Transcript net4

Module 4
Cable 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
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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
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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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