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Module 4:

Cable Testing

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Outline

Background for Studying Frequency-Based Cable Testing Waves

Sine waves and square waves

Exponents and logarithms

Decibels

Viewing signals in time and frequency

Analog and digital signals in time and frequency Noise in time and frequency

Bandwidth

Signals and Noise Signaling over copper and fiber optic cabling

Attenuation and insertion loss on copper media

Sources of noise on copper media

Types of crosstalk Cable testing standards

Other test parameters

Time-based parameters

Testing optical fiber

A new standard

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Background for Studying Frequency-

Based Cable Testing

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Waves

A wave is energy traveling from one place to another.

A bucket of water that is completely still. no waves, no disturbances

The ocean always has some sort of detectable waves. wind and tide

measured in meters.

How frequently the waves reach the shore ? period

It is the amount of time between each wave, measured in seconds.

frequency

It 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.

Electromagnetic waves voltage waves on copper media light waves in optical fiber

Pulse The disturbance is caused in a fixed or predictable duration.

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Sine waves and square waves

Sine waves, or sinusoids

periodical

repeat the same pattern at regular intervals

continuously varying with time

analog waves

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Sine waves and square waves (cont.)

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Sine waves and square waves (cont.)

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Sine waves and square waves (cont.)

Square waves

periodical

do not continuously vary with time

The wave holds one value for some time, and

then suddenly changes to a different value.

digital signals, or pulses

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Exponents and logarithms

In networking, there are three important number systems:

Base 2 binary

Base 10 decimal

Base 16 hexadecimal

Power and exponent

10 * 10 = 10^2 (10 raised to the second power,exponent = 2)

10 * 10 * 10 = 10^3 (10 raised to the third power, exponent =3)

Numbers with exponents are used to easily represent very

large or very small numbers. Logarithm

base 10 logarithms are often abbreviated ()log

log (10^9) = 9

log (10^-3) = -3

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Decibels

The first formula describes decibels in

terms of power (P), and the second interms of voltage (V).

dB measures the loss orgain of thepower of a wave.

negative values : a loss in power asthe wave travels

positive values : a gain in power if the

signal is amplified Light waves on optical fiber and radio

waves in the air are measured using thepower formula.

Electromagnetic waves on copper cablesare measured using the voltage formula.

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Viewing signals in time and frequency

An oscilloscope is an important electronic device used to viewelectrical signals such as voltage waves and pulses.

The x-axis represents time.

The y-axis represents voltage or current.

Time-domain analysis

Spectrum analyzer

The x-axis represents frequency.

Frequency-domain analysis

Electromagnetic signals use different frequencies fortransmission 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 listenerschange the station on a radio, they are changing the frequencythat the radio is receiving

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Viewing signals in time and frequency (cont.)

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Analog and digital signals in time and frequency

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

First, consider a single-frequency electrical sine wave, whosefrequency can be detected by the human ear. If this signal is

transmitted to a speaker, a tone can be heard. How would aspectrum analyzer display this pure tone?

Next, imagine the combination of several sine waves. Theresulting wave is more complex than a pure sine wave. Severaltones would be heard. How would a spectrum analyzer displaythis? The graph of several tones shows several individual linescorresponding to the frequency of each tone.

Finally, imagine a complex signal, like a voice or a musicalinstrument. What would its spectrum analyzer graph look like? Ifmany different tones are present, a continuous spectrum ofindividual tones would be represented.

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Analog and digital signals in time and frequency

(cont.)

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Noise in time and frequency

Noise is an important concept in communications systems

undesirable signals

Noise can originate from natural and technological sources.

Noise is added to the data signals in communications systems.

There are many possible sources of noise:

Nearby cables which carry data signals. Radio frequency interference (RFI)

Noise is from othersignals being transmitted nearby.

Electromagnetic interference (EMI) Noise is from nearby sources such as motors and lights

Laser noise at the transmitter or receiver of an optical signal

White noise Noise that affects all transmission frequencies equally.

Narrowband interference

Noise that only affects small ranges of frequencies.

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Bandwidth

Analog bandwidth

Analog bandwidth could be used to describe the range offrequencies transmitted by a radio station or an electronicamplifier.

measurement unit is Hertz

Digital bandwidth Digital bandwidth measures how much information can flow

from one place to another in a given amount of time.

measurement unit is bits per second (bps).

During cable testing, analog bandwidth is used to determine thedigital bandwidth of a copper cable.

Analog + Digital

Media that will support higher analog bandwidths withouthigh degrees of attenuation will also support higher digitalbandwidths.

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Signals and Noise

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Signaling over copper and fiber optic cabling

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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 greatersignal attenuation.

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

Smaller negative dB values are an indication ofbetter link performance.

There are several factors that contribute to attenuation. the resistance of the copper cable

the insulation of the cable

impedance of a Cat5 cable is 100 ohms.

Impedance discontinuity or an impedance mismatch. a portion of a transmitted signal will be reflected back to the

transmitting device, much like an echo.

jitter and results in data errors.

Insertion loss The combination of the effects of signal attenuation and impedance

discontinuities on a communications link.

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Sources of noise on copper media

Noise is any electrical energy on the transmission cable that makes itdifficult for a receiver to interpret the data sent from the transmitter.

TIA/EIA-568-B certification of a cable now requires testing for a varietyof types of noise.

Crosstalk involves the transmission of signals from one wire to a nearby

wire. Twisted-paircable is designed to take advantage of the effects of

crosstalk in order to minimize noise.

Higher categories of UTP require more twists on each wire pair in thecable to minimize crosstalk at high transmission frequencies.

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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) It 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.

Negative value of decibels (dB).

Low negative numbers indicate more noise, just as low negativetemperatures indicate more heat (closed to zero).

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 10dB.

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Types of crosstalk (cont.)

NEXT needs to be measured from each pair to each other pair ina UTP link, and from both ends of the link.

To shorten test times, some cable test instruments allow the userto test the NEXT performance of a link by using larger frequencystep sizes than specified by the TIA/EIA standard. The resulting

measurements may not comply with TIA/EIA-568-B, and mayoverlook ()link faults.

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Types of crosstalk (cont.)

Far-end crosstalk (FEXT)

Due to attenuation, crosstalk occurring further away from thetransmittercreates less noise on a cable than NEXT.

The noise caused by FEXT still travels back to the source,but it is attenuated as it returns. Thus, FEXT is not assignificant a problem as NEXT.

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Types of crosstalk (cont.)

Power Sum NEXT (PSNEXT)

It measures the cumulative effect of NEXT from other 3 pairs in thecable.

TIA/EIA-568-B certification now requires this PSNEXT test.

Some Ethernet standards such as 10BASE-T and 100BASE-TXreceive data from only one wire pair in each direction.

For newer technologies such as 1000BASE-T that receive datasimultaneously from multiple pairs in the same direction, power summeasurements are very important tests.

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Cable testing standards

The TIA/EIA-568-B standard specifiesten tests that a copper cable must pass ifit will be used for modern, high-speedEthernet LANs.

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-endcrosstalk (PSELFEXT)

Return loss Propagation delay

Cable length

Delay skew

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Cable testing standards (cont.)

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 pins3 and 6.

The wire map test insures that no open or short circuits exist on thecable.

An open circuit occurs if the wire does not attach properly at theconnector.

A short circuit occurs iftwo wires are connected to each other.

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Cable testing standards (cont.)

The wiring faults (TIA/EIA-568-B) :

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Other test parameters

Insertion loss 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.

Equal-level far-end crosstalk (ELFEXT) Pair-to-pair ELFEXT is expressed in dB as the difference between the measured

FEXT and the insertion loss of the wire pairwhose signal is disturbed by theFEXT.

ELFEXT is an important measurement in Ethernet networks using 1000BASE-Ttechnologies.

Power sum equal-level far-end crosstalk (PSELFEXT) It is the combined effect of ELFEXT from all wire pairs.

Return loss It is a measure in decibels ofreflections 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 theimpedance discontinuities will strike()the receiver at different intervalscausing signaljitter.

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Time-based parameters

Propagation delay is a simple measurement ofhow long it takes for asignal 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.

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.

Time Domain Reflectometry (TDR) test Since the wires inside the cable are twisted, signals actually travel

fartherthan the physical length of the cable.

It sends a pulse signal down a wire pair and measures the amount oftime required for the pulse to return on the same wire pair.

The TDR test is used not only to determine length, but also to identifythe 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.

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Time-based parameters (cont.)

Delay skew

The propagation delays of different wire pairs in a singlecable can differ slightly because of differences in the numberof twists and electrical properties of each wire pair.

The delay difference between pairs is called delay skew.

Delay skew is a critical parameterfor high-speed networks inwhich 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 atdifferent times and the data cannot be properly reassembled.

All cable links in a LAN must pass all of the tests in the TIA/EIA-

568-B standard. These tests ensure that the cable links will function reliably at

high speeds and frequencies.

High quality cable test instruments should be correctly used toensure that the tests are accurate.

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Testing optical fiber

A fiber link consists of two separate glass fibers.

There are no crosstalk problems on fiber optic cable.

External electromagnetic interference or noise has no affect on fiber cabling.

Optical discontinuity Some of the light signal is reflected back in the opposite direction.

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.

Improperly installed connectors are the main cause of light reflection and signalstrength loss in optical fiber.

The strength of the light signal that arrives at the receiver is important.

If attenuation weakens the light signal at the receiver, then data errors will result.

Optical link loss budget The acceptable amount of signal power loss that can occur without dropping

below the requirements of the receiver.

If the fiber fails the test, The problem usually is one or more improperly attachedconnectors.

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A new standard

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

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

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

Cat6 cable must be capable of carrying frequenciesup to 250 MHz and must have lowerlevels ofcrosstalk and return loss.

Fluke DSP-4000 series or Fluke OMNIScanner2 canperform all the test measurements required for Cat 5,Cat 5e, and Cat 6 cable certifications of bothpermanent links and channel links.

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Thank You