Copyright Goodheart-Willcox Co., Inc. 49 Chapter 2 Network Media—Copper-Core Cable Attenuation All electronic signals degenerate, or lose amplitude, over long distances. Losing sig- nal strength is referred to as attenuation. As an electronic signal travels across the media, the amplitude of the signal is lessened. Th e amplitude can be augmented to its original level with an amplifi er. An amplifi er is an electronic device designed to raise a signal’s amplitude. Interference In addition to attenuation, most electronic signals pick up interference from sources such as motors, fl uorescent lights, transformers, radio transmitters, and other con- ductors carrying electronic signals. Interference is an undesired electromagnetic signal imposed on a desired signal that distorts or corrupts the desired signal. Elec- tromagnetic interference is generally referred to as noise. Interference that comes from neighboring conductors inside a wire’s insulating jacket is called crosstalk. Crosstalk is covered later in detail. Noise and crosstalk are unwanted signals that need to be removed from the desired signal, especially before it is amplifi ed. When an analog or digital signal is amplifi ed, the unwanted signals caused by interference are also amplifi ed. To get rid of the unwanted signals, the signal must be fi ltered. Filtering electronic signals is the removal of unwanted signals. Amplifi ers often incorporate fi lters. Th e fi lter removes the unwanted signals before amplifi cation so when the signal is amplifi ed, it is an exact duplicate of the original signal. Networks use repeaters to amplify and fi lter signals. A repeater is not actually an amplifi er. However, in a networking environment, a repeater is often re- ferred to as an amplifi er because it counters attenuation by reshaping the signal to its original form. While both analog and digital signals can be used to express data in network communication, the digital signal is preferred. It is very diffi cult, and sometimes im- possible, to fi lter all of the unwanted signals from an analog signal without changing the analog signal. Amplifying an analog signal not only amplifi es the original signal, but also the unwanted signals, which results in a signal that no longer represents the original. However, when a digital signal is amplifi ed, the unwanted signals are more easily fi ltered to restore the original look of the original signal. Digital signals have a square or rectangular waveform and are produced at a fi xed frequency. When digital signals are regenerated, they are regenerated by digital devices that also produce a square or rectangular waveform. A typical signal from a radio station is an analog signal composed of various amplitudes and a mixture of frequencies inside a set range of frequencies. Because the radio signal comprises var- ious frequencies, the interference is sometimes impossible to separate or fi lter. Th is is a major drawback of using analog signals. Since interference can be easily removed from digital signals, the digital signal has become the norm for transmitting data over network cables. Look at Figure 2-2. At the top is the original signal in digital and analog form. Th ese signals represent transmitted data. Below each signal is a representation of the same signal after attenuation and added interference. Th e bottom drawings are a representation of the distorted signals after amplifi cation and fi ltering. Notice the analog signal is distorted after amplifi cation and is not an exact match of the original signal. Th e digital signal, however, looks exactly like the original signal after amplifi - cation. Th is is because the repeater is only capable of reproducing the original signal as 5-volt and 0-volt levels. Interference consists of many diff erent levels of analog 5.3 NET 5.3 NET