 it is transmitting a one or a zero. Digital transmission requires the use of modulation schemes, which are sometimes also called line-coding techniques. Modulation schemes convert the digital information onto the transmission medium (see Figure 5.2). Over time, many modulation schemes have been developed, and they vary in the speed at which they operate, the quality of wire they require, their immunity to noise, and their complexity. The variety of modulation schemes means that incompatibilities exist. Modem Modem Digital Analog Digital Figure 5.2 Modems Data Flow Components of Modulation Schemes Modems can vary any of the three main characteristics of analog waveforms- amplitude, frequency, and phase-to encode information (see Figure 5.3): Amplitude modulation-A modem that relies on amplitude modulation might associate ones with a high amplitude and zeros with a low amplitude. A compatible receiving modem can discriminate between the high and low amplitudes and properly interpret them SO that the receiving device can reproduce the message correctly. Frequency modulation-A frequency modulation-based modem alters the frequency value; in Figure 5.3, zero represents a low frequency and one rep- resents a high frequency. A complementary modem decodes the original bit patterns, based on the frequency of the received signal. Phase modulation-Phase modulation refers to the position of the wave- form at a particular instant in time (e.g., a 90-degree phase, a 180-degree phase, a 270-degree phase). A phase modulation-based modem uses the phases to differentiate between ones and zeros, so, for example, zeros can be transmitted beginning at a 90-degree phase, and ones may be transmit- ted beginning at a 270-degree phase. By using the three characteristics of a waveform, a modem can encode multiple bits within a single cycle of the waveform (see Figure 5.4). The more of these variables Amplitude Modulation Frequency Modulation Phase Modulation Figure 5.3 Amplitude, frequency, and phase modulation Chapter 5 Data Communicatio