Cable Testing - Module 4

                                             Module Overview 

  

  

4.1 Frequency-Based Cable Testing

4.1.1 Waves

4.1.2 Sine waves and square waves

4.1.3 Exponents and logarithms

4.1.4 Decibels

4.1.5 Time and frequency of signals

4.1.6 Analog and digital signals

4.1.7 Noise in time and frequency

4.1.8 Bandwidth

4.2 Signals and Noise

4.2.1 Signals over copper and fiber optic cables

4.2.2 Attenuation and insertion loss on copper media

4.2.3 Sources of noise on copper media

4.2.4 Types of crosstalk

4.2.5 Cable testing standards

4.2.6 Other test parameters

4.2.7 Time-based parameters

4.2.8 Testing optical fiber

4.2.9 A new standard

Module: Summary

Overview

Networking media is the backbone of a network. Networking media is literally and physically the backbone of a network. Inferior quality of network cabling results in network failures and unreliable performance. Copper, optical fiber, and wireless networking media all require testing to ensure that they meet strict specification guidelines. These tests involve certain electrical and mathematical concepts and terms such as signal, wave, frequency, and noise. These terms will help students understand networks, cables, and cable testing.

The first lesson in this module will provide some basic definitions to help students understand the cable testing concepts presented in the second lesson.

The second lesson of this module describes issues related to cable testing for physical layer connectivity in LANs. In order for the LAN to function properly, the physical layer medium should meet the industry standard specifications.

Attenuation, which is signal deterioration, and noise, which is signal interference, can cause problems in networks because the data sent may be interpreted incorrectly or not recognized at all after it has been received. Proper termination of cable connectors and proper cable installation are important. If standards are followed during installations, repairs, and changes, attenuation and noise levels should be minimized.

After a cable has been installed, a cable certification meter can verify that the installation meets TIA/EIA specifications. This module also describes some important tests that are performed.

This module covers some of the objectives for the CCNA 640-801, INTRO 640-821, and ICND 640-811 exams.  

Students who complete this module should be able to perform the following tasks:

• 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 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

4.1  Frequency-Based Cable Testing

4.1.1  Waves

This lesson provides definitions that relate to frequency-based cable testing. This page defines waves.

A wave is energy that travels from one place to another. There are many types of waves, but all can be described with similar vocabulary.

It is helpful to think of waves as disturbances. A bucket of water that is completely still does not have waves since there are no disturbances. Conversely, the ocean always has some sort of detectable waves due to disturbances such as wind and tide.

Ocean waves can be described in terms of their height, or amplitude, which could be measured in meters. They can also be described in terms of how frequently the waves reach the shore, which relates to period and frequency. The period of the waves is the amount of time between each wave, measured in seconds. The frequency is the number of waves that reach the shore each second, measured in hertz (Hz). 1 Hz is equal to 1 wave per second, or 1 cycle per second. To experiment with these concepts, adjust the amplitude and frequency in Figure .

Networking professionals are specifically 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 still represents height, but it is measured in volts (V) instead of meters (m). The period is the amount of time that it takes to complete 1 cycle. This is measured in seconds. The frequency is the number of complete cycles per second. This is measured in Hz.

If a disturbance is deliberately caused, and involves a fixed, predictable duration, it is called a pulse. Pulses are an important part of electrical signals because they are the basis of digital transmission. The pattern of the pulses represents the value of the data being transmitted.

The next page will introduce sine waves and square waves.

4.1  Frequency-Based Cable Testing

4.1.2  Sine waves and square waves

 

This page defines sine waves and square waves.

Sine waves, or sinusoids, are graphs of mathematical functions.  Sine waves are periodic, which means that they repeat the same pattern at regular intervals. Sine waves vary continuously, which means that no adjacent points on the graph have the same value.

Sine waves are graphical representations of many natural occurrences that change regularly over time. Some examples of these occurrences are the distance from the earth to the sun, the distance from the ground while riding a Ferris wheel, and the time of day that the sun rises. Since sine waves vary continuously, they are examples of analog waves.

Square waves, like sine waves, are periodic.  However, square wave graphs do not continuously vary with time. The wave maintains one value and then suddenly changes to a different value. After a short amount of time it 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.

The next page reviews exponents and logarithms.

4.1  Frequency-Based Cable Testing

4.1.3  Exponents and logarithms

 

This page explains exponents and logarithms.

In networking, there are three important number systems:

• Base 2 – binary
• Base 10 – decimal
• Base 16 – hexadecimal

Recall that the base of a number system refers to the number of different symbols that can occupy one position. For example, binary numbers have only two placeholders, which are zero and one. Decimal numbers have ten different placeholders, the numbers 0 to 9. Hexadecimal numbers have 16 different placeholders, the numbers 0 to 9 and the letters A to F.

Remember that 10 x 10 can be written as 102. 102 means ten squared or ten raised to the second power. 10 is the base of the number and 2 is the exponent of the number. 10 x 10 x 10 can be written as 103. 103 means ten cubed or ten raised to the third power. The base is ten and the exponent is three. Use the Interactive Media Activity to calculate exponents. Enter a value for x to calculate y or a value for y to calculate x.

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. This is equal to 2 for binary numbers, 10 for decimal numbers, and 16 for hexadecimal numbers. 

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 cable-testing calculations involve numbers that are very large and require exponents. Use the Interactive Media Activity to learn more about exponents.

One way to work with the very large and very small numbers is to transform the numbers based on the mathematical rule known as a logarithm. Logarithm is abbreviated as "log". Any number may be used as a base for a system of logarithms. However, base 10 has many advantages not obtainable in ordinary calculations with other bases. Base 10 is used almost exclusively for ordinary calculations. Logarithms with 10 as a base are called common logarithms. It is not possible to obtain the logarithm of a negative number.

To take the log of a number use a calculator or the Interactive Media Activity. For example, the log of (109) = 9. It is possible to take the logarithm of numbers that are not powers of ten. It is not possible to determine the logarithm of a negative number. The study of logarithms is beyond the scope of this course. However, the terminology is often used to calculate decibels and measure signal intensity on copper, optical, and wireless media.

The next page will explain how to calculate decibels.

4.1  Frequency-Based Cable Testing

4.1.4  Decibels

 

This page provides an overview of decibels.

The study of logarithms is beyond the scope of this course. However, the terminology is often used to calculate decibels and measure signals on copper, optical, and wireless media. The decibel is related to the exponents and logarithms described in prior sections. There are two formulas that are used to calculate decibels:

dB = 10 log10 (Pfinal / Pref)

dB = 20 log10 (Vfinal / Vref)

In these formulas, dB represents the loss or gain of the power of a wave. Decibels can be negative values which would represent a loss in power as the wave travels or a positive value to represent a gain in power if the signal is amplified.

The log10 variable implies that the number in parentheses will be transformed with the base 10 logarithm rule.

Pfinal is the delivered power measured in watts.

Pref is the original power measured in watts.

Vfinal is the delivered voltage measured in volts.

Vref is the original voltage measured in volts.

The first formula describes decibels in terms of power (P), and the second in terms of voltage (V). The power formula is often used to measure light waves on optical fiber and radio waves in the air. The voltage formula is used to measure electromagnetic waves on copper cables. These formulas have several things in common.

In the formula dB = 10 log10 (Pfinal / Pref), enter values for dB and Pref to discover the delivered power.  This formula could be used to see how much power is left in a radio wave after it travels through different materials and stages of electronic systems such as radios. Try the following examples with the Interactive Media Activities: 

• If the source power of the original laser, or Pref is seven microwatts (1 x 10-6 Watts), and the total loss of a fiber link is 13 dB, how much power is delivered?
• If the total loss of a fiber link is 84 dB and the source power of the original laser, or Pref is 1 milliwatt, how much power is delivered?
• If 2 microvolts, or 2 x 10-6 volts, are measured at the end of a cable and the source voltage was 1 volt, what is the gain or loss in decibels? Is this value positive or negative? Does the value represent a gain or a loss in voltage?

The next page will explain how an oscilloscope is used to analyze and view signals.

  4.1  Frequency-Based Cable Testing

  4.1.5  Time and frequency of signals

 

This page will teach students how to analyze and view signals.

One of the most important facts of the information age is that characters, words, pictures, video, or music can be represented electrically by voltage patterns on wires and in electronic devices. The data represented by these voltage patterns can be converted to light waves or radio waves, and then back to voltage waves. Consider the example of an analog telephone. The sound waves of the caller’s voice enter a microphone in the telephone. The microphone converts the patterns of sound energy into voltage patterns of electrical energy that represent the voice.

If the voltage is graphed over time, the patterns that represent the voice will be displayed.  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.

The analysis of signals with an oscilloscope is called time-domain analysis. The x-axis or domain of the mathematical function represents time. 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.

Electromagnetic signals use different frequencies for transmission so that different signals do not interfere with each other. Frequency modulation (FM) radio signals use frequencies that are different from television or satellite signals. When listeners change the station on a radio, they change the frequency that the radio receives.

The next page examines the variations of network signals.

  4.1  Frequency-Based Cable Testing

  4.1.6  Analog and digital signals

 

This page will explain how analog signals vary with time and with frequency.

First, consider a single-frequency electrical sine wave, whose frequency can be detected by the human ear. If this signal is transmitted to a speaker, a tone can be heard.

Next, imagine the combination of several sine waves.  This will create a wave that is more complex than a pure sine wave. This wave will include several tones. A graph of the tones will show several lines that correspond to the frequency of each tone.

Finally, imagine a complex signal, like a voice or a musical instrument. If many different tones are present, the graph will show a continuous spectrum of individual tones.

The Interactive Media Activity draws sine waves and complex waves based on amplitude, frequency, and the phase.

  4.1  Frequency-Based Cable Testing

  4.1.7  Noise in time and frequency

 

This page will describe the sources and effects of noise.

Noise is an important concept in networks such as LANs.  Noise usually refers to sounds. However, noise related to communications refers to undesirable signals. Noise can originate from natural or 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. There are many possible sources of noise:

• Nearby cables that carry data signals
• RFI from other signals that are transmitted nearby
• EMI from nearby sources such as motors and lights
• Laser noise at the transmitter or receiver of an optical signal

Noise that affects all transmission frequencies equally is called white noise. Noise that only affects small ranges of frequencies is called narrowband interference. White noise on a radio receiver would interfere with all radio stations. Narrowband interference would affect only a few stations whose frequencies are close together. When detected on a LAN, white noise could affect all data transmissions, but narrowband interference might disrupt only certain signals.

The Interactive Media Activity will allow students to generate white noise and narrowband noise.

The next page will describe analog bandwidth and digital bandwidth.

  4.1  Frequency-Based Cable Testing

  4.1.8  Bandwidth 

 

This page will describe bandwidth, which is an extremely important concept in networks.

Two types of bandwidth that are important for the study of LANs are analog and digital.

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 unit of measurement for analog bandwidth is hertz (Hz), the same as the unit of frequency.

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 bps. Since LANs are capable of speeds of thousands or millions of bits per second, measurement is expressed in kbps or Mbps. Physical media, current technologies, and the laws of physics limit bandwidth.

During cable testing, analog bandwidth is used to determine the digital bandwidth of a copper cable. The digital waveforms are made up of many sinewaves (analog waves). 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.

This page concludes this lesson. The next lesson will discuss signals and noise. The first page describes copper and fiber optic cables.

  4.2  Signals and Noise

  4.2.1  Signals over copper and fiber optic cables

This page discusses signals over copper and fiber optic cables.

On copper cable, data signals are represented by voltage levels that represent binary ones and zeros. The voltage levels are measured based on a reference level of 0 volts at both the transmitter and the receiver. This reference level is called the signal ground. It is important for devices that transmit and receive data to have the same 0-volt reference point. When they do, they are said to be properly grounded. 

For a LAN to operate properly, the devices that receive data 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 bps, each bit must be recognized and the duration of each bit is very small. This means that as much of the original signal strength as possible must be retained, as the signal moves through the cable and passes through the connectors. In anticipation of faster Ethernet protocols, new cable installations should be made with the best cable, connectors, and interconnect devices such as punch-down blocks and patch panels.

The two basic types of copper cable are 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.

Coaxial cable is a type of shielded cable.  It consists of a solid copper conductor surrounded by insulating material and a braided conductive shield. In LAN applications, the braided shielding is electrically grounded to protect the inner conductor from external electrical noise. The shield also keeps the transmitted signal confined to the cable, which reduces signal loss. This helps make coaxial cable less noisy than other types of copper cabling, but also makes it more expensive. The need to ground the shielding and the bulky size of coaxial cable make it more difficult to install than other copper cabling.

Two types of twisted-pair cable are shielded twisted-pair (STP) and 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. STP cable is sometimes called screened twisted pair (ScTP) in error. ScTP generally refers to Category 5 or Category 5e twisted pair cabling, while STP refers to an IBM specific cable containing only two pairs of conductors. ScTP cable is more expensive, more difficult to install, and less frequently used than 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.

Fiber-optic cable increases and decreases the intensity of light to represent binary ones and zeros in data transmissions.  The strength of a light signal does not diminish as much as the strength of an electrical signal does over an identical run length. Optical signals are not affected by electrical noise and optical fiber does not need to be grounded unless the jacket contains a metal or a metalized strength member. Therefore, optical fiber is often used between buildings and between floors within a building. As costs decrease and speeds increase, optical fiber may become a more commonly used LAN media.

The next page explains the concept of insertion loss.

  4.2  Signals and Noise

  4.2.2  Attenuation and insertion loss on copper media

This page explains insertion loss caused by signal attenuation and impedance discontinuities.

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. For this reason, attenuation on a cable is measured by a cable tester with the highest frequencies that the cable is rated to support. Attenuation is expressed in dBs with negative numbers. Smaller negative dB values are an indication of better link performance.

There are several 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.

Impedance is a measurement of the resistance of the cable to alternating current (AC) and is measured in ohms. The normal impedance of a Category 5 cable is 100 ohms. If a connector is improperly installed on Category 5, it will have a different impedance value than the cable. This is called an impedance discontinuity or an impedance mismatch.

Impedance discontinuities cause attenuation because a portion of a transmitted signal is reflected back, like an echo, and does not reach the receiver. This effect is compounded if multiple discontinuities cause additional portions of the signal to be reflected back to the transmitter. When the reflected signal strikes the first discontinuity, some of the signal rebounds in the original direction, which creates multiple echo effects. The echoes strike the receiver at different intervals. This makes it difficult for the receiver to detect data values. This is called jitter and results in data errors.

The combination of the effects of signal attenuation and impedance discontinuities on a communications link is called insertion loss. Proper network operation depends on constant characteristic impedance in all cables and connectors, with no impedance discontinuities in the entire cable system.

The next page will discuss sources of noise on copper cable.

  4.2  Signals and Noise

  4.2.3  Sources of noise on copper media

   

This page will describe the sources of noise on copper cables.

Noise is any electrical energy on the transmission cable that makes it difficult for a receiver to interpret the data sent from the transmitter. TIA/EIA-568-B certification now requires cables to be tested for a variety of types 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 radiates outward from the wire like a radio signal from a transmitter. Adjacent wires in the cable act like antennas and receive the transmitted energy, which interferes with data on those wires. Crosstalk can also be caused by signals on separate, nearby cables. When crosstalk is caused by a signal on another cable, it is called alien crosstalk. Crosstalk is more destructive at higher transmission frequencies.

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 on the other wire pairs in the cable.

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. 

Twisted wire pairs in a cable are also more resistant to crosstalk or noise signals from adjacent wire pairs. Higher categories of UTP require more twists on each wire pair in the cable to minimize crosstalk at high transmission frequencies. When connectors are attached to the ends of UTP cable, the wire pairs should be untwisted as little as possible to ensure reliable LAN communications.

The next page will explain the three types of crosstalk.

  4.2  Signals and Noise

  4.2.4  Types of crosstalk

  

This page defines the three types of crosstalk:

• Near-end Crosstalk (NEXT)
• Far-end Crosstalk (FEXT)
• Power Sum Near-end Crosstalk (PSNEXT)

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.  This difference is expressed in a negative value of decibels (dB). Low negative numbers indicate more noise, just as low negative temperatures indicate more heat. By tradition, cable testers do not show the minus sign indicating the negative NEXT values. A NEXT reading of 30 dB (which actually indicates -30 dB) indicates less NEXT noise and a cleaner signal than does a NEXT reading of 10 dB.

NEXT needs to be measured from each pair to each other pair in a UTP link, and from both ends of the link. To shorten test times, some cable test instruments allow the user to test the NEXT performance of a link by using larger frequency step sizes than specified by the TIA/EIA standard. The resulting measurements may not comply with TIA/EIA-568-B, and may overlook link faults. To verify proper link performance, NEXT should be measured from both ends of the link with a high-quality test instrument. This is also a requirement for complete compliance with high-speed cable specifications.

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 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. TIA/EIA-568-B certification now requires this PSNEXT test.

Some Ethernet standards such as 10BASE-T and 100BASE-TX receive data from only one wire pair in each direction. However, for newer technologies such as 1000BASE-T that receive data simultaneously from multiple pairs in the same direction, power sum measurements are very important tests.

The next page will discuss cable testing standards.

  4.2  Signals and Noise

  4.2.5  Cable testing standards

 

This page will describe the TIA/EIA-568-B standard. This 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.

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.  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 detect.  The reversed-pair fault occurs when a wire pair is correctly installed on one connector, but reversed on the other connector. If the white/orange wire is terminated on pin 1 and the orange wire is terminated on pin 2 at one end of a cable, but reversed at the other end, then the cable has a reversed-pair fault. This example is shown in the graphic.

A split-pair wiring fault occurs when one wire from one pair is switched with one wire from a different pair at both ends. Look carefully at the pin numbers in the graphic to detect the wiring fault. A split pair creates two transmit or receive pairs each with two wires that are not twisted together. This mixing hampers the cross-cancellation process and makes the cable more susceptible to crosstalk and interference. Contrast this with a reversed-pair, where the same pair of pins is used at both ends.

Other test parameters will be described on the next page.

  4.2  Signals and Noise

  4.2.6  Other test parameters

 

 

This page will explain how cables are tested for crosstalk and attenuation.

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. The TIA/EIA standard requires that a cable and its connectors pass an insertion loss test before the cable can be used as a communications link in a LAN. 

Crosstalk is measured in four separate tests. A cable tester measures NEXT by applying a test signal to one cable pair and measuring the amplitude of the crosstalk signals received by the other cable pairs. The NEXT value, expressed in decibels, is computed as the difference in amplitude between the test signal and the crosstalk signal measured at the same end of the cable. Remember, because the number of decibels that the tester displays is a negative number, the larger the number, the lower the NEXT on the wire pair. As previously mentioned, the PSNEXT test is actually a calculation based on combined NEXT effects.

The equal-level far-end crosstalk (ELFEXT) test measures FEXT. Pair-to-pair ELFEXT is expressed in dB as the difference between the measured FEXT and the insertion loss of the wire pair whose signal is disturbed by the FEXT. ELFEXT is an important measurement in Ethernet networks using 1000BASE-T technologies. Power sum equal-level far-end crosstalk (PSELFEXT) is the combined effect of ELFEXT from all wire pairs.

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.

The next page will describe the concept of propagation delay.

  4.2  Signals and Noise

  4.2.7  Time-based parameters

 

This page will discuss propegation delay and how it is measured.

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.

Propagation delay measurements are the basis of the cable length measurement. TIA/EIA-568-B.1 specifies that the physical length of the link shall be calculated using the wire pair with the shortest electrical delay. 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. When a cable tester makes a TDR measurement, it sends a pulse signal down a wire pair and measures the amount of time required for the pulse to return on the same wire pair.

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. This can be used to calculate the approximate distance to the wiring fault. The approximate distance can be helpful in locating a faulty connection point along a cable run, such as a wall jack.

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. The delay difference between pairs is called delay skew. Delay skew is a critical parameter for high-speed networks in which data is simultaneously transmitted over multiple wire pairs, such as 1000BASE-T Ethernet. If the delay skew between the pairs is too great, the bits arrive at different times and the data cannot be properly reassembled. Even though a cable link may not be intended for this type of data transmission, testing for delay skew helps ensure that the link will support future upgrades to high-speed networks.

All cable links in a LAN must pass all of the tests previously mentioned as specified in the TIA/EIA-568-B standard in order to be considered standards compliant. A certification meter must be used to ensure that all of the tests are passed in order to be considered standards compliant. These tests ensure that the cable links will function reliably at high speeds and frequencies. Cable tests should be performed when the cable is installed and afterward on a regular basis to ensure that LAN cabling meets industry standards. High quality cable test instruments should be correctly used to ensure that the tests are accurate. Test results should also be carefully documented.

The next page will explain how optical media is tested.

  4.2  Signals and Noise

  4.2.8  Testing optical fiber

 

This page will explain how optical fiber is tested.

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. 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.

Fiber links are subject to the optical equivalent of UTP impedance discontinuities.  When light encounters an optical discontinuity, like an impurity in the glass or a micro-fracture, some of the light signal is reflected back in the opposite direction. This means only a fraction of the original light signal will continue 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.

Because noise is not an issue when transmitting on optical fiber, 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.

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. A fiber test instrument, known as a light source and power meter, checks whether the optical link loss budget has been exceeded.  If the fiber fails the test, another cable test instrument can be used to indicate where the optical discontinuities occur along the length of the cable link. An optical TDR known as an OTDR is capable of locating these discontinuities. Usually, the problem is one or more improperly attached connectors. The OTDR will indicate the location of the faulty connections that must be replaced. When the faults are corrected, the cable must be retested.

The standards for testing are updated regularly. The next page will introduce a new standard.

  4.2  Signals and Noise

  4.2.9  A new standard

 

This page discusses the new test standards for Category 6 cable.

On June 20, 2002, the Category 6 addition to the TIA-568 standard was published. The official title of the standard is ANSI/TIA/EIA-568-B.2-1. This new standard specifies the original set of performance parameters that need to be tested for Ethernet cabling as well as the passing scores for each of these tests. Cables certified as Category 6 cable must pass all ten tests.

Although the Category 6 tests are essentially the same as those specified by the Category 5 standard, Category 6 cable must pass the tests with higher scores to be certified. Category 6 cable must be capable of carrying frequencies up to 250 MHz and must have lower levels of crosstalk and return loss.

A quality cable tester similar to the Fluke DSP-4000 series or Fluke OMNIScanner2 can perform all the test measurements required for Category 5, Category 5e, and Category 6 cable certifications of both permanent links and channel links. Figure  shows the Fluke DSP-4100 Cable Analyzer with a DSP-LIA013 Channel/Traffic Adapter for Category 5e.

The Lab Activities will teach students how to use a cable tester.

This page concludes this lesson. The next page will summarize the main points from the module.

Summary

This page summarizes the topics discussed in this module.

Data symbolizing characters, words, pictures, video, or music can be represented electrically by voltage patterns on wires and in electronic devices. The data represented by these voltage patterns can be converted to light waves or radio waves, and then back to voltage patterns. Waves are energy traveling from one place to another, and are created by disturbances. All waves have similar attributes such as amplitude, period, and frequency. Sine waves are periodic, continuously varying functions. Analog signals look like sine waves. Square waves are periodic functions whose values remain constant for a period of time and then change abruptly. Digital signals look like square waves.

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.

Decibels are measurements of a gain or loss in the power of a signal. Negative values represent losses and positive values represent gains. Time and frequency analysis can both be used to graph the voltage or power of a signal.

Undesirable signals in a communications system are called noise. Noise originates from other cables, radio frequency interference (RFI), and electromagnetic interference (EMI). Noise may affect all signal frequencies or a subset of frequencies.

Analog bandwidth is the frequency range that is associated with certain analog transmission, such as television or FM radio. Digital bandwidth measures how much information can flow from one place to another in a given amount of time. Its units are in various multiples of bits per second.

On copper cable, data signals are represented by voltage levels that correspond to binary ones and zeros. In order for the LAN to operate properly, the receiving device must be able to accurately interpret the bit signal. Proper cable installation according to standards increases LAN reliability and performance.

Signal degradation is due to various factors such as attenuation, impedance mismatch, noise, and several types of crosstalk. Attenuation is the decrease in signal amplitude over the length of a link. Impedance is a measurement of resistance to the electrical signal. Cables and the connectors used on them must have similar impedance values or some of the data signal may be reflected back from a connector. This is referred to as impedance mismatch or impedance discontinuity. 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. There are three distinct types of crosstalk: Near-end Crosstalk (NEXT), Far-end Crosstalk (FEXT), Power Sum Near-end Crosstalk (PSNEXT).

STP and UTP cable are designed to take advantage of the effects of crosstalk in order to minimize noise. Additionally, STP contains an outer conductive shield and inner foil shields that make it less susceptible to noise. 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.

Fiber-optic cable is used to transmit data signals by increasing and decreasing the intensity of light to represent binary ones and zeros. The strength of a light signal does not diminish like the strength of an electrical signal does over an identical run length. 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 a building.

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. Optical fiber must also be tested according to networking standards. Category 6 cable must meet more rigorous frequency testing standards than Category 5 cable.