Tài liệu học CCNA kỳ 1 mod4 cabletesting

NIIT-ICT Hanoi info@niithanoi.vn 5Analog Signal Digital Signals • Square waves, like sine waves, are periodic.. NIIT-ICT Hanoi info@niithanoi.vn 13Analog and digital signals in time and

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Module 4 – Cable Testing

Overview

Students completing this module should be able to:

• Differentiate between sine wavesand 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 noiselevels 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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NIIT-ICT Hanoi info@niithanoi.vn 3

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 traveling 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 the

original value

• Square waves represent digital signals, or pulses Like all waves,

square waves can be described in terms of amplitude, period, and

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Numbering Systems

1×10 + 7×101 0

1×2 + 1×24 0

+

1×16 + 1×161 0

+

• Three important number systems:

– Binary(base 2),

– Decimal(base 10),

– Hexadecimal(base 16)

• Binary system uses 2 symbols: 0,1

• Decimal system uses

10 symbols:

0,1,2,3,4,5,6,7,8,9

• Hexadecimal system uses 16 symbols:

0,1, ,9,A,B,C,D,E,F

• Example: using the Windows program

CALCto perform conversion among number systems

Exponents and Logarithms

x

y

y = x

0 1

1

y = 10 x

y = log 10x

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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)

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

• log 10implies that the number in parenthesis will be transformed using

the base 10 logarithm rule

• P final is the delivered power measured in Watts

• P ref is the original power measured in Watts

• V final is the delivered voltage measured in Volts

• V reference is the original voltage measured in Volts

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Code and Encode

• Information (text, voice, picture, video, ) can be represented

electrically by voltage patterns

• The data represented by these voltage patterns can be converted to

light waves or radio waves, etc (signals) to be transmitted over a

media, 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

Infomation

is 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 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

• Frequency-domain analysis has the x-axis representing frequency Spectrum

analyzer is the device that produde such the graph.

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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

• Imagine the combination of several sine waves

a

2

f ( x ) ∼ F ( x ) = 0 + Σn = 1∞ [ a sin (n x ) + b cos (n x ) ]

Example

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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

• Even though noise cannot be eliminated, its effects can be minimized if

the 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 from

other signals being transmitted nearby

• Electromagnetic interference (EMI), which is noise from

nearby sources such as motors and lights

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Narrowband Interference and White Noice

Narrowband Interference White Noice

• White noiceaffects all transmission frequencies

• Narowband interferenceonly 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 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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Signaling over coper and fiber optic cabling

• On copper cable, data signals are represented by voltage levels that represent binary

ones and zeros.

• The zero-voltage reference is

called the signal ground.

• If the transmitter and the receiver volt reference point are equal, they are said to be

properly grounded.

• Fiber optic cable is used to

transmit 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 optical fiber does NOT need to be grounded.

• As price decrease and speed increase, optical fiber is more and more popular.

Coaxial

• Coaxial cable consists of a

solid copper conductor surrounded by insulating material, and then braided conductive shielding

• 10Base-2: (thinnet)

Ethernet 10 Mpbs, 185 m, BNC connector, 50Ω.

• 10Base-5: (thicknet)

Ethernet 10 Mbps, 500 m,

50Ω.

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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 Ω.

• ScTP (Screened Pair) or FTP (Foiled

Twisted-Pair) is similar with STP, with exeption that there is no inner foil shields Connector:

shielded RJ45, 100m, 100 Ω

UTP

• UTP(Unshielded Twisted-Pair) has no shield This is the most popularcable used

in LAN.

• Connector: RJ45.

• 100 m.

• 100 Ω.

• 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 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

“good” original signal attenuation

Impedance

• Impedance is a measurement

of the resistance of the cable

to alternating current (AC) and is measured in ohms (Ω).

• If the cable is improperly installed, there may happen

an situation called impedance discontinuity or impedance mismatch, which causes reflection.

• The combination of the effects

of signal attenuation and impedance discontinuities on

a 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 pair of wires is used to transmit one signal The wire pair is twisted so

that each wire experiences similar crosstalk.

• Twisting wires also helps to reduce 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 wire pairs 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 the

transmission of signals from one wire to a nearby wire

Adjacent wires in the cable act like antennas, receiving the transmitted energy.

• Crosstalk is more destructive

at higher transmission frequencies.

• When crosstalk is caused by a signal on another cable, it is

called alien crosstalk.

• 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

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 of

voltage 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 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 Near-end Crosstalk (PSNEXT)

• 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

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Cable testing standards

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

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• 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

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Time-based parameters

• 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

Testing optical fiber

• 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 backin 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 connectorsare the main

cause 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 fiberoptic cable

• External electromagnetic interference or noise has no affecton fiber

cabling

• Attenuation doesoccur on fiber links, but to a lesserextent than on

copper cabling

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

• 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 crosstalkand return loss

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Summary

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