James Chen ydjames@ydu.edu.tw Module 4: Cable Testing James Chen ydjames@ydu.edu.tw 2018/11/10 Yuda college of business

Yuda college of business Outline Background for Studying Frequency-Based Cable Testing Waves Sine waves and square waves Exponents and logarithms Decibels Viewing signals in time and frequency Analog and digital signals in time and frequency Noise in time and frequency Bandwidth Signals and Noise Signaling over copper and fiber optic cabling Attenuation and insertion loss on copper media Sources of noise on copper media Types of crosstalk Cable testing standards Other test parameters Time-based parameters Testing optical fiber A new standard 2018/11/10 Yuda college of business

Yuda college of business Background for Studying Frequency-Based Cable Testing 2018/11/10 Yuda college of business

Yuda college of business Waves A wave is energy traveling from one place to another. A bucket of water that is completely still. no waves, no disturbances The ocean always has some sort of detectable waves. wind and tide(潮汐) measured in meters. How frequently the waves reach the shore ? period It is the amount of time between each wave, measured in seconds. frequency It is the number of waves that reach the shore each second, measured in Hertz. One Hertz is equal to one wave per second, or one cycle per second.  Electromagnetic waves voltage waves on copper media light waves in optical fiber Pulse The disturbance is caused in a fixed or predictable duration. 2018/11/10 Yuda college of business

Sine waves and square waves Sine waves, or sinusoids periodical repeat the same pattern at regular intervals continuously varying with time analog waves 2018/11/10 Yuda college of business

Sine waves and square waves (cont.) 2018/11/10 Yuda college of business

Sine waves and square waves (cont.) 2018/11/10 Yuda college of business

Sine waves and square waves (cont.) periodical do not continuously vary with time The wave holds one value for some time, and then suddenly changes to a different value. digital signals, or pulses 2018/11/10 Yuda college of business

Exponents and logarithms In networking, there are three important number systems: Base 2 – binary Base 10 – decimal Base 16 – hexadecimal Power and exponent 10 * 10 = 10^2 (10 raised to the second power, exponent = 2) 10 * 10 * 10 = 10^3 (10 raised to the third power, exponent = 3) Numbers with exponents are used to easily represent very large or very small numbers. Logarithm base 10 logarithms are often abbreviated (縮短的)log log (10^9) = 9 log (10^-3) = -3 2018/11/10 Yuda college of business

Yuda college of business Decibels(分貝) The first formula describes decibels in terms of power (P), and the second in terms of voltage (V). dB measures the loss or gain of the power of a wave. negative values : a loss in power as the wave travels positive values : a gain in power if the signal is amplified 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. 2018/11/10 Yuda college of business

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 represents time. The y-axis represents voltage or current. Time-domain analysis Spectrum analyzer The x-axis represents frequency. 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 are changing the frequency that the radio is receiving 2018/11/10 Yuda college of business

Viewing signals in time and frequency (cont.) 2018/11/10 Yuda college of business

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. 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. How would a spectrum analyzer display this pure tone?  Next, imagine the combination of several sine waves. The resulting wave is more complex than a pure sine wave. Several tones would be heard. How would a spectrum analyzer display this? The graph of several tones shows several individual lines corresponding to the frequency of each tone. Finally, imagine a complex signal, like a voice or a musical instrument. What would its spectrum analyzer graph look like? If many different tones are present, a continuous spectrum of individual tones would be represented. 2018/11/10 Yuda college of business

Analog and digital signals in time and frequency (cont.) 2018/11/10 Yuda college of business

Noise in time and frequency Noise is an important concept in communications systems undesirable signals Noise can originate from natural and technological sources. Noise is added to the data signals in communications systems. There are many possible sources of noise: Nearby cables which carry data signals. Radio frequency interference (RFI) Noise is from other signals being transmitted nearby. Electromagnetic interference (EMI) Noise is from nearby sources such as motors and lights Laser noise at the transmitter or receiver of an optical signal White noise Noise that affects all transmission frequencies equally. Narrowband interference Noise that only affects small ranges of frequencies. 2018/11/10 Yuda college of business

Yuda college of business Bandwidth Analog bandwidth Analog bandwidth could be used to describe the range of frequencies transmitted by a radio station or an electronic amplifier. measurement unit is Hertz Digital bandwidth Digital bandwidth measures how much information can flow from one place to another in a given amount of time. measurement unit is bits per second (bps). During cable testing, analog bandwidth is used to determine the digital bandwidth of a copper cable. Analog + Digital Media that will support higher analog bandwidths without high degrees of attenuation will also support higher digital bandwidths. 2018/11/10 Yuda college of business

Yuda college of business Signals and Noise 2018/11/10 Yuda college of business

Signaling over copper and fiber optic cabling 2018/11/10 Yuda college of business

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. Attenuation is expressed in decibels (dB) using 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 the insulation of the cable impedance of a Cat5 cable is 100 ohms. Impedance discontinuity or an impedance mismatch. a portion of a transmitted signal will be reflected back to the transmitting device, much like an echo. jitter and results in data errors. Insertion loss The combination of the effects of signal attenuation and impedance discontinuities on a communications link. 2018/11/10 Yuda college of business

Sources of noise on copper media 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 of a cable now requires testing for a variety of types of noise. Crosstalk involves the transmission of signals from one wire to a nearby wire. Twisted-pair cable is designed to take advantage of the effects of crosstalk in order to minimize noise. Higher categories of UTP require more twists on each wire pair in the cable to minimize crosstalk at high transmission frequencies. 2018/11/10 Yuda college of business

Yuda college of business Types of crosstalk There are three distinct types of crosstalk: Near-end Crosstalk (NEXT) Far-end Crosstalk (FEXT) Power Sum Near-end Crosstalk (PSNEXT) Near-end crosstalk (NEXT) It 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. Negative value of decibels (dB). Low negative numbers indicate more noise, just as low negative temperatures indicate more heat (closed to zero). 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 better cable than does a NEXT reading of 10 dB. 2018/11/10 Yuda college of business

Types of crosstalk (cont.) 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 (忽略)link faults. 2018/11/10 Yuda college of business

Types of crosstalk (cont.) Far-end crosstalk (FEXT) Due to attenuation, crosstalk occurring further away from the transmitter creates less noise on a cable than NEXT. 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. 2018/11/10 Yuda college of business

Types of crosstalk (cont.) Power Sum NEXT (PSNEXT) It measures the cumulative effect of NEXT from other 3 pairs in the cable. 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. 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. 2018/11/10 Yuda college of business

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. 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 2018/11/10 Yuda college of business

Cable testing standards (cont.) 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 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. 2018/11/10 Yuda college of business

Cable testing standards (cont.) The wiring faults (TIA/EIA-568-B) : 2018/11/10 Yuda college of business

Yuda college of business Other test parameters 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. Equal-level far-end crosstalk (ELFEXT) 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) It is the combined effect of ELFEXT from all wire pairs. Return loss It 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(侵襲)the receiver at different intervals causing signal jitter. 2018/11/10 Yuda college of business

Time-based parameters 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. 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. Time Domain Reflectometry (TDR) test Since the wires inside the cable are twisted, signals actually travel farther than the physical length of the cable. 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 calculate the approximate distance to the wiring fault. 2018/11/10 Yuda college of business

Time-based parameters (cont.) Delay skew 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. All cable links in a LAN must pass all of the tests in the TIA/EIA-568-B standard. These tests ensure that the cable links will function reliably at high speeds and frequencies. High quality cable test instruments should be correctly used to ensure that the tests are accurate. 2018/11/10 Yuda college of business

Yuda college of business Testing optical fiber A fiber link consists of two separate glass fibers. There are no crosstalk problems on fiber optic cable. External electromagnetic interference or noise has no affect on fiber cabling. Optical discontinuity Some of the light signal is reflected back in the opposite direction. 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. Improperly installed connectors are the main cause of light reflection and signal strength loss in optical fiber. The strength of the light signal that arrives at the receiver is important. If attenuation weakens the light signal at the receiver, then data errors will result. Optical link loss budget (預算) The acceptable amount of signal power loss that can occur without dropping below the requirements of the receiver. If the fiber fails the test, The problem usually is one or more improperly attached connectors. 2018/11/10 Yuda college of business

Yuda college of business 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. 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. 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. 2018/11/10 Yuda college of business

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