Bài giảng Mạng máy tính căn bản: Chương 4 - Phan Vĩnh Thuần

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Bài giảng Mạng máy tính căn bản - Chương 4: Kiểm tra cáp (Cable testing) trình bày tổng quan về networking, cung cấp một số định nghĩa cơ bản để về kiểm tra cáp, mô tả các vấn đề liên quan đến việc thử nghiệm phương tiện truyền thông được sử dụng cho kết nối trong mạng cục bộ (LAN),...

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Nội dung Text: Bài giảng Mạng máy tính căn bản: Chương 4 - Phan Vĩnh Thuần

Chương 4 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 1
• 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 2
• 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 3
• 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
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 5
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. Phan Vĩnh Thuần Computer Network 6
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• 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 Phan Vĩnh Thuần Computer Network 8
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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. Phan Vĩnh Thuần Computer Network 10
• 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. Phan Vĩnh Thuần Computer Network 11
• 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. 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) Phan Vĩnh Thuần Computer Network 13
• 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 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. 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. 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 Computer Network 17
– 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 Phan Vĩnh Thuần Computer Network 18
• 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 Phan Vĩnh Thuần Computer Network 19
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