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Module 4Cable Testing

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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.

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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.

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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.

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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.

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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 Indicates a loss in power

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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.

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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 Indicates a loss in Voltage

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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.

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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

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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

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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.

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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).

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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

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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.

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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.

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Attenuation

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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.

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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.

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Cable Testing

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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.

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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.

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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.

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