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. 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 induced 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. Twisting one pair of wires in a cable also helps to reduce crosstalk of data or noise signals from an adjacent wire pair. Higher categories of UTP require more twists on each wire pair in the cable to minimize crosstalk at high transmission frequencies. When attaching connectors to the ends of UTP cable, untwisting of wire pairs must be kept to an absolute minimum to ensure reliable LAN communications. Web Links The Basics of Datacom: Copper Media http://www.ecmweb.com/ar/ electric_basics_ datacom_ copper_4
Content 4.2 Signals and Noise 4.2.4 Types of crosstalk There are three distinct types of crosstalk: 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 better cable 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. Web Links Power Sum NEXT (PSNEXT) http://www.cabletesting.com/CableTesting/ Testing/Definitions/ Definitions_ Power+Sum+NEXT.htm
Content 4.2 Signals and Noise 4.2.5 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. 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: 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 orange striped wire is on pin 1 and the orange wire on pin 2 at one end, 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 two wires from different wire pairs are connected to the wrong pins on both ends of the cable. 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. Transposed-pair wiring faults occur when a wire pair is connected to completely different pins at both ends. Contrast this with a reversed-pair, where the same pair of pins is used at both ends. Transposed pairs also occur when two different color codes on punchdown blocks, representing T568-A and T568-B, are used at different locations on the same link. Web Links Cable Testing http://www.cabletesting.com/CableTesting/ default.htm
Content 4.2 Signals and Noise 4.2.6 Other test parameters 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