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Module 4 Cable Testing

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. 4.1.3

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. 4.1.4

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. 4.1.4

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. 4.1.4

dB = 10 * Log10 ( Pfinal / Pref )
Decibels Example If Pfinal is one microWatt (1 x or 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 ( / .001 ) dB = 10 * Log10 ( .001 ) dB = 10 * -3 dB = Indicates a loss in power 4.1.4

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. 4.1.4

dB = 20 * Log10 ( Vfinal / Vref )
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 = Indicates a loss in Voltage 4.1.4

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. 4.1.7

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 4.1.7

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 4.1.8

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. 4.1.8

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). 4.1.8

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 4.1.8

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. 4.1.8

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. 4.2.2

Attenuation 4.2.2

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. 4.2.2

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. 4.2.3

Cable Testing 4.2.5

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. 4.2.7

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. 4.2.8

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. 4.2.8

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