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CCNA 1

Module 4: Cable Testing

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4.1 Background

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Basic Understandings

• Waves•Sine waves and Square waves•Decibels

Measure loss or gain of the power of a wave

•Viewing signals in time and frequency

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Noise

Noise is an important concept in communications systems, including LANS. Noise usually refers to undesirable sounds. In communications, it refers to undesirable signals. Noise can originate from natural and technological sources, and is added to the data signals in communications systems.Source of Noise:

Nearby cables carrying data signalsRFI – noise from other signalsEMI – noise from nearby motors, light …Laser noise for optical signal

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Bandwidth

•Very important concept in communications.

•2 types:

1) Analog : refers to the frequency range of an

analog electronic system. Units of measurement is

Hertz, the same as the unit of frequency.

Examples of analog bandwidth values are 3 kHz

for telephony, 20 kHz for audible signals, 5 kHz for

AM radio stations, and 200 MHz for FM radio

stations.

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Bandwidth

2) Digital - measures how much information can

flow from one place to another in a given amount of

time. Unit of measurement is bits per second (bps).

Since LANs are capable of speeds of millions of bits

per second, measurement is expressed in kilobits

per second (Kbps) or megabits per second (Mbps).

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Bandwidth

During cable testing, analog bandwidth is used to

determine the digital bandwidth of a copper cable.

Analog frequencies are transmitted from one end

and received on the opposite end. The two signals

are then compared, and the amount of attenuation

of the signal is calculated. In general, media that

will support higher analog bandwidths without high

degrees of attenuation will also support higher

digital bandwidths.

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4.2 Signals and Noise

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Signaling over copper and fibre optic cabling

•On copper cable, data signals are represented by voltage

levels that represent binary ones and zeros.

•For the LAN to operate properly, the receiving device must

be able to accurately interpret the binary ones and zeros

transmitted as voltage levels.

•2 basic types of copper cable: shielded and unshielded. In

shielded cable, shielding material protects the data signal

from external sources of noise and from noise generated by

electrical signals within the cable.

•2 types of twisted-pair cable: STP and UTP

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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.

•Several factors that contribute to attenuation:

-signal energy is lost when it leaks through the cable

-resistance of the copper cable

-Impedance caused by defective connectors

(jitter : echoes making receiver difficult to accurate detect the data

value)

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Sources of Noise

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

When voltages change on a wire, electromagnetic energy is generated. This energy interferes with data on those wires.

Crosstalk can also be caused by signals on separate, nearby cables. It is more destructive at higher transmission frequencies.

Types of Crosstalk

-Near-end crosstalk (NEXT)

-Far-end crosstalk (FEXT)

-Power Sum Near-End crosstalk (PSNEXT)

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Sources of noise

Cable testing instruments measure crosstalk by applying a test signal to one wire pair.

Twisted-pair cable 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 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.

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Cable testing standard

The TIA/EIA-568-B standard specifies ten tests that a

copper cable must pass if it will be used for modern, high-

speed Ethernet LANs.

All cable links should be tested to the maximum rating that

applies for the category of cable being installed.

There are ten primary test parameters that must be verified

for a cable link to meet TIA/EIA standards such as:

•Wire map, Return loss,Propagation delay,Cable length,

CrossTalk

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Testing optical fibre

• Consists of two separate glass fibers for bi-directional signal.• Each glass fiber is surrounded by a sheath that light cannot

pass through, so there are no crosstalk problems.• External electromagnetic interference or noise has no affect on

fiber cabling. • Attenuation does occur on fiber links, but to a lesser extent than

on copper cabling. • Noise is not an issue, the main concern with a fiber link is the

strength of the light signal that arrives at the receiver. • If attenuation weakens the light signal at the receiver, then data

errors will result. • Testing fiber optic cable primarily involves shining a light down

the fiber and measuring whether a sufficient amount of light reaches the receiver.

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A new standardOn 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.

Cables certified as Cat 6 cable must pass all ten tests and Cat 6 cable must pass the tests with higher scores to be certified.

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

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End of Module 4