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Measuring Frequency The amplitude of an electrical signal represents height and is measured in volts. The period is the amount of time to complete one cycle, measured in seconds. The frequency is the # of complete cycles per second, measured in Hertz. Exponents & Logarithms In networking, there are three important number systems: Base 2 – binary Base 10 – decimal Base 16 – hexadecimal One way to work with the very large and very small numbers that occur in networking is to use logarithms. While the study of logarithms is beyond the scope of this course, the terminology is used commonly in calculating decibels, a way of measuring signals on copper, optical, and wireless media. Exponents & Logarithms In networking, there are three important number systems: Base 2 – binary Base 10 – decimal Base 16 – hexadecimal One way to work with the very large and very small numbers that occur in networking is to use logarithms. While the study of logarithms is beyond the scope of this course, the terminology is used commonly in calculating decibels, a way of measuring signals on copper, optical, and wireless media. Decibels The decibel (dB) is a measurement unit important in describing networking signals. 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 Pref is the original power measured in Watts Vfinal is the delivered voltage measured in Volts Vreference is the original voltage measured in Volts Noise 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. 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 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 Noise that affects all transmission frequencies equally is called white noise. Noise that only affects small ranges of frequencies is called narrowband interference. Bandwidth Bandwidth is an extremely important concept in communications systems. Two ways of considering bandwidth that are important for the study of LANs are analog bandwidth and digital 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. Its units of measurement is Hertz, 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 bits per second (bps). 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. 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. Signals Over Copper Cabling On copper cable, data signals are represented by voltage levels that represent binary ones and zeros. The voltage levels are measured with respect to a reference level of zero volts at both the transmitter and the receiver. This reference level is called the signal ground. It is important that both transmitting and receiving devices refer to the same zero volt reference point. When they do, they are said to be properly grounded. Signaling Over Fiber Optic Cabling 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 the building. As costs decrease and demand for speed increases, optical fiber may become a more commonly used LAN media. Attenuation & Insertion Loss on Copper Media Attenuation is the decrease in signal amplitude over the length of a link. Attenuation is expressed in decibels (dB) using negative numbers. Smaller negative dB values are an indication of better link performance. Impedance is a measurement of the resistance of the cable to alternating current (AC) and is measured in ohms. The normal, or characteristic, impedance of a Cat5 cable is 100 ohms. If a connector is improperly installed on Cat5, it will have a different impedance value than the cable. This is called an impedance discontinuity or an impedance mismatch. Impedance discontinuities cause attenuation because a portion of a transmitted signal will be reflected back to the transmitting device rather than continuing to the receiver, much like an echo. The combination of the effects of signal attenuation and impedance discontinuities on a communications link is called insertion loss. 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. 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. Twisting one pair of wires in a cable also helps to reduce crosstalk of data or noise signals from adjacent wires. 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) 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. 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. Power Sum Near-end Crosstalk 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. 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: • • • • • • • • • • 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 Ethernet 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 & 2, and it receives signals on pins 3 & 6. Remember… A straight-thru cable has T568B on both ends. A crossover (or cross-connect) cable has T568B on one end and T568A on the other. A console cable had T568B on one end and reverse T568B on the other, which is why it is also called a rollover cable. Wiring Faults 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. One nanosecond is one-billionth (0.000000001) of a second. The TIA/EIA-568-B standard sets a limit for propagation delay for the various categories of UTP. The delay difference between pairs is called the delay skew. 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 tests for certifying Cat 6 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.