CCNA 1 Module 4 Cable Testing. 2 Waves Frequency: Frequency: –the number of waves per second...

CCNA 1 Module 4CCNA 1 Module 4

Cable TestingCable Testing

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Waves Waves

Frequency:Frequency:– the number of waves per secondthe number of waves per second

Period:Period:– the amount of time that it takes to the amount of time that it takes to

complete 1 cycle, in secondscomplete 1 cycle, in seconds Amplitude:Amplitude:

– the height of the wave, measured in the height of the wave, measured in voltsvolts

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Analog signal – sine Analog signal – sine wavewave

Varies continuously with time

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Analog signal – sine Analog signal – sine wavewave

Phase = 1

Phase = .5

Phase = 0

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Digital signal – square Digital signal – square wavewave

Maintains one value and then suddenly changes to a different value

maintains one value and then suddenly changes to a different value

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Signals Signals

Bits (0s and 1s) need to be transmitted Bits (0s and 1s) need to be transmitted from one host to anotherfrom one host to another

Each bit is placed on the cable as a Each bit is placed on the cable as a signal or signal or pulsepulse

On copper cable the pulses are On copper cable the pulses are electrical signals of different voltage electrical signals of different voltage levelslevels

The simplest signalling schemes have The simplest signalling schemes have only two voltage levelsonly two voltage levels

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A pulseA pulse

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Fourier Synthesis of a Fourier Synthesis of a Square WaveSquare Wave

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NoiseNoise

Noise is unwanted Noise is unwanted additional additional electrical signals electrical signals on a cableon a cable

The extra signals The extra signals are added to the are added to the data signals and data signals and distort themdistort them

Original signal

Noise

Resulting signal

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Possible sources of noise:Possible sources of noise:

– Nearby cables which carry data signals Nearby cables which carry data signals

– Radio frequency interference (RFI), which is noise Radio frequency interference (RFI), which is noise from other signals being transmitted nearby from other signals being transmitted nearby

– Electromagnetic interference (EMI), which is noise Electromagnetic interference (EMI), which is noise from nearby sources such as motors and lights from nearby sources such as motors and lights

– Laser noise at the transmitter or receiver of an Laser noise at the transmitter or receiver of an optical signal optical signal

NoiseNoise

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BandwidthBandwidth

Two ways of considering bandwidth in Two ways of considering bandwidth in communications systems are analog communications systems are analog and digital. and digital.

– Analog bandwidth describes the range of Analog bandwidth describes the range of frequencies transmitted (frequencies transmitted (hertz; Hz)hertz; Hz)

– Digital bandwidth measures how much Digital bandwidth measures how much information can flow from one place to information can flow from one place to another in a given amount of time (bps)another in a given amount of time (bps)

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Signals and NoiseSignals and Noise

Bits are represented by either no Bits are represented by either no voltage (0) or +3 to +6 Volts (1).voltage (0) or +3 to +6 Volts (1).

A Signal Reference Ground attached A Signal Reference Ground attached close to a computer’s digital circuits close to a computer’s digital circuits establishes the baseline for no voltage.establishes the baseline for no voltage.

The reference level is called a The reference level is called a signal signal groundground

Bits must arrive at the destination Bits must arrive at the destination undistorted in order to be properly undistorted in order to be properly interpreted.interpreted.

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Shielded vs Unshielded Shielded vs Unshielded CableCable Shielding material protects the data signal from Shielding material protects the data signal from

external sources of noise and from noise generated external sources of noise and from noise generated by electrical signals within the cableby electrical signals within the cable

Benefits of shieldingBenefits of shielding

– protects the inner conductor from external electrical noiseprotects the inner conductor from external electrical noise

– keeps the transmitted signal confined to the cable keeps the transmitted signal confined to the cable

– protects each wire pair from noise generated by the other protects each wire pair from noise generated by the other pairspairs

Shielded cable examples: coaxial, STP, ScTPShielded cable examples: coaxial, STP, ScTP Unshielded cable: UTPUnshielded cable: UTP

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Shielded cable helps insulate the Shielded cable helps insulate the signal from external noise.signal from external noise.

Shielding makes the cable thicker Shielding makes the cable thicker and therefore more difficult to and therefore more difficult to install.install.

Twisted pair cable uses Twisted pair cable uses twistingtwisting of pairs of wires to of pairs of wires to cancelcancel signals signals from adjacent wires.from adjacent wires.

Shielded vs Unshielded Shielded vs Unshielded CableCable

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Optical CableOptical Cable

Fiber optic cable:Fiber optic cable:– transmits data signals by increasing and transmits data signals by increasing and

decreasing the intensity of light.decreasing the intensity of light.– needs no insulation as it is not needs no insulation as it is not

affected by electrical noise.affected by electrical noise.– does not need to be grounded.does not need to be grounded.– is often used between buildings and is often used between buildings and

between floors within the building.between floors within the building.

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What six things can What six things can distort a bit?distort a bit? PropagationPropagation AttenuationAttenuation ReflectionReflection NoiseNoise Timing problemsTiming problems CollisionsCollisions

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PropagationPropagation

The travelling of bits down a wireThe travelling of bits down a wire– Devices (eg printer) at receiving end Devices (eg printer) at receiving end

cannot handle the speed, and data cannot handle the speed, and data is lostis lost

– Solve with buffers on the device or a Solve with buffers on the device or a message to slow down the message to slow down the transmission ratetransmission rate

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Bits are distorted by:Bits are distorted by:

PropagationPropagation AttenuationAttenuation ReflectionReflection NoiseNoise Timing problemsTiming problems CollisionsCollisions

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AttenuationAttenuation

Loss of signal strength Loss of signal strength as bits move down a as bits move down a wire (dB)wire (dB)– 1s look like 0s, and the 1s look like 0s, and the

message is unreadablemessage is unreadable– Test by using the Test by using the

highest frequencies highest frequencies possible with the cable.possible with the cable.

– Solved by using Solved by using repeaters, hubs to repeaters, hubs to amplify the signalamplify the signal

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Impedance is caused:Impedance is caused:– Exceeding the maximum recommended Exceeding the maximum recommended

cable lengthcable length Use repeaters or don’t exceed the 100mUse repeaters or don’t exceed the 100m

– by defective connectorsby defective connectors– when signal energy is lost, when it leaks when signal energy is lost, when it leaks

through the insulation of the cablethrough the insulation of the cable– by the resistance of the copper cable by the resistance of the copper cable

converting some of the electrical energy of converting some of the electrical energy of the signal to heat the signal to heat

ImpedanceImpedance

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Reflection Reflection

Reflected energy moves back towards signal, Reflected energy moves back towards signal, distorting it as bits run into each other.distorting it as bits run into each other.– Caused by impedance mismatch (NIC and media)Caused by impedance mismatch (NIC and media)– Jitter is created as the reflected signal bounces back and Jitter is created as the reflected signal bounces back and

forth.forth.– Solve by correct wiringSolve by correct wiring

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Noise Noise

Unwanted additions to a signalUnwanted additions to a signal Results in 1s turned into 0s and Results in 1s turned into 0s and

0s into 1s. 0s into 1s. Too much noise can Too much noise can destroy the message.destroy the message.

It is important to keep the signal It is important to keep the signal to noise ratio as high as possible to noise ratio as high as possible (little noise)(little noise)

It is not possible to avoid noise It is not possible to avoid noise altogetheraltogether

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Four kindsFour kinds– ThermalThermal– Reference ground noise Reference ground noise – EMI/RFIEMI/RFI– Crosstalk Crosstalk

NoiseNoise

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Thermal noise is everywhere but Thermal noise is everywhere but isn’t a problem.isn’t a problem.

Reference ground noise is AC line Reference ground noise is AC line noise, fixed by rewiring.noise, fixed by rewiring.

Electromagnetic interference (EMI)Electromagnetic interference (EMI) Radio Frequency interference (RFI)Radio Frequency interference (RFI)

External NoiseExternal Noise

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EMI/RFIEMI/RFI

EMI and RFI attack the quality of EMI and RFI attack the quality of electrical signals on the cable.electrical signals on the cable.

Sources of EMI/RFI include:Sources of EMI/RFI include:– EMI – fluorescent lights; electric EMI – fluorescent lights; electric

motorsmotors– RFI – Radio systemsRFI – Radio systems

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EMI/RFI noiseEMI/RFI noise

Source computer Source computer sends out a digital sends out a digital signal.signal.

EMI noise occurs EMI noise occurs along the path.along the path.

The extra signal is The extra signal is added to the data added to the data signals and distorts it.signals and distorts it.

Original signal

Noise

Resulting signal

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Two ways to prevent EMI/RFI Two ways to prevent EMI/RFI Noise:Noise:– shieldingshielding the wires in the cable with the wires in the cable with

a metal braid or foil. (Increases cost a metal braid or foil. (Increases cost and diameter of the cable)and diameter of the cable)

– cancellationcancellation – the wires are – the wires are twistedtwisted together in pairs providing self-together in pairs providing self-shielding within the network media.shielding within the network media.


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UTP Cat 5 has eight UTP Cat 5 has eight wires twisted into wires twisted into four pairs.four pairs.

In each pair, one wire In each pair, one wire is sending data and is sending data and the other is receiving.the other is receiving.

Electrons flowing Electrons flowing down the wire create down the wire create a small, circular a small, circular magnetic field around magnetic field around the wire.the wire.

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Since the two wires Since the two wires are close together, are close together, their opposing their opposing magnetic fields magnetic fields cancel each other.cancel each other.

They also cancel out They also cancel out outside magnetic outside magnetic fields (EMI/RFI).fields (EMI/RFI).

Twisting of the wires Twisting of the wires enhances enhances cancellationcancellation

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Noise on Copper MediaNoise on Copper Media

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Internal Noise - Internal Noise - CrosstalkCrosstalk1.1. A voltage signal passes along a wireA voltage signal passes along a wire

2.2. It generates energy that radiates out, It generates energy that radiates out, like a radio signallike a radio signal

3.3. Other nearby wires can pick up the Other nearby wires can pick up the signalsignal

4.4. This signal adds a small voltage to the This signal adds a small voltage to the signal on the nearby wiressignal on the nearby wires

5.5. This effect is called crosstalkThis effect is called crosstalk

6.6. Crosstalk is cancelled by the twisting Crosstalk is cancelled by the twisting of pairs of wires.of pairs of wires.

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Testing for CrosstalkTesting for Crosstalk

The cable testing instrument puts a The cable testing instrument puts a signal on one wire pair in a cable.signal on one wire pair in a cable.

It then measures, in dB the amplitude It then measures, in dB the amplitude of the crosstalk signals.of the crosstalk signals.

The lower (smaller) the negative The lower (smaller) the negative number, the more noise there is.number, the more noise there is.– -30dB is a CLEARER signal than -10dB.-30dB is a CLEARER signal than -10dB.

Each pair should be measured from Each pair should be measured from each other pair in a UTP link, and from each other pair in a UTP link, and from both ends of the link.both ends of the link.

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Types of Crosstalk - Types of Crosstalk - NEXTNEXT Near-end CrosstalkNear-end Crosstalk NEXT NEXT is the test signal measured from the is the test signal measured from the

same end of the link (cable)same end of the link (cable)..

Radiated EM energy

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Types of Crosstalk - Types of Crosstalk - FEXTFEXT Far-end CrosstalkFar-end Crosstalk FEXT occurs further away from the source, FEXT occurs further away from the source,

like at the other end of the cable.like at the other end of the cable. Due to attenuation, FEXT creates less noise Due to attenuation, FEXT creates less noise

than NEXT.than NEXT.

Generates weak FEXT at other pairs

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Power Sum Near-end Crosstalk Power Sum Near-end Crosstalk PSNEXT is the combined effect of NEXT PSNEXT is the combined effect of NEXT

on one pair of wires from the other 3 on one pair of wires from the other 3 pairs of wires in the cable.pairs of wires in the cable.

Cables need to be tested to measure Cables need to be tested to measure the amount of PSNEXT.the amount of PSNEXT.

It is particularly important when all the It is particularly important when all the wires in a cable are being used for wires in a cable are being used for transmission, as in Gigabit Ethernet.transmission, as in Gigabit Ethernet.

Types of Crosstalk - Types of Crosstalk - PSNEXTPSNEXT

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Timing ProblemsTiming Problems

DispersionDispersion – similar to attenuation; is the – similar to attenuation; is the broadening of a signal as it travels down the broadening of a signal as it travels down the media.media.

JitterJitter – – the reflected signal strikes the first the reflected signal strikes the first discontinuity, and some of the signal discontinuity, and some of the signal rebounds in the original direction, creating rebounds in the original direction, creating multiple echo effects. The echoes strike the multiple echo effects. The echoes strike the receiver at different intervals. This is called receiver at different intervals. This is called jitter and can result in data errors. jitter and can result in data errors.

LatencyLatency – is the – is the delaydelay of a network signal of a network signal caused by:caused by:– The time it takes a bit to travel to its destinationThe time it takes a bit to travel to its destination– Devices the bit travels throughDevices the bit travels through

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Collisions Collisions

Collisions occur in broadcast Collisions occur in broadcast topologies where devices share access topologies where devices share access to the network media.to the network media.

A collision happens when two devices A collision happens when two devices attempt to communicate on the attempt to communicate on the shared-medium at the same time.shared-medium at the same time.

Collisions destroy data requiring the Collisions destroy data requiring the source to retransmit.source to retransmit.

The prevention of collisions will be The prevention of collisions will be discussed in more detail later in the discussed in more detail later in the semester.semester.

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Cable Testing Cable Testing StandardsStandards TIA/EIA-568-B standard applies to TIA/EIA-568-B standard applies to

copper cable in a LAN.copper cable in a LAN. Each pair of pins on an RJ45 Each pair of pins on an RJ45

connector have a specific purpose.connector have a specific purpose.– A NIC transmits signals on pins 1 and A NIC transmits signals on pins 1 and

2, and receives signals on pins 3 and 2, and receives signals on pins 3 and 6.6.

– The other four pins are not used The other four pins are not used except in gigabit ethernet.except in gigabit ethernet.

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TIA/EIA-568-B TIA/EIA-568-B Standard – the ten Standard – the ten cable testscable tests

1.1. Wire map Wire map 2.2. Insertion loss Insertion loss 3.3. Near-end crosstalk (NEXT) Near-end crosstalk (NEXT) 4.4. Power sum near-end crosstalk (PSNEXT) Power sum near-end crosstalk (PSNEXT) 5.5. Equal-level far-end crosstalk (ELFEXT) Equal-level far-end crosstalk (ELFEXT) 6.6. Power sum equal-level far-end crosstalk Power sum equal-level far-end crosstalk

(PSELFEXT) (PSELFEXT) 7.7. Return loss Return loss 8.8. Propagation delay Propagation delay 9.9. Cable length Cable length 10.10. Delay skewDelay skew

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Wire MapWire Map ensures that no open or short circuits ensures that no open or short circuits

exist on the cable (Good Wire Map)exist on the cable (Good Wire Map) an ‘an ‘OpenOpen’ circuit occurs if a wire does ’ circuit occurs if a wire does

not attach properly at the connector. not attach properly at the connector. an ‘an ‘ShortShort’ circuit occurs if two wires are ’ circuit occurs if two wires are

connected to each other. connected to each other.

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Wire Map and Wiring Wire Map and Wiring FaultsFaults The wire map verifies The wire map verifies

that all eight wires that all eight wires are connected to the are connected to the correct pins on both correct pins on both ends of the cableends of the cable

The reversed-pair The reversed-pair fault occurs when a fault occurs when a wire pair is correctly wire pair is correctly installed on one installed on one connector, but connector, but reversed on the reversed on the other connector. other connector.

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Wire Map and Wiring Wire Map and Wiring FaultsFaults A split-pair wiring A split-pair wiring

fault occurs when fault occurs when one wire from one one wire from one pair is switched with pair is switched with one wire from a one wire from a different pair at both different pair at both ends. Less ends. Less cancellation means cancellation means more crosstalk.more crosstalk.

A transposed pair A transposed pair fault occurs when two fault occurs when two wire pairs are correct wire pairs are correct at one end but at one end but reversed at the other reversed at the other end. end.

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– Wires inside the cable are twistedWires inside the cable are twisted– Signals actually travel farther than the physical Signals actually travel farther than the physical

length of the cable length of the cable TDR measurement:TDR measurement:

– it sends a pulse signal down a wire pair and it sends a pulse signal down a wire pair and measures the amount of time required for the measures the amount of time required for the pulse to return on the same wire pairpulse to return on the same wire pair

TDR test TDR test – determine lengthdetermine length– identify the distance to wiring faults such as shorts identify the distance to wiring faults such as shorts

&opens&opens – When the pulse encounters an open, short, or poor When the pulse encounters an open, short, or poor

connection, all or part of the pulse energy is connection, all or part of the pulse energy is reflected back to the testerreflected back to the tester

Time Domain Time Domain Reflectometry-TDR Reflectometry-TDR

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A New StandardA New Standard

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

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

Cables certified as Cat 6 cable must Cables certified as Cat 6 cable must pass all 10 tests pass all 10 tests

Cat 6 cable must pass the tests with Cat 6 cable must pass the tests with higher scores to be certified. higher scores to be certified.

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CCNA 1 Module 4 Cable Testing. 2 Waves Frequency: Frequency: –the number of waves per second...

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