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Overview
Networking media is literally and physically the backbone of a
network. Inferior quality of network cabling results in network failures
and unreliable performance. 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.
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.
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.
After cable has been installed, it must be tested with quality cable
testers to verify that the specifications of the TIA/EIA standards are
met. This module also describes the various important tests that are
performed.
Students completing this module should be able to:
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Differentiate between sine waves and square waves.
Define and calculate exponents and logarithms.
Define and calculate decibels.
Define basic terminology related to time, frequency, and noise.
Differentiate between digital bandwidth and analog bandwidth.
Compare and contrast noise levels on various types of cabling.
 Define and describe the affects of attenuation and impedance
mismatch.
 Define crosstalk, near-end crosstalk, far-end crosstalk, and power
sum near-end crosstalk.
 Describe how crosstalk and twisted pairs help reduce noise.
 Describe the ten copper cable tests defined in TIA/EIA-568-B.
 Describe the difference between Category 5 and Category 6 cable.
Module 4: Cable Testing
4.1 Background for Studying Frequency-Based Cable Testing
4.1.1 Waves
A wave is energy traveling from one place to another. There are
many types of waves, but all can be described with similar
vocabulary.
It is helpful to think of waves as disturbances. A bucket of water that
is completely still does not have waves, because there are no
disturbances. Conversely, the ocean always has some sort of
detectable waves due to disturbances such as wind and tide.
Ocean waves can be described in terms of their height, or amplitude,
which could be measured in meters. They can also be described in
terms of how frequently the waves reach the shore, using period and
frequency. The period of the waves is the amount of time between
each wave, measured in seconds. The frequency is the number of
waves that reach the shore each second, measured in Hertz. One
Hertz is equal to one wave per second, or one cycle per second.
Experiment with these concepts by adjusting the amplitude and
frequency in Figure .
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.
If a disturbance is deliberately caused, and involves a fixed,
predictable duration, it is called a pulse. Pulses are important in
electrical signals because they determine the value of the data being
transmitted.
Amplitude and Frequency
4.1.2 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. Some examples of
these occurrences are the distance from the earth to the sun, the
distance from the ground while riding a Ferris wheel, and the time of
day that the sun rises. Since sine waves are continuously varying,
they are examples of analog waves.
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.
Analog Signals
Digital Signals
4.1.3 Exponents and logarithms
In networking, there are three important number systems:
 Base 2 – binary
 Base 10 – decimal
 Base 16 – hexadecimal
Recall that the base of a number system refers to the number of
different symbols that can occupy one position. For example, binary
numbers have only two different placeholders, 0 and 1. Decimal
numbers have 10 different placeholders, the numbers 0-9.
Hexadecimal numbers have 16 different placeholders, the numbers
0-9 and the letters A-F.
Remember that 10x10 can be written as 102. 102 means ten squared
or ten raised to the second power. When written this way, it is said
that 10 is the base of the number and 2 is the exponent of the
number. 10x10x10 can be written as 103. 103 means ten cubed or
ten raised to the third power. The base is still 10, but the exponent is
now 3. Use the Media Activity below to practice calculating exponents.
Enter x, and y is calculated, or enter y, and x is calculated.
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.
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.
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.
To take the “log” of a number use a calculator or the flash activity. For
example, log (109) equals 9, log (10-3) = -3. You can also take the
logarithm of numbers that are not powers of 10, but you cannot take
the logarithm of a negative number. 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.
Numbering Systems
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)
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
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.
Enter values for dB and Pref to discover the correct power. This
formula could be used to see how much power is left in a radio wave
after it has traveled over a distance through different materials, and
through various stages of electronic systems such as a radio. To
explore decibels further, try the following examples using the flash
activities:
 If Pfinal is one microWatt (1 x 10-6 Watts) and Pref is one milliWatt
(1 x 10-3 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?
 If the total loss of a fiber link is -84 dB, and the source power of the
original laser (Pref) is one milliWatt (1 x 10-3 Watts), how much
power is delivered?
 If two microVolts (2 x 10-6 Volts) are measured at the end of a
cable and the source voltage was one volt, what is the gain or loss
in decibels? Is this value positive or negative? Does the value
represent a gain or a loss in voltage?
Caculating Decibels
4.1.5 Viewing signals in time and frequency
One of the most important facts of the information age is that data
symbolizing characters, words, pictures, video, or music can be
represented electrically by voltage patterns on wires and in electronic
devices. The data represented by these voltage patterns can be
converted to light waves or radio waves, and then back to voltage
waves. Consider the example of an analog telephone. The sound
waves of the caller’s voice enter a microphone in the telephone. The
microphone converts the patterns of sound energy into voltage
patterns of electrical energy that represent the voice.
If the voltage patterns were graphed over time, the distinct patterns
representing the voice would be displayed. An oscilloscope is an
important electronic device used to view electrical signals such as
voltage waves and pulses. The x-axis on the display represents time,
and the y-axis represents voltage or current. There are usually two
y-axis inputs, so two waves can be observed and measured at the
same time.
Analyzing signals using an oscilloscope is called time-domain
analysis, because the x-axis or domain of the mathematical function
represents time. Engineers also use frequency-domain analysis to
study signals. In frequency-domain analysis, the x-axis represents
frequency. An electronic device called a spectrum analyzer creates
graphs for frequency-domain analysis. Experiment with this graphic
by adding several signals, and try to predict what the output will look
like on both the oscilloscope and the spectrum analyzer.
Electromagnetic signals use different frequencies for transmission so
that different signals do not interfere with each other. Frequency
modulation (FM) radio signals use frequencies that are different from
television or satellite signals. When listeners change the station on a
radio, they are changing the frequency that the radio is receiving.
Oscilloscope
4.1.6 Analog and digital signals in time and frequency
To understand the complexities of networking signals and cable
testing, examine how analog signals vary with time and with
frequency. First, consider a single-frequency electrical sine wave,
whose frequency can be detected by the human ear. If this signal is
transmitted to a speaker, a tone can be heard. How would a spectrum
analyzer display this pure tone?
Next, imagine the combination of several sine waves. The resulting
wave is more complex than a pure sine wave. Several tones would
be heard. How would a spectrum analyzer display this? The graph of
several tones shows several individual lines corresponding to the
frequency of each tone. Finally, imagine a complex signal, like a
voice or a musical instrument. What would its spectrum analyzer
graph look like? If many different tones are present, a continuous
spectrum of individual tones would be represented.
Fourier Synthesis of a Square Wave
4.1.7 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.
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
 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
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.
Digital Signal and Electrical Noise
4.1.8 Bandwidth
Bandwidth is an extremely important concept in communications
systems. Two ways of considering bandwidth that are important for
the study of LANs are analog bandwidth and digital bandwidth.
Analog bandwidth typically refers to the frequency range of an analog
electronic system. Analog bandwidth could be used to describe the
range of frequencies transmitted by a radio station or an electronic
amplifier. The units of measurement for analog bandwidth is Hertz,
the same as the unit of frequency. Examples of analog bandwidth
values are 3 kHz for telephony, 20 kHz for audible signals, 5 kHz for
AM radio stations, and 200 MHz for FM radio stations.
Digital bandwidth measures how much information can flow from one
place to another in a given amount of time. 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). Physical media, current technologies, and the
laws of physics limit 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. In general, media that will support higher analog
bandwidths without high degrees of attenuation will also support
higher digital bandwidths.
Units of Digital Bandwidth
4.2 Signals and Noise
4.2.1 Signaling over copper and fiber optic cabling
On copper cable, data signals are represented by voltage levels that
represent binary ones and zeros. The voltage levels are measured
with respect to a reference level of zero volts at both the transmitter
and the receiver. This reference level is called the signal ground. It is
important that both transmitting and receiving devices refer to the
same zero volt reference point. When they do, they are said to be
properly grounded.
In order for the LAN to operate properly, the receiving device must be
able to accurately interpret the binary ones and zeros transmitted as
voltage levels. Since current Ethernet technology supports data rates
of billions of bits per second, each bit must be recognized, even
though duration of the bit is very small. The voltage level cannot be
amplified at the receiver, nor can the bit duration be extended in order
to recognize the data. This means that as much of the original signal
strength must be retained, as the signal moves through the cable and
passes through the connectors. In anticipation of ever-faster Ethernet
protocols, new cable installations should be made with the best
available cable, connectors, and interconnect devices such as
punch-down blocks and patch panels.
There are two basic types of copper cable: shielded and unshielded.
In shielded cable, shielding material protects the data signal from
external sources of noise and from noise generated by electrical
signals within the cable.
Coaxial cable is a type of shielded cable. It consists of a solid
copper conductor surrounded by insulating material, and then braided
conductive shielding. In LAN applications, the braided shielding is
electrically grounded to protect the inner conductor from external
electrical noise. The shielding also helps eliminate signal loss by
keeping the transmitted signal confined to the cable. This helps make
coaxial cable less noisy than other types of copper cabling, but also
makes it more expensive. The need to ground the shielding and the
bulky size of coaxial cable make it more difficult to install than other
copper cabling.
There are two types of twisted-pair cable: shielded twisted-pair (STP)
and unshielded twisted pair (UTP).
STP cable contains an outer conductive shield that is electrically
grounded to insulate the signals from external electrical noise. STP
also uses inner foil shields to protect each wire pair from noise
generated by the other pairs. STP cable is sometimes called
screened twisted pair (ScTP). STP cable is more expensive, more
difficult to install, and less frequently used than UTP. UTP contains
no shielding and is more susceptible to external noise but is the most
frequently used because it is inexpensive and easier to install.
Fiber optic cable is used to transmit data signals by increasing and
decreasing the intensity of light to represent binary ones and zeros.
The strength of a light signal does not diminish like the strength of an
electrical signal does over an identical run length. Optical signals are
not affected by electrical noise, and optical fiber does not need to be
grounded. Therefore, optical fiber is often used between buildings
and between floors within the building. As costs decrease and
demand for speed increases, optical fiber may become a more
commonly used LAN media.
Coaxial Cable
Shielded Twisted-Pair
Unshielded Twisted Pair
Fiber Optic Cable
4.2.2 Attenuation and insertion loss on copper media
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. For this reason, attenuation on a cable is
measured by a cable tester using the highest frequencies that the
cable is rated to support. Attenuation is expressed in decibels (dB)
using negative numbers. Smaller negative dB values are an
indication of better link performance.
There are several factors that contribute to attenuation. The
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 and by impedance caused by defective
connectors.
Impedance is a measurement of the resistance of the cable to
alternating current (AC) and is measured in ohms. The normal, or
characteristic, impedance of a Cat5 cable is 100 ohms. If a connector
is improperly installed on Cat5, it will have a different impedance
value than the cable. This is called an impedance discontinuity or an
impedance mismatch.
Impedance discontinuities cause attenuation because a portion of a
transmitted signal will be reflected back to the transmitting device
rather than continuing to the receiver, much like an echo. This effect
is compounded if there are multiple discontinuities causing additional
portions of the remaining signal to be reflected back to the transmitter.
When this returning reflection strikes the first discontinuity, some of
the signal rebounds in the direction of the original signal, creating
multiple echo effects. The echoes strike the receiver at different
intervals making it difficult for the receiver to accurately detect data
values on the signal. This is called jitter and results in data errors.
The combination of the effects of signal attenuation and impedance
discontinuities on a communications link is called insertion loss.
Proper network operation depends on constant characteristic
impedance in all cables and connectors, with no impedance
discontinuities in the entire cable system.
Attenuation
4.2.3 Sources of noise on copper media
Noise is any electrical energy on the transmission cable that makes
it difficult for a receiver to interpret the data sent from the transmitter.
TIA/EIA-568-B certification of a cable now requires testing for a
variety of types of noise.
Crosstalk involves the transmission of signals from one wire to a
nearby wire. When voltages change on a wire, electromagnetic
energy is generated. This energy radiates outward from the
transmitting wire like a radio signal from a transmitter. Adjacent wires
in the cable act like antennas, receiving the transmitted energy, which
interferes with data on those wires. Crosstalk can also be caused by
signals on separate, nearby cables. When crosstalk is caused by a
signal on another cable, it is called alien crosstalk. Crosstalk is more
destructive at higher transmission frequencies.
Cable testing instruments measure crosstalk by applying a test signal
to one wire pair. The cable tester then measures the amplitude of the
unwanted crosstalk signals induced on the other wire pairs in the
cable.
Twisted-pair cable is designed to take advantage of the effects of
crosstalk in order to minimize noise. In twisted-pair cable, a pair of
wires is used to transmit one signal. The wire pair is twisted so that
each wire experiences similar crosstalk. Because a noise signal on
one wire will appear identically on the other wire, this noise be easily
detected and filtered at the receiver.
Twisting one pair of wires in a cable also helps to reduce crosstalk of
data or noise signals from an adjacent wire pair. Higher categories of
UTP require more twists on each wire pair in the cable to minimize
crosstalk at high transmission frequencies. When attaching
connectors to the ends of UTP cable, untwisting of wire pairs must be
kept to an absolute minimum to ensure reliable LAN communications.
Wire Connections
4.2.4 Types of crosstalk
There are three distinct types of crosstalk:
 Near-end Crosstalk (NEXT)
 Far-end Crosstalk (FEXT)
 Power Sum Near-end Crosstalk (PSNEXT)
Near-end crosstalk (NEXT) is computed as the ratio of voltage
amplitude between the test signal and the crosstalk signal when
measured from the same end of the link. This difference is expressed
in a negative value of decibels (dB). Low negative numbers indicate
more noise, just as low negative temperatures indicate more heat. By
tradition, cable testers do not show the minus sign indicating the
negative NEXT values. A NEXT reading of 30 dB (which actually
indicates -30 dB) indicates less NEXT noise and a better cable than
does a NEXT reading of 10 dB.
NEXT needs to be measured from each pair to each other pair in a
UTP link, and from both ends of the link. To shorten test times, some
cable test instruments allow the user to test the NEXT performance of
a link by using larger frequency step sizes than specified by the
TIA/EIA standard. The resulting measurements may not comply with
TIA/EIA-568-B, and may overlook link faults. To verify proper link
performance, NEXT should be measured from both ends of the link
with a high-quality test instrument. This is also a requirement for
complete compliance with high-speed cable specifications.
Due to attenuation, crosstalk occurring further away from the
transmitter creates less noise on a cable than NEXT. This is called
far-end crosstalk, or FEXT. The noise caused by FEXT still travels
back to the source, but it is attenuated as it returns. Thus, FEXT is
not as significant a problem as NEXT.
Power Sum NEXT (PSNEXT) measures the cumulative effect of
NEXT from all wire pairs in the cable. PSNEXT is computed for
each wire pair based on the NEXT effects of the other three pairs.
The combined effect of crosstalk from multiple simultaneous
transmission sources can be very detrimental to the signal.
TIA/EIA-568-B certification now requires this PSNEXT test.
Some Ethernet standards such as 10BASE-T and 100BASE-TX
receive data from only one wire pair in each direction. However, for
newer technologies such as 1000BASE-T that receive data
simultaneously from multiple pairs in the same direction, power sum
measurements are very important tests.
Near-End Crosstalk
Far-End Crosstalk
Power Sum NEXT (PSNEXT)
4.2.5 Cable testing standards
The TIA/EIA-568-B standard specifies ten tests that a copper cable
must pass if it will be used for modern, high-speed Ethernet LANs. All
cable links should be tested to the maximum rating that applies for
the category of cable being installed.
The ten primary test parameters that must be verified for a cable link
to meet TIA/EIA standards are:
Wire map
Insertion loss
Near-end crosstalk (NEXT)
Power sum near-end crosstalk (PSNEXT)
Equal-level far-end crosstalk (ELFEXT)
Power sum equal-level far-end crosstalk (PSELFEXT)
Return loss
Propagation delay
Cable length
Delay skew
The Ethernet standard specifies that each of the pins on an RJ-45
connector have a particular purpose. A NIC transmits signals on
pins 1 and 2, and it receives signals on pins 3 and 6. The wires in
UTP cable must be connected to the proper pins at each end of a
cable. The wire map test insures that no open or short circuits exist
on the cable. An open circuit occurs if the wire does not attach
properly at the connector. A short circuit occurs if two wires are
connected to each other.
The wire map test also verifies that all eight wires are connected to
the correct pins on both ends of the cable. There are several different
wiring faults that the wire map test can detect. The reversed-pair
fault occurs when a wire pair is correctly installed on one connector,
but reversed on the other connector. If the orange striped wire is on
pin 1 and the orange wire on pin 2 at one end, but reversed at the
other end, then the cable has a reversed-pair fault. This example is
shown in the graphic.
A split-pair wiring fault occurs when two wires from different wire pairs
are connected to the wrong pins on both ends of the cable. Look
carefully at the pin numbers in the graphic to detect the wiring fault. A
split pair creates two transmit or receive pairs each with two wires
that are not twisted together.
Transposed-pair wiring faults occur when a wire pair is connected to
completely different pins at both ends. Contrast this with a
reversed-pair, where the same pair of pins is used at both ends.
Transposed pairs also occur when two different color codes on
punchdown blocks, representing T568-A and T568-B, are used at
different locations on the same link.
Ethernet Standards
Cable Tesing Standard
Wiring Fault
4.2.6 Other test parameters
The combination of the effects of signal attenuation and impedance
discontinuities on a communications link is called insertion loss.
Insertion loss is measured in decibels at the far end of the cable. The
TIA/EIA standard requires that a cable and its connectors pass an
insertion loss test before the cable can be used as a communications
link in a LAN.
Crosstalk is measured in four separate tests. A cable tester measures
NEXT by applying a test signal to one cable pair and measuring the
amplitude of the crosstalk signals received by the other cable pairs.
The NEXT value, expressed in decibels, is computed as the
difference in amplitude between the test signal and the crosstalk
signal measured at the same end of the cable. Remember, because
the number of decibels that the tester displays is a negative number,
the larger the number, the lower the NEXT on the wire pair. As
previously mentioned, the PSNEXT test is actually a calculation
based on combined NEXT effects.
The equal-level far-end crosstalk (ELFEXT) test measures FEXT.
Pair-to-pair ELFEXT is expressed in dB as the difference between the
measured FEXT and the insertion loss of the wire pair whose signal
is disturbed by the FEXT. ELFEXT is an important measurement in
Ethernet networks using 1000BASE-T technologies. Power sum
equal-level far-end crosstalk (PSELFEXT) is the combined effect of
ELFEXT from all wire pairs.
Return loss is a measure in decibels of reflections that are caused by
the impedance discontinuities at all locations along the link. Recall
that the main impact of return loss is not on loss of signal strength.
The significant problem is that signal echoes caused by the
reflections from the impedance discontinuities will strike the receiver
at different intervals causing signal jitter.
Crosstalk
4.2.7 Time-based parameters
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. Delays
are measured in hundredths of nanoseconds. One nanosecond is
one-billionth of a second, or 0.000000001 second. The
TIA/EIA-568-B standard sets a limit for propagation delay for the
various categories of UTP.
Propagation delay measurements are the basis of the cable length
measurement. TIA/EIA-568-B-1 specifies that the physical length of
the link shall be calculated using the wire pair with the shortest
electrical delay. Testers measure the length of the wire based on the
electrical delay as measured by a Time Domain Reflectometry (TDR)
test, not by the physical length of the cable jacket. Since the wires
inside the cable are twisted, signals actually travel farther than the
physical length of the cable. When a cable tester makes a TDR
measurement, it sends a pulse signal down a wire pair and measures
the amount of time required for the pulse to return on the same wire
pair.
The TDR test is used not only to determine length, but also to identify
the distance to wiring faults such as shorts and opens. When the
pulse encounters an open, short, or poor connection, all or part of the
pulse energy is reflected back to the tester. This can calculate the
approximate distance to the wiring fault. The approximate distance
can be helpful in locating a faulty connection point along a cable run,
such as a wall jack.
The propagation delays of different wire pairs in a single cable can
differ slightly because of differences in the number of twists and
electrical properties of each wire pair. The delay difference between
pairs is called delay skew. Delay skew is a critical parameter for
high-speed networks in which data is simultaneously transmitted over
multiple wire pairs, such as 1000BASE-T Ethernet. If the delay skew
between the pairs is too great, the bits arrive at different times and
the data cannot be properly reassembled. Even though a cable link
may not be intended for this type of data transmission, testing for
delay skew helps ensure that the link will support future upgrades to
high-speed networks.
All cable links in a LAN must pass all of the tests previously
mentioned as specified in the TIA/EIA-568-B standard. These tests
ensure that the cable links will function reliably at high speeds and
frequencies. Cable tests should be performed when the cable is
installed and afterward on a regular basis to ensure that LAN cabling
meets industry standards. High quality cable test instruments should
be correctly used to ensure that the tests are accurate. Test results
should also be carefully documented.
Time-based Parameters
4.2.8 Testing 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. 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.
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. This results in a reduced amount of light energy arriving
at the receiver, making signal recognition difficult. Just as with UTP
cable, improperly installed connectors are the main cause of light
reflection and signal strength loss in optical fiber.
Because noise is not an issue when transmitting on optical fiber, the
main concern with a fiber link is the strength of the light signal that
arrives at the receiver. If attenuation weakens the light signal at the
receiver, then data errors will result. Testing fiber optic cable primarily
involves shining a light down the fiber and measuring whether a
sufficient amount of light reaches the receiver.
On a fiber optic link, the acceptable amount of signal power loss that
can occur without dropping below the requirements of the receiver
must be calculated. This calculation is referred to as the optical link
loss budget. A fiber test instrument checks whether the optical link
loss budget has been exceeded. If the fiber fails the test, the cable
test instrument should indicate where the optical discontinuities occur
along the length of the cable link. Usually, the problem is one or more
improperly attached connectors. The cable test instrument will
indicate the location of the faulty connections that must be replaced.
When the faults are corrected, the cable must be retested.
Discontinuity
4.2.9 A new standard
On June 20, 2002, the Category 6 (or Cat 6) addition to the TIA-568
standard was published. The official title of the standard is
ANSI/TIA/EIA-568-B.2-1. This new standard specifies the original set
of performance parameters that need to be tested for Ethernet
cabling as well as the passing scores for each of these tests. Cables
certified as Cat 6 cable must pass all ten tests.
Although the Cat 6 tests are essentially the same as those specified
by the Cat 5 standard, Cat 6 cable must pass the tests with higher
scores to be certified. Cat6 cable must be capable of carrying
frequencies up to 250 MHz and must have lower levels of crosstalk
and return loss.
A quality cable tester similar to the Fluke DSP-4000 series or Fluke
OMNIScanner2 can perform all the test measurements required for
Cat 5, Cat 5e, and Cat 6 cable certifications of both permanent links
and channel links. Figure shows the Fluke DSP-LIA013
Channel/Traffic Adapter for Cat 5e.
Fluke DSP-LIA013 Channel/Traffic Adapter
Summary
An understanding of the following key points should have been
achieved:
Waves are energy traveling from one place to another, and are
created by disturbances. All waves have similar attributes such as
amplitude, period, and frequency.
Sine waves are periodic, continuously varying functions. Analog
signals look like sine waves.
Square waves are periodic functions whose values remain constant
for a period of time and then change abruptly. Digital signals look like
square waves.
Exponents are used to represent very large or very small numbers.
The base of a number raised to a positive exponent is equal to the
base multiplied by itself exponent times. For example, 103 =
10x10x10 = 1000.
Logarithms are similar to exponents. A logarithm to the base of 10 of
a number equals the exponent to which 10 would have to be raised in
order to equal the number. For example, log10 1000 = 3 because 103
= 1000.
Decibels are measurements of a gain or loss in the power of a signal.
Negative values represent losses and positive values represent
gains.
Time-domain analysis is the graphing of voltage or current with
respect to time using an oscilloscope. Frequency-domain analysis is
the graphing of voltage or power with respect to frequency using a
spectrum analyzer.
Undesirable signals in a communications system are called noise.
Noise originates from other cables, RFI, and EMI. White noise affects
all frequencies, while narrowband interference affects only a certain
subset of frequencies.
Analog bandwidth is the frequency range that is associated with
certain analog transmission, such as television or FM radio.
Digital bandwidth measures how much information can flow from one
place to another in a given amount of time. Its units are in various
multiples of bits per second.
Most LAN problems occur at the physical layer. The only way to
prevent or troubleshoot many of these problems is through the use of
cable testers.
Proper cable installation according to standards increases LAN
reliability and performance.
Copper media is available in shielded and unshielded forms.
Unshielded cable is more susceptible to noise.
Signal degradation is due to various factors such as noise,
attenuation, impedance mismatch, and several types of crosstalk.
These factors cause decreased network performance.
The TIA/EIA-568-B standard specifies ten tests that a copper cable
must pass if it will be used for modern, high-speed Ethernet LANs.
Optical fiber must also be tested according to networking standards.
Category 6 cable must meet more rigorous frequency testing
standards than Category 5 cable.
Module 4: Summary