Copyright Goodheart-Willcox Co., Inc. 55 Chapter 2 Network Media—Copper-Core Cable determined by the amount of current traveling through the conductor. Frequency also aff ects the strength of the magnetic fi eld. When another conductor is placed in close proximity to a current-carrying conductor, the magnetic fi eld encircles the other conductor and induces current, as demonstrated in Figure 2-7B. Th is electron- ic phenomenon is referred to as magnetic induction, or mutual induction. When a conductor in a cable assembly carries digital pulses, the digital pulses create a mag- netic fi eld pattern similar to the digital pulses. Th e magnetic fi eld induces in a neigh- boring conductor an electrical current resembling the digital pulse pattern of the original conductor, thus creating crosstalk. Analog telephone systems also suff er from crosstalk. In fact, the term crosstalk originated in telephone communication to describe the eff ect of hearing an addi- tional telephone conversation from a neighboring telephone line while talking to someone else. Network conductors, such as twisted pair, are designed to limit the eff ects of crosstalk by reducing the amount of contact between a pair of conductors, as shown in Figure 2-7C. Th is is why twisted pair must be used in place of the older style of tele- phone cable that has no twist. However, one place where crosstalk can be generated on twisted-pair cable is at the ends near the connectors. When cables are made by hand, the technician may leave too much untwisted conductor near the connector, leaving the cable vulnerable to crosstalk. Figure 2-8 shows a twisted-pair cable properly connected to a connector and a twisted-pair cable improperly connected to a connector. Note that the improperly connected twisted-pair cable has too much wire exposed and that the pairs are not twisted. Th e improperly connected cable will produce crosstalk. Twisted-pair cable segments are limited to 100 meters (328 ft.) maximum in length. Longer segment lengths produce crosstalk, as well as a reduction in signal strength. Cable shielding of wire mesh or foil also reduces crosstalk. Th e shielding absorbs and contains the magnetic fi eld generated by a conductor and protects the pairs from electromagnetic interference generated by an outside source. Four types of measurement can be taken on twisted-pair cable to measure the eff ects of crosstalk: near-end crosstalk (NEXT), far-end crosstalk (FEXT), equal level Magnetic field B C A Goodheart-Willcox Publisher Figure 2-7 The occurrence of crosstalk. A—An energized conductor is surrounded by a magnetic field. B—Any conductor placed near the energized conductor and in the magnetic field will have electrical energy transferred to it. C—Twisting the pairs of conductors reduces the amount of electrical energy transferred between the pairs.