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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 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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Sine waves and square waves

Sine waves-Analog signals

A = Amplitude (height or depth of wave)

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

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Exponents and logarithms

Exponents are used to represent very large or very small numbers. The base of a number raised to a positive exponent is equal to the base multiplied by itself exponent times. For example, 103 = 10x10x10 = 1000.

Logarithms are similar to exponents. A logarithm to the base of 10 of a number equals the exponent to which 10 would have to be raised in order to equal the number. For example, log10 1000 = 3 because 103 = 1000.

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Decibels

Decibels are measurements of a gain or loss in 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 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.

Analyzing signals using an oscilloscope is called time-domain analysis, because the x-axis or domain of the mathematical function represents time.

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Spectrum analyzer

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

In frequency-domain analysis, the x-axis represents 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 to

undesirable signals. Noise can originate from natural and

technological sources, and is added to the data signals in communications systems.

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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.

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

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Types of noise

White noiseAffects all transmission frequencies equally

Narrowband interference Only affects small ranges of frequencies

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Bandwidth Analog bandwidth

Refers to the frequency range of an analog electronic system.

The units of measurement is Hertz Digital bandwidth

Digital bandwidth measures how much information can flow from one place to another in a given amount of time.

The unit of measurement is bits per second (bps).

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Various types of cabling

Shielded twisted-pair (STP) STP cable is more expensive, more difficult to install, and less frequently used than UTP.

Unshielded twisted pair (UTP)UTP contains no shielding and is more susceptible to external noise but is the most frequently used because it is inexpensive and easier to install.

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Various types of cabling

Coaxial cable Coaxial cable is a type of shielded cable

Fiber optic cable Fiber optic cable is used to transmit data signals by increasing and decreasing the intensity of light to represent binary ones and zeros.

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Shielding material protects the data signal from external sources of noise and from noise generated 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 electrical noise, and optical fiber does not need to be grounded.

Therefore, optical fiber is often used between buildings and between floors within the building.

As costs decrease and demand for speed increases, optical fiber may become a more commonly used LAN media.

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Cables Comparison

Kind of Cable

Speed and throughput

Media & connector size

Max Cable Lenght

Cost

STP 10-100Mps Medium - Large

100m Expensive

UTP 10-100Mps Medium - Large

100m Least Expensive

Coaxial 10-100Mps Medium 500m Inexpensive

Fiber optic

100+ Mps Small Up to 3000m

Most Expensive

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Attenuation

Attenuation is the decrease in signal amplitude over the length of a link.

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Factors that contribute to attenuation

The resistance of the copper cable converts some of the electrical energy of the signal to heat.

Signal energy is also lost when it leaks through the insulation of the cable and by impedance caused by defective connectors.

Long cable lengths and high signal frequencies contribute to greater signal attenuation

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How to measure the attenuation

Attenuation on a cable is measured by a cable tester using the highest frequencies that the cable is rated to support.

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

Smaller negative dB values are an indication of better link performance.

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Impedance and impedance mismatch

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

The normal, or characteristic, impedance of a Cat5 cable is 100 ohms. If a connector is improperly installed on Cat5, it will have a different impedance value than the cable. This is called an impedance discontinuity or an 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 one wire to a nearby wire Poorly terminated network cabling is a cause of 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 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.

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Power Sum Near-end Crosstalk (PSNEXT)

Power Sum NEXT (PSNEXT) measures the cumulative effect 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 also helps to reduce crosstalk of data or noise signals from an adjacent wire pair

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. Because a noise signal on one wire will appear identically on the other wire, this noise be easily detected and filtered at the receiver

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

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RJ-45 PINOUTSThe 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 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

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Wire Map test

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

Good wiremap Open circuit. The wire does not attach properly at the connector

Short circuit.Two wires are connected to each other

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Wire Map test

The wire map test also verifies that all eight wires are connected to the correct pins on both ends of the cable.

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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.

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Crosstalk

Crosstalk is meassure in four separate tests A cable tester measures NEXT by applying a

test 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 ELFEXT from all wire pairs

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Return loss

Return loss is a measure in decibels of reflections that are caused by the impedance discontinuities at all locations along the link.

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Propagation delay

Propagation delay is a simple measurement of how long it takes for a signal 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. One nanosecond is one-billionth of a second, or 0.000000001 second. The TIA/EIA-568-B standard sets a limit for propagation delay for the various categories of UTP.

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TDR Test

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

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Delay skew

The propagation delay difference between pairs is called delay skew.

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Category 6 and category 5 cable

Category 6 cable must meet more rigorous frequency testing standards than Category 5 cable.

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Discontinuity

A pulse is a deliberate, fixed disturbances of predictable duration used to…•Measure propagation delay (delay skew) to determine 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 loss budget has been exceeded. If the fiber fails the test, the cable test instrument should indicate where the optical discontinuities occur along the length of the cable link. Usually, the problem is one or more improperly attached connectors

Calibrated Light Source and Power Meter

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Testing Cat5 and Cat6 cable

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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Fluke DSP-LIA013 Channel/Traffic Adapter for Cat 5e