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 Phan Vĩnh Thuần 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 Phan Vĩnh Thuần 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 Phan Vĩnh Thuần 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 Phan Vĩnh Thuần 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 Phan Vĩnh Thuần Computer Network 4.1.2 Sóng sin sóng vuông (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 Phan Vĩnh Thuần Computer Network Phan Vĩnh Thuần 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, andNetwork frequency Phan Vĩnh Thuần Computer Phan Vĩnh Thuần 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 Phan Vĩnh Thuần 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 Phan Vĩnh Thuần Computer Network 34 Phan Vĩnh Thuần 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 Phan Vĩnh Thuần 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 Phan Vĩnh Thuần 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 Phan Vĩnh Thuần 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 Phan Vĩnh Thuần 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 Phan Vĩnh Thuần 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 Phan Vĩnh Thuần Computer Network 41 Phan Vĩnh Thuần 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/EIA568-B certification of a cable now requires testing for a variety of types of noise Phan Vĩnh Thuần 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 Phan Vĩnh Thuần 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 Phan Vĩnh Thuần 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 Phan Vĩnh Thuần 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 Phan Vĩnh Thuần Computer Network 47 Phan Vĩnh Thuần Computer Network 48 [...]... performance of the entire LAN would be compromised Phan Vĩnh Thuần Computer Network 19 Phan Vĩnh Thuần Computer Network 20 4. 1.6 Băng thông (Bandwidth) • Bandwidth is an extremely important concept in communications systems Two ways of considering bandwidth that are important for the study of LANs are analog bandwidth and digital bandwidth Phan Vĩnh Thuần Computer Network 21 • Analog bandwidth typically... cable is more expensive, more difficult to install, and less frequently used than UTP Phan Vĩnh Thuần Computer Network 31 Phan Vĩnh Thuần Computer Network 32 • UTP contains no shielding and is more susceptible to external noise but is the most frequently used because it is inexpensive and easier to install Phan Vĩnh Thuần Computer Network 33 • Fiber optic cable is used to transmit data signals by increasing... optical, and wireless media Phan Vĩnh Thuần Computer Network 12 4. 1 .4 Decibels • The decibel (dB) is a measurement unit important in describing networking signals The decibel is related to the exponents and logarithms described in prior sections There are two formulas for calculating decibels: dB = 10 log10 (Pfinal / Pref) dB = 20 log10 (Vfinal / Vreference) Computer Network Phan Vĩnh Thuần 13 • The variables... Phan Vĩnh Thuần Computer Network 14 • 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 These formulas have several things in common Phan Vĩnh Thuần Computer Network 15 4. 1.5... 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 Phan Vĩnh Thuần Computer Network 34 Phan Vĩnh Thuần Computer Network 35 4. 2.2 Tổn thất do suy giảm và can nhiễu trên đường truyền cáp đồng (Attenuation and insertion loss on copper media) • Attenuation is the decrease in signal amplitude... signal moves through the cable and passes through the connectors Phan Vĩnh Thuần Computer Network 26 • There are two basic types of copper cable: shielded and unshielded In shielded cable, shielding material protects the data signal from external sources of noise and from noise generated by electrical signals within the cable Phan Vĩnh Thuần Computer Network 27 • Coaxial cable is a type of shielded... noise Phan Vĩnh Thuần Computer Network 28 • The shielding also helps eliminate signal loss by keeping the transmitted signal confined to the cable This helps make coaxial cable less noisy than other types of copper cabling, but also makes it more expensive The need to ground the shielding and the bulky size of coaxial cable make it more difficult to install than other copper cabling Phan Vĩnh Thuần. .. systems Phan Vĩnh Thuần Computer Network 16 • All communications systems have some amount of noise Even though noise cannot be eliminated, its effects can be minimized if the sources of the noise are understood There are many possible sources of noise: – Nearby cables which carry data signals – Radio frequency interference (RFI), which is noise from other signals being transmitted nearby Phan Vĩnh Thuần. .. signal is calculated In general, media that will support higher analog bandwidths without high degrees of attenuation will also support higher digital bandwidths Phan Vĩnh Thuần Computer Network 24 4.2 Tín hiệu và tạp âm (Signals and Noise) 4. 2.1 Phát tín hiệu qua dây đồng và cáp quang (Signaling over copper and fiber optic cabling) • On copper cable, data signals are represented by voltage levels that... reference level is called the signal ground It is important that both transmitting and receiving devices refer to the same zero volt reference point When they do, they are said to be properly grounded Phan Vĩnh Thuần Computer Network 25 • In order for the LAN to operate properly, the receiving device must be able to accurately interpret the binary ones and zeros transmitted as voltage levels Since current ... with no impedance discontinuities in the entire cable system Phan Vĩnh Thuần Computer Network 41 Phan Vĩnh Thuần Computer Network 42 4. 2.3 Nguồn phát sinh tạp âm đường truyền cáp đồng (Sources... increases, optical fiber may become a more commonly used LAN media Phan Vĩnh Thuần Computer Network 34 Phan Vĩnh Thuần Computer Network 35 4. 2.2 Tổn thất suy giảm can nhiễu đường truyền cáp đồng (Attenuation... install than other copper cabling Phan Vĩnh Thuần Computer Network 29 Phan Vĩnh Thuần Computer Network 30 • There are two types of twisted-pair cable: shielded twisted-pair (STP) and unshielded twisted