Copyright Goodheart-Willcox Co., Inc. 54 Networking Fundamentals form of electrical energy. DC means that the current is in one steady direction. One of the most common sources of direct current is a battery. Electrical energy from a battery fl ows in a steady direction from the negative (–) terminal to the positive (+) terminal. AC does not fl ow in one direction. It alternates from a negative charge to a positive charge. Th e most common source of alternating current is the wall outlet. Since the two forms of electrical energy fl ow diff erently, they produce diff erent electrical characteristics. Because the two systems have diff erent characteristics, dif- ferent terminology is used to describe their characteristics. Two terms applied to these characteristics are resistance and impedance. Th ese terms are inherited from the electronics industry and are applied to networking cable. Resistance and Impedance Resistance is the opposition to direct current (DC). Every conductor has a certain amount of resistance that aff ects current—the longer the conductor, the greater the resistance. Resistance is a DC term and should not be confused with impedance, which refers to the opposition in alternating current (AC). Impedance increases as frequency increases. In other words, the higher the frequency, the faster the speed of data transmission. However, the quality or integrity of the data will diminish because of impedance. Th is is why there are limitations to the speed at which data can be de- livered across a conductor. Th e length of a conductor also infl uences impedance. As the length of a conductor increases, so does the total impedance. Digital signals change current direction just as analog signals do. Digital signals consist of a series of square or rectangular waveforms, which sharply change current direction after a set time period. For this reason, digital signals are aff ected by im- pedance the same way analog signals are aff ected by impedance. See Figure 2-6 for a comparative summary of resistance and impedance. Electrical Term Defi nition Symbol Comments Resistance Opposition to direct current Ω (ohm) Current flow affected by cable length. Resistance is present in battery-powered circuits. Impedance Opposition to alternating current Z Current flow is affected by frequency, cable length, induction, and capacitance. Impedance is present in AC-powered circuits. Figure 2-6 Comparison of resistance and impedance. Goodheart-Willcox Publisher Refl ected Loss In high-speed networks, data is transmitted in full-duplex mode. Th is means that data can fl ow in both directions at the same time. Th e original signal enters the ca- ble and travels to the end. When it reaches the end, part of the signal is refl ected. Refl ected loss is the amount of signal refl ected from the end of the cable. If it is of suffi cient value, the refl ected signal can disrupt communication. Crosstalk Crosstalk is a type of interference that occurs when one pair of conductors imposes a signal on another pair of parallel conductors, as shown in Figure 2-7. All energized conductors are surrounded by a magnetic fi eld, as shown in Figure 2-7A. Th e mag- netic fi eld runs the length of the conductor. Th e strength of the magnetic fi eld is 5.3 NET