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CCNA SEMESTER 1

V 3.0

CHAPTER 4 Cable testing

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Students completing this chapter

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 impedancemismatch.

Define crosstalk, near-end crosstalk, far-end crosstalk, andpower 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 6cable.

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Sine waves and square waves

Sine waves-Analog signals

A = Amplitude (height or depth ofwave)

Square waves-Digital signals

A= Amplitude (Height of Pulses)

A wave is energy traveling from one place to another.

T= Period (time to complete 1 wave cycle

F= Frequency (cycles per second) = 1/T

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Exponents and logarithms

Exponents are used to represent very largeor very small numbers. The base of a numberraised to a positive exponent is equal to the

base multiplied by itself exponent times. Forexample, 103 = 10x10x10 = 1000.

Logarithms are similar to exponents. Alogarithm to the base of 10 of a number

equals the exponent to which 10 would haveto be raised in order to equal the number. Forexample, log10 1000 = 3 because 10

3 = 1000.

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Decibels

Decibels are measurements of a gain or lossin the power of a signal.

Negative values represent losses

Positive values represent gains.

dB in form of Power (P)dB = 10 log10 (Pfinal / Pref)

dB in form of Voltage (V)dB = 20 log10 (Vfinal / Vref)

delivered power

original power

delivered voltage

original voltage

Formulas for calculating decibels

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Oscilloscope

Oscilloscope is an important electronic deviceused to view electrical signals such asvoltage waves and pulses.

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

Analyzing signals using an oscilloscope iscalled time-domain analysis, because the x-axis or domain of the mathematical functionrepresents time.

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Spectrum analyzer

An electronic device called a spectrumanalyzer creates graphs for frequency-domain analysis.

In frequency-domain analysis, the x-axisrepresents frequency.

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Oscilloscope and Spectrum analyser

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Noise

Noise usually refers to undesirable sounds.

Noise related to communications refers toundesirable signals.

Noise can originate from natural andtechnological sources, and is added to thedata signals in communications systems.

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Possible sources of noise

Nearby cables which carry data signals.

Radio frequency interference (RFI), which isnoise from other signals being transmitted

nearby. Electromagnetic interference (EMI), which is

noise from nearby sources such as motorsand lights.

Laser noise at the transmitter or receiver ofan optical signal.

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Various types of cabling

Shielded twisted-pair (STP)

STP cable is more expensive,more difficult to install, and lessfrequently used than UTP.

Unshielded twisted pair (UTP)

UTP contains no shielding andis more susceptible to externalnoise but is the most frequentlyused because it is inexpensive

and easier to install.

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Various types of cabling

Coaxial cable

Coaxial cable is a type ofshielded cable

Fiber optic cable

Fiber optic cable is used to

transmit data signals byincreasing and decreasingthe intensity of light torepresent binary ones andzeros.

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Shielding material protects the data signalfrom external sources of noise and from noisegenerated by electrical signals within the

cable.Shielded cable

Coaxial cable

Shielded twisted-pair (STP)

Ushielded cable

unshielded twisted pair (UTP).

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Fiber Optic Cable

Optical signals are not affected by electricalnoise, and optical fiber does not need to begrounded.

Therefore, optical fiber is often used betweenbuildings and between floors within thebuilding.

As costs decrease and demand for speedincreases, optical fiber may become a morecommonly used LAN media.

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Cables Comparison

Kind ofCable

Speed andthroughput

Media &connectorsize

MaxCableLenght

Cost

STP 10-100Mps Medium -

Large

100m Expensive

UTP 10-100Mps Medium -Large

100m LeastExpensive

Coaxial 10-100Mps Medium 500m Inexpensive

Fiberoptic

100+ Mps Small Up to3000m

MostExpensive

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Attenuation

Attenuation is the decrease in signal amplitudeover the length of a link.

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Factors that contribute to attenuation

The resistance of the copper cable convertssome of the electrical energy of the signal toheat.

Signal energy is also lost when it leaksthrough the insulation of the cable and byimpedance caused by defective connectors.

Long cable lengths and high signal frequenciescontribute to greater signal attenuation

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How to measure the attenuation

Attenuation on a cable is measured by acable tester using the highest frequenciesthat the cable is rated to support.

Attenuation is expressed in decibels (dB)using negative numbers.

Smaller negative dB values are an indicationof better link performance.

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Impedance and impedance mismatch

Impedance is a measurement of theresistance of the cable to alternating current(AC) and is measured in ohms.

The normal, or characteristic, impedance of aCat5 cable is 100 ohms. If a connector isimproperly installed on Cat5, it will have adifferent impedance value than the cable.This is called an impedance discontinuity oran impedance mismatch.

Impedance discontinuities cause attenuation

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Crosstalk and types of crosstalk

Crosstalk is another from of noice.

Involves the transmission of signals from onewire to a nearby wire

Poorly terminated network cabling is a causeof crosstalk

Types of crosstalk are

Near-end Crosstalk (NEXT) Far-end Crosstalk (FEXT)

Power Sum Near-end Crosstalk (PSNEXT)

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Near-end Crosstalk (NEXT)

Near-end Crosstalk (NEXT) is the ratio of voltageamplitude between the test signal and the crosstalksignal when measured from the same end of the link.

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Far-end Crosstalk (FEXT)

Due to attenuation, crosstalk occurring further awayfrom the transmitter creates less noise on a cable thanNEXT. This is called far-end crosstalk, or FEXT.

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Power Sum Near-end Crosstalk

(PSNEXT)

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

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How to minimize noise

Twisting one pair of wires in a cable alsohelps to reduce crosstalk of data or noisesignals from an adjacent wire pair

In twisted-pair cable, a pair of wires is used totransmit one signal. The wire pair is twistedso that each wire experiences similarcrosstalk. Because a noise signal on one wire

will appear identically on the other wire, thisnoise be easily detected and filtered at thereceiver

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Untwisting of wire pairs must be kept to an absolute minimumto reduce crosstalk of data or noise signals from an adjacentwire pair and to ensure reliable LAN communications.

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RJ-45 PINOUTS

The Ethernet standard specifies that each of the pins onan RJ-45 connector have a particular purpose. A NICtransmits signals on pins 1 and 2, and it receives signalson pins 3 and 6. The wires in UTP cable must be

connected to the proper pins at each end of a cable.

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Wire Map test

The wire map test insures that no open or shortcircuits exist on the cable.

Good wiremap Open circuit. Thewire does notattach properly

at the connector

Short circuit.Twowires areconnected to each

other

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Wire Map test

The wire map test also verifies that all eight wiresare connected to the correct pins on both ends of thecable.

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Insertion loss

The combination of the effects of signalattenuation and impedance discontinuities ona communications link is called insertion loss.

Insertion loss is measured in decibels at thefar end of the cable.

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Crosstalk

Crosstalk is meassure in four separate tests

A cable tester measures NEXT by applying atest signal to one cable pair.

The equal-level far-end crosstalk (ELFEXT)test measures FEXT

Power sum equal-level far-end crosstalk

(PSELFEXT) is a compine effect of ELFEXTfrom all wire pairs

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Return loss

Return loss is a measure in decibels ofreflections that are caused by the impedancediscontinuities at all locations along the link.

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Propagation delay

Propagation delay is a simple measurementof how long it takes for a signal to travel alongthe cable being tested. The delay in a wire

pair depends on its length, twist rate, andelectrical properties. Delays are measured inhundredths of nanoseconds. Onenanosecond is one-billionth of a second, or

0.000000001 second. The TIA/EIA-568-Bstandard sets a limit for propagation delay forthe various categories of UTP.

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TDR Test

The TDR test is used not only to determinelength, but also to identify the distance towiring faults such as shorts and opens. When

the pulse encounters an open, short, or poorconnection, all or part of the pulse energy isreflected back to the tester.

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Delay skew

The propagation delay difference betweenpairs is called delay skew.

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Category 6 and category 5 cable

Category 6 cable must meet more rigorousfrequency testing standards than Category 5cable.

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Discontinuity

A pulse is a deliberate, fixed disturbances ofpredictable duration used toMeasure propagation delay (delay skew) todetermine the value of the data being transmitted

Find discontinuities (reflections, jitter)Measure transmission length

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Testing optical fiber

A fiber test instrument checks whether the optical link lossbudget has been exceeded. If the fiber fails the test, thecable test instrument should indicate where the opticaldiscontinuities occur along the length of the cable link.

Usually, the problem is one or more improperly attachedconnectors

Calibrated Light Sourceand Power Meter

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Testing Cat5 and Cat6 cable

A quality cable tester similar to the Fluke DSP-4000series or Fluke OMNIScanner2 can perform all thetest measurements required for Cat 5, Cat 5e, andCat 6 cable certifications of both permanent links

and channel links..

Fluke DSP-LIA013Channel/Traffic Adapter forCat 5e