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Transcript

Module 4 Cable Testing Version 3.0 1 Number Systems and Exponents • In networking, there are three important number systems: – Base 2 – binary – Base 10 – decimal – Base 16 – hexadecimal • The number system refers to the number of different symbols that can occupy one position (single digit). • 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. Version 3.0 4.1.3 2 Decibels • The decibel (dB) is a measurement unit important in describing networking signals. • The common units of measurement used in formulas for calculating the amount of gain or loss in networking signals are: – Decibels – Watts – Volts • They are used to describe all networking signals, whether voltage waves on copper, optical pulses in fiber, or microwaves in a wireless system. Version 3.0 4.1.4 3 Decibels • The decibel is related to the exponents and logarithms • There are two formulas for calculating decibels: – dB = 10 log10 (Pfinal / Pref) – dB = 20 log10 (Vfinal / Vreference) • Students are not expected to master the formula, just to recognize that decibels are the key measure of signal and noise in all communications systems. Version 3.0 4.1.4 4 Decibels • The first formula describes decibels in terms of power (P) – dB = 10 log10 (Pfinal / Pref) • The variables represent the following values: – dB measures the loss or gain of the power of a wave. – 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 • Typically, light waves on optical fiber and radio waves in the air are measured using the power formula. Version 3.0 4.1.4 5 Decibels Example • If Pfinal is one microWatt (1 x 10-6 or .000001 Watts) and Pref is one milliWatt (1 x 10-3 or .001 Watts), what is the gain or loss in decibels? Is this value positive or negative? Does the value represent a gain or a loss in power? dB = 10 * Log10 ( Pfinal / Pref ) dB = 10 * Log10 (.000001 / .001 ) dB = 10 * Log10 ( .001 ) dB = 10 * -3 dB = -30 Version 3.0 4.1.4 Indicates a loss in power 6 Decibels • The second formula describes decibels in terms of Volts (V) – dB = 20 log10 (Vfinal / Vreference) • The variables represent the following values: – dB measures the loss or gain of the power of a wave. – log10 implies that the number in parenthesis will be transformed using the base 10 logarithm rule – Vfinal is the delivered Voltage measured in Volts – Vref is the original Voltage measured in Volts • Typically, electromagnetic waves on copper cables are measured using the voltage formula. Version 3.0 4.1.4 7 Decibels • 10 millivolts (10 * .001 = .01) are measured at the end of a cable. The source voltage was 1 Volt. What is the gain or loss in decibels? dB = 20 * Log10 ( Vfinal / Vref ) dB = 20 * Log10 (.01 / 1 ) dB = 20 * Log10 ( .01 ) dB = 20 * -2 dB = -40 Version 3.0 4.1.4 Indicates a loss in Voltage 8 Noise • Noise is an important concept in communications systems, including LANS. • 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. Version 3.0 4.1.7 9 Noise • 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 (crosstalk) – 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 Version 3.0 4.1.7 10 Bandwidth • Bandwidth is an extremely important concept in communications systems. • Physical media, current technologies, and the laws of physics limit bandwidth. • Two ways of considering bandwidth that are important for the study of LANs are: – analog bandwidth – digital bandwidth Version 3.0 4.1.8 11 Bandwidth • Analog bandwidth typically refers to the frequency range of an analog electronic system. • The units of measurement for analog bandwidth is Hertz, the same as the unit of frequency — for example, 6MHz or 20KHz. • One hertz is equivalent to one cycle per second. Version 3.0 4.1.8 12 Bandwidth • Digital bandwidth measures how much information can flow from one place to another in a given amount of time (the speed of transmission). • 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). Version 3.0 4.1.8 13 Bandwidth • 1.6 megabits per second is different from 1.6 megabytes per second. • Eight bits make a byte, so 1.6 megabits per second is equal to 0.2 megabytes per second. 1.6 Mbps / 8 = 0.2 MBps Version 3.0 4.1.8 14 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. Version 3.0 4.1.8 15 Attenuation • 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. • Attenuation is expressed in decibels (dB) using negative numbers. • Smaller negative dB values are an indication of better link performance. Version 3.0 4.2.2 16 Attenuation Version 3.0 4.2.2 17 Attenuation • There are several factors that contribute to attenuation. – Long cable lengths – 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. – By impedance caused by defective connectors. Version 3.0 4.2.2 18 Crosstalk (Noise) • Noise is any electrical energy on the transmission cable that makes it difficult for a receiver to interpret the data sent from the transmitter. • Crosstalk involves the transmission of signals from one wire to a nearby wire. • Crosstalk can also be caused by signals on separate, nearby cables. • Crosstalk is more destructive at higher transmission frequencies. Version 3.0 4.2.3 19 Cable Testing Version 3.0 4.2.5 20 Propagation Delay • 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. • Propagation delay measurements are the basis of the cable length measurement. Version 3.0 4.2.7 21 Optical Fiber • A fiber link consists of two separate glass fibers functioning as independent data pathways. • One fiber carries transmitted signals in one direction, while the second carries signals in the opposite direction (this allows for full-duplex transmission). • Each glass fiber is surrounded by a sheath that light cannot pass through, so there are no crosstalk problems on fiber optic cable. • External electromagnetic interference or noise has no affect on fiber cabling. • Attenuation does occur on fiber links, but to a lesser extent than on copper cabling. Version 3.0 4.2.8 22 Optical Fiber • Fiber links are subject to the optical equivalent of UTP impedance discontinuities. • When light encounters an optical discontinuity, some of the light signal is reflected back in the opposite direction with only a fraction of the original light signal continuing down the fiber towards the receiver. • Improperly installed connectors are the main cause of light reflection and signal strength loss in optical fiber. Version 3.0 4.2.8 23