Chương 4 Kiểm tra cáp (Cable testing)

Chương 4KIỂM 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. 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. 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. 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. 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.  4.1.2. Sóng sin và 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. Square waves, like sine waves, are periodic. However, square wave graphs do 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 4.1.3. Lũy thừa và logarith (Exponents and logarithms) In networking, there are three important number systems: Base 2 – 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 2 for binary numbers, 10 for decimal numbers, and 16 for hexadecimal numbers.  Numbers with exponents are used to easily represent very large or very small numbers. It is much easier and less error-prone to represent one billion numerically as 109 than as 1000000000. Many calculations involved in cable testing involve numbers that are very large, so exponents are the preferred format. Exponents can be explored in the flash activity. One way to work with the very large and very small numbers that occur in networking is to transform the numbers according to the rule, or mathematical function, known as the logarithm. Logarithms are referenced to the base of the number system being used. For example, base 10 logarithms are often abbreviated log. While the study of logarithms is beyond the scope of this course, the terminology is used commonly in calculating decibels, a way of measuring signals on copper, optical, and wireless media. 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) The variables represent the following values: dB measures the loss or gain of the power of a wave. Decibels are usually negative numbers representing a loss in power as the wave travels, but can also be positive values representing a gain in power if the signal is amplified log10 implies that the number in parenthesis will be transformed using the base 10 logarithm rule Pfinal is the delivered power measured in Watts Pref is the original power measured in Watts Vfinal is the delivered voltage measured in Volts Vreference is the original voltage measured in Volts 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. 4.1.5. Tạp âm trong miền tần số và miền thời gian (Noise in time and frequency) Noise is an important concept in communications systems, including LANS. While noise usually refers to undesirable sounds, noise related to communications refers to undesirable signals. Noise can originate from natural and technological sources, and is added to the data signals in communications systems. 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 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 Noise that affects all transmission frequencies equally is called white noise. Noise that only affects small ranges of frequencies is called narrowband interference. When detected on a radio receiver, white noise would interfere with all radio stations. Narrowband interference would affect only a few stations whose frequencies are close together. When detected on a LAN, white noise would affect all data transmissions, but narrowband interference might disrupt only certain signals. If the band of frequencies affected by the narrowband interference included all frequencies transmitted on the LAN, then the performance of the entire LAN would be compromised 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. Analog bandwidth typically refers to the frequency range of an analog electronic system. Analog bandwidth could be used to describe the range of frequencies transmitted by a radio station or an electronic amplifier. 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. 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). Physical media, current technologies, and the laws of physics limit 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 is calculated. In general, media that will support higher analog bandwidths without high degrees of attenuation will also support higher digital bandwidths 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 represent binary ones and zeros. The voltage levels are measured with respect to a reference level of zero volts at both the transmitter and the receiver. This 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.  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 Ethernet technology supports data rates of billions of bits per second, each bit must be recognized, even though duration of the bit is very small. The voltage level cannot be amplified at the receiver, nor can the bit duration be extended in order to recognize the data. This means that as much of the original signal strength must be retained, as the signal moves through the cable and passes through the connectors. 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. Coaxial cable is a type of shielded cable. It consists of a solid copper conductor surrounded by insulating material, and then braided conductive shielding. In LAN applications, the braided shielding is electrically grounded to protect the inner conductor from external electrical noise. 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 There are two types of twisted-pair cable: shielded twisted-pair (STP) and unshielded twisted pair (UTP). STP cable contains an outer conductive shield that is electrically grounded to insulate the signals from external electrical noise. STP also uses inner foil shields to protect each wire pair from noise generated by the other pairs. STP cable is sometimes called screened twisted pair (ScTP). STP cable is more expensive, more difficult to install, and less frequently used than UTP 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. 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. 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 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. 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. 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. 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. 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. 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. 4.2.3. Nguồn phát sinh tạp âm trên 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. 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. 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. 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.  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.