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Mod 4 – Cable Testing. CCNA 1 version 3.0 Rick Graziani Cabrillo College. Overview. Students completing this module should be able to: Differentiate between sine waves and square waves. Define and calculate exponents and logarithms. Define and calculate decibels.

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Overview

Students completing this module 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.

Rick Graziani [email protected]

Background for Studying Frequency-Based Cable Testing

- 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.

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Amplitude and Frequency

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Analog Signal

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Other information

- For the next several slides we will explain analog signals from information from the following sources:

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Digital and Analog Bandwidth

Bandwidth = The width or carrying capacity of a communications circuit.

Digital bandwidth = the number of bits per second (bps) the circuit can carry

- used in digital communications such as T-1 or DDS
- measure in bps
- T-1 -> 1.544 Mbps
Analog bandwidth = the range of frequencies the circuit can carry

- used in analog communications such as voice (telephones)
- measured in Hertz (Hz), cycles per second
- voice-grade telephone lines have a 3,100 Hz bandwidth

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Digital Signals

- digital signal = a signal whose state consists of discrete elements such as high or low, on or off
Analog Signals

- analog signal = a signal which is “analogous” to sound waves
- telephone lines are designed to carry analog signals

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Sound Waves

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Analog Signals, Modulation and Modem Standards

- A perfect or steady tone makes a wave with consistent height (amplitude) and pitch (frequency) which looks like a sine wave. (Figure 4-15)
- A cycle or one complete cycle of the wave
- The frequency (the number of cycles) of the wave is measured in Hertz
- Hertz (Hz) = the number of cycles per second

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Transmission Terminology (whatis.com)

Broadband transmission =

- In general, broadband refers to telecommunication in which a wide band of frequencies is available to transmit information.
- Because a wide band of frequencies is available, information can be multiplexed and sent on many different frequencies or channels within the band concurrently, allowing more information to be transmitted in a given amount of time (much as more lanes on a highway allow more cars to travel on it at the same time).
Baseband transmission

1) Describing a telecommunication system in which information is carried in digital (or analog) form on a single unmultiplexed signal channel on the transmission medium. This usage pertains to a baseband network such as Ethernet and token ringlocal area networks.

Narrowband transmission

- Generally, narrowband describes telecommunication that carries voice information in a narrow band of frequencies.
- More specifically, the term has been used to describe a specific frequency range set aside by the U.S. Fcc for mobile or radio services, including paging systems, from 50 cps to 64 Kbps.

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Carrier Signal

Carrier Signal or Analog Wave = An electronic signal used to modulate data in broadband transmission, usually a sine wave.

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Carrier Signal

Three parts of any analog wave are:

1. amplitude - the height of the wave

2. frequency - the pitch of the wave

3. phase - the shift or position of the wave

These are the three parts we can modulate or change the carrier signal or wave!

Modulate = Change

More in a moment.

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Telephone Lines, Modems, and PSTN

- Voice grade telephone lines and equipment are designed to transmit tones between 300 and 3,400 Hertz
- bandwidth = 3,100 Hz or 3.1 KHz
- “most” of our human voice falls into this range
- Economics dictated the size of this bandwidth
- (Keyboard example)
- The “maximum” number of cycles (highest frequency) of an analog signal over voice grade telephone lines is 3,400 Hz (cycles per second)

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Telephone Lines, Modems, and PSTN

Modem

- MOdulator/DEModulator
- converts analog signals to digital and digital signals to analog
- used for transmitting digital information between computers over voice-grade telephone lines
- Computers use transmission interface standards such as RS-232-C using positive and negative voltages which form square waves, whereas the PSTN is designed to carry analog signals (sine waves)

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Modulation

modulation =

1. the process of varying the characteristic of an electrical carrier wave (analog, sine wave) as the information on that wave varies

Three types of modulation

1. amplitude modulation

2. frequency modulation

3. phase modulation

2. the process of converting digital signals to analog

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Amplitude Modulation

Amplitude Modulation (AM)

- a modulation technique to vary the height the electrical signal (the sine wave or carrier wave with modems) to transmit ones and zeroes, while the frequency of the wave remains constant
- different amplitudes for 0’s and 1’s
- a.k.a. amplitude shift keying, ASK
- Figure 4-22
- frequency for each bit remains constant
- volume = amplitude

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Amplitude Modulation

Different amplitudes for 0’s and 1’s, while the frequency of the wave remains constant

Full duplex

- different amplitudes and frequencies are used for different directions
Disadvantage

- Voice-grade telephone lines are susceptible to distortions which affect amplitudes, as volume fades, the amplitude lowers
- Amplitude modulation only effective for low speed transmissions

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Frequency Modulation

Frequency Modulation

- a modulation technique to vary the frequency of the sine wave (or carrier wave) to transmit ones and zeroes, while the amplitude remains constant
- different frequencies for 0’s and 1’s
- a.k.a. frequency shift keying, FSK
- Figure 4-23
- two separate frequencies for ones and zeroes

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Frequency Modulation

Full Duplex

- requires a minimum of four frequencies, two frequencies for each direction
- i.e. CCITT V.21 for 300 baud modems:
Originating Sending

Modem Modem

1270 Hz 1 2225 Hz

1070 Hz 0 2025 Hz

- loss of amplitude will not cause errors in transmission

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Frequency Modulation

Conceptually:

If voice-grade telephone lines can transmit a “maximum” of 3,400 Hz (cycles per second), between 300 Hz and 3,400 Hz,

AND

If one cycle = 1 bit,

Then a maximum of 3,400 bits per second can be transmitted over voice grade telephone lines? (Hold that thought!)

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Phase Modulation

Phase Modulation (PM)

- a modulation technique to vary the phase of the sine wave (or carrier wave) to transmit ones and zeroes, while the amplitude and the frequency remains constant
- sine waves repeat themselves indefinitely
- shifting the wave breaks the wave abruptly and starts it again a few degrees forward or backward
- A different phase shift, 0 to 360 degrees, is used to transmit one or more bits

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Phase Modulation

A different phase shift, 0 to 360 degrees, is used to transmit one or more bits

Full Duplex

- requires a minimum of two frequencies, one frequency for each direction

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Bits per second vs. Baud and High-speed modems

- So far, discussed transmission of one bit at a time, via high or low amplitude, high or low frequency, phase shift or no phase shift
- older modems sent only one bit per signal change, bps = baud
- baud rate = the number of these signal changes per second
- What if we could transmit more than one bit with each signal change (baud), amplitude, frequency of phase shift?
- Remember, voice-grade phone lines limit transmission to 3,400 Hz or 3,400 bps with 1 cycle per bit

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Dibit Modulation

Dibit Modulation

- 2 bits per baud, per cycle
- Two bits or dibit modulation:
00, 01, 10, 11

Using Amplitude Modulation

- use four different amplitudes (wave heights)
Using Frequency Modulation

- use four different frequencies
Using Phase Modulation

- use four different phases

Dibit Amplitude modulation

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Summary of Modulations

- Amplitude Modulation (AM)
- Frequency Modulation (FM)
- Phase Shift Keying (PSK)

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Back to Cisco Curriculum….

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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 frequency.

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Exponents and logarithms (Not testable)

- Numbers with exponents are used to easily represent very large or very small numbers.
- It is much easier and less error-prone to represent one billion numerically as 109 than as 1000000000.
- Many calculations involved in cable testing involve numbers that are very large, so exponents are the preferred format.
- Exponents can be explored in the flash activity.

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Exponents and logarithms (Not testable)

- One way to work with the very large and very small numbers that occur in networking is to transform the numbers according to the rule, or mathematical function, known as the logarithm.
- Logarithms are referenced to the base of the number system being used.
- For example, base 10 logarithms are often abbreviated log.
- To take the “log” of a number use a calculator or the flash activity.
- For example, log (109) equals 9, log (10-3) = -3.

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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)

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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
- log10 implies that the number in parenthesis will be transformed using the base 10 logarithm rule
- Pfinal is the delivered power measured in Watts
- Prefis the original power measured in Watts
- Vfinal is the delivered voltage measured in Volts
- Vreference is the original voltage measured in Volts

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Decibels (Not testable)

- The first formula describes decibels in terms of power (P), and the second in terms of voltage (V).
- Typically, light waves on optical fiber and radio waves in the air are measured using the power formula.
- Electromagnetic waves on copper cables are measured using the voltage formula.

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Decibels (Not testable)

- Enter values for dB and Pref to discover the correct power.
- This formula could be used to see how much power is left in a radio wave after it has traveled over a distance through different materials, and through various stages of electronic systems such as a radio.

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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.

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Viewing signals in time and frequency

- Engineers also use frequency-domain analysis to study signals.
- In frequency-domain analysis, the x-axis represents frequency.
- An electronic device called a spectrum analyzer creates graphs for frequency-domain analysis.

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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.

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

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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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Bandwidth

- Analog bandwidth typically refers to the frequency range of an analog electronic system.
- Analog bandwidth could be used to describe the range of frequencies transmitted by a radio station or an electronic amplifier.
- The units of measurement for analog bandwidth is Hertz, the same as the unit of frequency.
- Example of analog bandwidth values are 3 kHz for telephony

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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 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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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.

Rick Graziani [email protected]

Signaling over copper and fiber optic cabling

- 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.
- Since current Ethernet technology supports data rates of billions of bits per second, each bit must be recognized, even though duration of the bit is very small.
- The voltage level cannot be amplified at the receiver, nor can the bit duration be extended in order to recognize the data.
- This means that as much of the original signal strength must be retained, as the signal moves through the cable and passes through the connectors.
- In anticipation of ever-faster Ethernet protocols, new cable installations should be made with the best available cable, connectors, and interconnect devices such as punch-down blocks and patch panels.

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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.

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Sources of noise on copper media

- 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 radiates outward from the transmitting wire like a radio signal from a transmitter.
- Adjacent wires in the cable act like antennas, receiving the transmitted energy, which interferes with data on those wires.

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Sources of noise on copper media

- Twisted-pair cable is designed to take advantage of the effects of crosstalk in order to minimize 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.
- 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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Types of crosstalk

There are three distinct types of crosstalk:

- 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 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.

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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.
- 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

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Cable testing standards

- 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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Cable testing standards

- 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.

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Cable testing standards

- 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 detect.

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Other test parameters

- Return loss is a measure in decibels of reflections 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 the impedance discontinuities will strike the receiver at different intervals causing signal jitter.

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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.

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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 back in 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 connectors are 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 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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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 crosstalk and return loss.

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Summary

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