The second lesson of this module describes the issues relating to the testing of media used for physical layer connectivity in local-area networks (LANs) In order for the LAN to function properly, the physical layer medium must meet the industry standard specifications Computer Network • 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 Computer Network 4.1 Cơ sở nghiên cứu kiểm tra cáp dựa vào tần số (Background for Studying Frequency-Based Cable Testing) 4.1.1 Sóng (Wave) • 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 The amplitude of an electrical signal still represents height, but it is measured in volts instead of meters The period is the amount of time to complete one cycle, measured in seconds The frequency is the number of complete cycles per second, measured in Hertz.  Computer Network 4.1.2 Sóng sin sóng vng (Sine waves and square waves) • Sine waves, or sinusoids, are graphs of mathematical functions Sine waves have certain characteristics Sine waves are periodic, which means that they repeat the same pattern at regular intervals Sine waves are continuously varying, which means that no two adjacent points on the graph have the same value • Sine waves are graphical representations of many natural occurrences that change regularly over time Since sine waves are continuously varying, they are examples of analog waves Computer Network Computer Network • Square waves, like sine waves, are periodic However, square wave graphs not continuously vary with time The wave holds one value for some time, and then suddenly changes to a different value This value is held for some time, and then quickly changes back to the original value Square waves represent digital signals, or pulses Like all waves, square waves can be described in terms of amplitude, period, and frequency Computer Network Computer Network 4.1.3 Lũy thừa logarith (Exponents and logarithms) • In networking, there are three important number systems: – Base – binary – Base 10 – decimal – Base 16 – hexadecimal • Exponent of the base of a number system also refers to the value of each digit The least significant digit has a value of base0, or one The next digit has a value of base1 This is equal to for binary numbers, 10 for decimal numbers, and 16 for hexadecimal numbers.  Computer Network 10 • Fiber optic cable is used to transmit data signals by increasing and decreasing the intensity of light to represent binary ones and zeros The strength of a light signal does not diminish like the strength of an electrical signal does over an identical run length Optical signals are not affected by electrical noise, and optical fiber does not need to be grounded Therefore, optical fiber is often used between buildings and between floors within the building As costs decrease and demand for speed increases, optical fiber may become a more commonly used LAN media Computer Network 34 Computer Network 35 4.2.2 Tổn thất suy giảm can nhiễu đường truyền cáp đồng (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 For this reason, attenuation on a cable is measured by a cable tester using the highest frequencies that the cable is rated to support Computer Network 36 • 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 converts some of the electrical energy of the signal to heat Signal energy is also lost when it leaks through the insulation of the cable and by impedance caused by defective connectors Computer Network 37 • Impedance is a measurement of the resistance of the cable to alternating current (AC) and is measured in ohms The normal, or characteristic, impedance of a Cat5 cable is 100 ohms If a connector is improperly installed on Cat5, it will have a different impedance value than the cable This is called an impedance discontinuity or an impedance mismatch Computer Network 38 • Impedance discontinuities cause attenuation because a portion of a transmitted signal will be reflected back to the transmitting device rather than continuing to the receiver, much like an echo This effect is compounded if there are multiple discontinuities causing additional portions of the remaining signal to be reflected back to the transmitter Computer Network 39 • When this returning reflection strikes the first discontinuity, some of the signal rebounds in the direction of the original signal, creating multiple echo effects The echoes strike the receiver at different intervals making it difficult for the receiver to accurately detect data values on the signal This is called jitter and results in data errors Computer Network 40 • The combination of the effects of signal attenuation and impedance discontinuities on a communications link is called insertion loss Proper network operation depends on constant characteristic impedance in all cables and connectors, with no impedance discontinuities in the entire cable system Computer Network 41 Computer Network 42 4.2.3 Nguồn phát sinh tạp âm đường truyền cáp đồng (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 Computer Network 43 • Crosstalk involves the transmission of signals from one wire to a nearby wire When voltages change on a wire, electromagnetic energy is generated This energy radiates outward from the transmitting wire like a radio signal from a transmitter Adjacent wires in the cable act like antennas, receiving the transmitted energy, which interferes with data on those wires Computer Network 44 • Crosstalk 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 Computer Network 45 • Twisted-pair cable is designed to take advantage of the effects of crosstalk in order to minimize noise In twisted-pair cable, a pair of wires is used to transmit one signal The wire pair is twisted so that each wire experiences similar crosstalk Because a noise signal on one wire will appear identically on the other wire, this noise be easily detected and filtered at the receiver.  Computer Network 46 • Twisting one pair of wires in a cable also helps to reduce crosstalk of data or noise signals from an adjacent wire pair Higher categories of UTP require more twists on each wire pair in the cable to minimize crosstalk at high transmission frequencies When attaching connectors to the ends of UTP cable, untwisting of wire pairs must be kept to an absolute minimum to ensure reliable LAN communications Computer Network 47 Computer Network 48 ... and noise levels are minimized Computer Network 4.1 Cơ sở nghiên cứu kiểm tra cáp dựa vào tần số (Background for Studying Frequency-Based Cable Testing) 4.1.1 Sóng (Wave) • Networking professionals... truyền cáp đồng (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... other signals being transmitted nearby Computer Network 17 – Electromagnetic interference (EMI), which is noise from nearby sources such as motors and lights – Laser noise at the transmitter or receiver