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Module 4Cable Testing

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Waves

• Networking professionals are interested in voltage waves on copper media, light waves in optical fiber, and alternating electric and magnetic fields called electromagnetic waves.

• The amplitude of an electrical signal represents height, it is measured in volts .

• The period is the amount of time to complete one cycle, measured in seconds.

• The frequency is the number of complete cycles per second, measured in Hertz. 

• If a disturbance is deliberately caused, and involves a fixed, predictable duration, it is called a pulse. Pulses are important in electrical signals because they determine the value of the data being transmitted.

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Waves

What has changed in

each of these

graphs?

Amplitude

Frequency

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Sine Waves• Sine waves, or sinusoids, are graphs of mathematical functions.

• Sine waves have certain characteristics. – Sine waves are periodic, which means that they repeat the same

pattern at regular intervals. – Sine waves are continuously varying, which means that no two

adjacent points on the graph have the same value.– Sine waves are graphical representations of many natural

occurrences that change regularly over time. – Examples of analog waves

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Square Waves

• Square waves, like sine waves, are periodic.

• 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.

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Number Systems and Exponents

• In networking, there are three important number systems: – Base 2 – binary

– Base 10 – decimal

– Base 16 – hexadecimal

• The number system refers to the number of different symbols that can occupy one position (single digit).

• The base of a number system also refers to the value of each digit.

• The least significant digit has a value of base0, or one. The next digit has a value of base1.

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Decibels

• The decibel (dB) is a measurement unit important in describing networking signals.

• The common units of measurement used in formulas for calculating the amount of gain or loss in networking signals are:– Decibels

– Watts

– Volts

• They are used to describe all networking signals, whether voltage waves on copper, optical pulses in fiber, or microwaves in a wireless system.

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Decibels

• The decibel is related to the exponents and logarithms

• There are two formulas for calculating decibels:

– dB = 10 log10 (Pfinal / Pref)

– dB = 20 log10 (Vfinal / Vreference)

• Students are not expected to master the formula, just to recognize that decibels are the key measure of signal and noise in all communications systems.

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Decibels

• The first formula describes decibels in terms of power (P)– dB = 10 log10 (Pfinal / Pref)

• The variables represent the following values:– dB measures the loss or gain of the power of a wave.

– log10 implies that the number in parenthesis will be transformed

using the base 10 logarithm rule

– Pfinal is the delivered power measured in Watts

– Pref is the original power measured in Watts

• Typically, light waves on optical fiber and radio waves in the air are measured using the power formula.

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Decibels Example

• If Pfinal is one microWatt (1 x 10-6 or .000001 Watts) and Pref is

one milliWatt (1 x 10-3 or .001 Watts), what is the gain or loss in decibels? Is this value positive or negative? Does the value represent a gain or a loss in power?

dB = 10 * Log10 ( Pfinal / Pref )

dB = 10 * Log10 (.000001 / .001 )

dB = 10 * Log10 ( .001 )

dB = 10 * -3

dB = -30 Indicates a loss in power

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Decibels

• The second formula describes decibels in terms of Volts (V)– dB = 20 log10 (Vfinal / Vreference)

• The variables represent the following values:– dB measures the loss or gain of the power of a wave.

– log10 implies that the number in parenthesis will be transformed

using the base 10 logarithm rule

– Vfinal is the delivered Voltage measured in Volts

– Vref is the original Voltage measured in Volts

• Typically, electromagnetic waves on copper cables are measured using the voltage formula.

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Decibels

• 10 millivolts (10 * .001 = .01) are measured at the end of a cable. The source voltage was 1 Volt. What is the gain or loss in decibels?

dB = 20 * Log10 ( Vfinal / Vref )

dB = 20 * Log10 (.01 / 1 )

dB = 20 * Log10 ( .01 )

dB = 20 * -2

dB = -40 Indicates a loss in Voltage

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Oscilloscopes

• 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

• 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

What type of wave is displayed on the oscilloscope?

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Oscilloscopes

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

What type of wave is displayed on the oscilloscope?

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Noise

• Noise is an important concept in communications systems, including LANS.

• 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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Noise

• All communications systems have some amount of noise.

• Even though noise cannot be eliminated, its effects can be minimized if the sources of the noise are understood.

• There are many possible sources of noise: – Nearby cables which carry data signals (crosstalk)

– 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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Noise

• Noise that affects all transmission frequencies equally is called white noise.

• Noise that only affects small ranges of frequencies is called narrowband interference.

• When detected on a LAN, white noise would affect all data transmissions, but narrowband interference might disrupt only certain signals.

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Bandwidth

• Bandwidth is an extremely important concept in communications systems.

• Physical media, current technologies, and the laws of physics limit bandwidth.

• Two ways of considering bandwidth that are important for the study of LANs are:– analog bandwidth

– digital bandwidth

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Bandwidth

• Analog bandwidth typically refers to the frequency range of an analog electronic system.

• The units of measurement for analog bandwidth is Hertz, the same as the unit of frequency — for example, 6MHz or 20KHz.

• One hertz is equivalent to one cycle per second.

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Bandwidth

• Digital bandwidth measures how much information can flow from one place to another in a given amount of time (the speed of transmission).

• The fundamental unit of measurement for digital bandwidth 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

• 1.6 megabits per second is different from 1.6 megabytes per second.

• Eight bits make a byte, so 1.6 megabits per second is equal to 0.2 megabytes per second.

1.6 Mbps / 8 = 0.2 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.

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Signaling Over Copper

• On copper cable, data signals are represented by voltage levels that represent binary ones and zeros.

• In order for the LAN to operate properly, the receiving device must be able to accurately interpret the binary ones and zeros transmitted as voltage levels.

• There are two basic types of copper cable: – shielded

– 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.

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Coaxial Cable

• Coaxial cable is a type of shielded cable.

• It consists of a solid copper conductor surrounded by insulating material, and then braided conductive shielding.

• In LAN applications, the braided shielding is electrically grounded to protect the inner conductor from external electrical noise and to eliminate signal loss by keeping the transmitted signal confined to the cable.

• The need to ground the shielding and the bulky size of coaxial cable make it more difficult to install than other copper cabling.

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Twisted-pair Cable

• There are two types of twisted-pair cable: – shielded twisted-pair (STP) – unshielded twisted pair (UTP)

• STP 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.

• 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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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.

• Optical signals are not affected by electrical noise.

• Optical fiber does not need to be grounded.

• Optical fiber is often used between buildings and between floors within the building.

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Attenuation

• 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.

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

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

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Attenuation

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Attenuation

• There are several factors that contribute to attenuation.– Long cable lengths

– 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.

– By impedance caused by defective connectors.

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Crosstalk (Noise)

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

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

• Crosstalk can also be caused by signals on separate, nearby cables.

• Crosstalk is more destructive at higher transmission frequencies.

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Cable Testing Standards

• 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.

• 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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Cable Testing

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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.

• Propagation delay measurements are the basis of the cable length measurement.

• 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.

• The propagation delays of different wire pairs in a single cable can differ slightly because of differences in the number of twists and electrical properties of each wire pair (delay skew).

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Optical Fiber

• A fiber link consists of two separate glass fibers functioning as independent data pathways.

• One fiber carries transmitted signals in one direction, while the second carries signals in the opposite direction (this allows for full-duplex transmission).

• Each glass fiber is surrounded by a sheath that light cannot pass through, so there are no crosstalk problems on fiber optic cable.

• 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.

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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 back in the opposite direction with only a fraction of the original light signal continuing down the fiber towards the receiver.

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

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Testing Optical Fiber

• Testing fiber optic cable primarily involves shining a light down the fiber and measuring whether a sufficient amount of light reaches the receiver.

• On a fiber optic link, the acceptable amount of signal power loss that can occur without dropping below the requirements of the receiver must be calculated.

• This calculation is referred to as the optical link loss budget.

• Usually, the problem is one or more improperly attached connectors.

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New Cabling Standards

• Category 6 (or Cat 6) has been added to the TIA-568 standard.

• 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.

• Cat 6 must have lower levels of crosstalk and return loss.

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