ccna1-mod4-CableTesting

7/27/2019 ccna1-mod4-CableTesting

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Module 4 Cable Testing

CCNA 1 version 3.1

Hc vin mng Cisco BchKhoa - Website: www.ciscobachkhoa.com 2

Overview

Students completing this module should be able to:

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.


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Background for Studying Frequency

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.


Amplitude and Frequency

A wave is type of energy travel ing from one place to another. Example: voltage waves in copper wires, light waves in fiber optic cables

electromagnetic waves in wireless communication.


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Analog Signal


Digital Signals

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 theoriginal value.

Square waves represent digital signals, or pulses. Like all waves,square waves can be described in terms of amplitude, period, andfrequency.


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Numbering Systems

= + = 17 (decimal)

110 + 7101 0

12 + 124 0

+= = 10001 (binary)

116 + 1161 0

+= = 11 (hexadecimal)

Three important numbersystems: Binary (base 2),

Decimal (base 10),

Hexadecimal (base16).

Binary system uses 2symbols: 0,1.

Decimal system uses 10symbols:0,1,2,3,4,5,6,7,8,9.

Hexadecimal system uses16 symbols:0,1,...,9,A,B,C,D,E,F

Example: using theWindows program CALCto perform conversionamong number systems


Exponents and Logarithms

x

yy = x

0

1

1

y = 10x

y = log x10


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Decibels

The decibel (dB) is a measurement unit important indescribing networking signals.

The decibel is related to the exponents and logarithmsdescribed in prior sections.

There are two formulas for calculating decibels:dB = 10 log10 (Pfinal / Pref)

dB = 20 log10 (Vfinal / Vreference)


Decibels

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


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Code and Encode

Information (text, voice, picture, video, ...) can be representedelectrically by voltage patterns.

The data represented by these voltage patterns can be converted tolight waves or radio waves, etc. (signals) to be transmitted over amedia, and then back to voltage waves at the receiver.

Carrierrefers to the signal/waves that carries the information. Example: phone system transmitting voice, wireless communication,

Ethernet.

Infomationis travelling

in digital

signals

Infomation

is travelling

in analog

signals


Viewing signals in time and frequency

An oscilloscope is an important electronic device used to view electrical signalssuch as voltage waves and pulses.

The x-axis on the display represents time, and the y-axis represents voltage orcurrent.

There are usually two y-axis inputs, so two waves can be observed andmeasured 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.

Frequency-domain analysis has the x-axis representing frequency. Spectrumanalyzer is the device that produde such the graph.


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Analog and digital signals in time and f requency

To understand the complexities of networking signals and cabletesting, examine how analog signals vary with time and with frequency.

Imagine the combination of several sine waves.

a

2

f(x) F(x) = + [ a . sin (nx) + b . cos (nx) ]n = 1

0

n n


Example


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Noise in time and frequency

Noise is an important concept in communications systems, includingLANS.

While noise usually refers to undesirable sounds, noise related tocommunications refers to undesirable signals.

Noise can originate from natural and technological sources, and is

added to the data signals in communications systems. Even though noise cannot be eliminated, its effects can be minimized ifthe sources of the noise are understood.


Noise in time and frequency

There are many possible sources of noise:

Nearby cables which carry data signals

Radio frequency interference (RFI), which is noise fromother signals being transmitted nearby

Electromagnetic interference (EMI), which is noise fromnearby sources such as motors and lights


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Narrowband Interference and White Noice

Narrowband Interference White Noice

White noice affects all transmission frequencies.

Narowband interference only affects small ranges of frequencies.


Signals and Noise

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 sumnear-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.


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Signaling over coper and fiber optic cabling

On coppercable, data signals

are represented by voltage

levels that represent binary

ones and zeros.

The zero-voltage reference iscalled the signal ground.

If the transmitter and thereceiver volt reference point are

equal, they are said to be

properly grounded.

Fiber optic cable is used totransmit data signals by

increasing and decreasing the

intensity of light to represent

binary ones and zeros.

Optical signals are NOT affected

by electrical noise, and opticalfiber does NOT need to be

grounded.

As price decrease and speedincrease, optical fiber is more

and more popular.


Coaxial

Coaxial cable consists of asolid copper conductor

surrounded by insulating

material, and then braided

conductive shielding.

10Base-2: (thinnet)Ethernet 10 Mpbs, 185 m,

BNC connector, 50 .

10Base-5: (thicknet)Ethernet 10 Mbps, 500 m,

50 .


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STP and ScTP (FTP)

STP (Shielded Twisted-Pair)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. Connector: shielded

RJ45, 100m, 150 .

ScTP (Screened Twisted-Pair) orFTP (Foiled Twisted-

Pair) is similar with STP, with

exeption that there is no inner

foil shields. Connector:

shielded RJ45, 100m, 100 .


UTP

UTP (Unshielded Twisted-Pair) has no shield. This isthe most popularcable used

in LAN.

Connector: RJ45. 100 m. 100 . There are several categories:

Cat-1, Cat-2, Cat-3, Cat-4,

Cat-5, Cat-5e, Cat-6, Cat-7,

...


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Attenuation and inser tion loss on copper media

Attenuation is the decrease in signal amplitude over thelength of a link.

Long cable lengths and high signal frequencies contributeto greater signal attenuation.

good original signal attenuation


Impedance

Impedance is a measurementof the resistance of the cable

to alternating current (AC)

and is measured in ohms ().

If the cable is improperlyinstalled, there may happen

an situation called impedance

discontinuity orimpedance

mismatch, which causes

reflection.

The combination of the effectsof signal attenuation and

impedance discontinuities ona communications link is

called insertion loss.

good original signal reflection

dispersion jitter


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Twisted-pair Cable

TIA/EIA-568-B certification of a

cable requires testing for a variety

of types of noise.

In twisted-pair cable, a pairofwires is used to transmit one

signal . The wire pair is twisted so

that each wire experiences similar

crosstalk.

Twisting wires also helps toreduce crosstalk or noise 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 wirepairs must be kept to an absolute

minimum to ensure reliable LAN

communications.

GOOD connector

BAD connector:

wires are untwisted too long


Cross Talk

Crosstalk involves thetransmission of signals from

one wire to a nearby wire.

Adjacent wires in the cable act

like antennas, receiving the

transmitted energy.

Crosstalk is more destructiveat higher transmission

frequencies.

When crosstalk is caused by asignal on another cable, it is

called alien c rosstalk.

Cable testing instrumentsmeasure 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.


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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)

Near-end crosstalk (NEXT) is computed as the ratio ofvoltage amplitude between the test signal and the crosstalk

signal when measured from the same end of the link.


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Far-end Crosstalk (FEXT)

Due to attenuation, crosstalk occurring further away fromthe 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 Near-end Crosstalk (PSNEXT)

Power Sum NEXT (PSNEXT) measures the cumulativeeffect of NEXT from all wire pairs in the cable.

PSNEXT is computed for each wire pair based on theNEXT effects of the other three pairs. The combined effect of crosstalk from multiple

simultaneous transmission sources can be very detrimentalto the signal.


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Cable testing standards

The ten primary test parameters that must be verified for acable 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 anRJ-45 connector have a particular purpose.

A NIC transmits signals on pins 1 and 2, and it receivessignals on pins 3 and 6.

The wires in UTP cable must be connected to the properpins at each end of a cable.


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The wire map test insures that no open or short circuitsexist on the cable.

An open circuit occurs if the wire does not attach properlyat the connector.

A short circuit occurs if two wires are connected to eachother.



The wire map test also verifies that all eight wires are connected to thecorrect pins on both ends of the cable.

There are several different wiring faults that the wire map test candetect.


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Time-based parameters

Testers measure the length of the wire based on theelectrical 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, signalsactually travel farther than the physical length of the cable.


Testing optical fiber

Fiber links are subject to the optical equivalent of UTP impedancediscontinuities.

When light encounters an optical discontinuity, some of the light signalis 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 maincause of light reflection and signal strength loss in optical fiber.


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Testing optical fiber

Absence of electrical signals. 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 oncopper cabling.


A new standard

On June 20, 2002, the Category 6 (or Cat 6) addition to the TIA-568standard was published.

The official title of the standard is ANSI/TIA/EIA-568-B.2-1. 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 scoresto be certified.

Cat6 cable must be capable of carrying frequencies up to 250 MHz andmust have lower levels of crosstalk and return loss.


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Summary

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