erized by a frequency, described in Section 2.3.3. This single radio fre- quency carries no useful information for the user. Useful information may include voice, 2.4 ELECTRICAL SIGNALS data, or image (typically facsimile or television). Modulation is the process of impinging that useful information on the carrier, and demodulation is the recovery of that information from the carrier at the distant end near the destination user. The IEEE defines modulation as "a process whereby certain characteristics of a wave, often called the carrier, are varied or selected in accordance with a modulating function." The modulating function is the information baseband described above. There are three generic forms of modulation: 1. Amplitude modulation (AM) 2. Frequency modulation (FM) 3. Phase modulation (PM). Item 1 (amplitude modulation) is where a carrier is varied in amplitude in accordance with information baseband signal. In the case of item 2 (frequency modulation), a carrier is varied in frequency in accordance with the baseband signal. For item 3 (phase modulation) a carrier is varied in its phase in accordance with the information baseband signal. Figure 2.9 graphically illustrates amplitude, frequency, and phase modulation. The modulating signal is a baseband stream of bits: 1s and Os. We deal with digital transmission (e.g., 1s and Os) extensively in Chapters 6 and 10. Prior to 1960, all transmission systems were analog. Today, in the PSTN, all telecom- munication systems are digital, except for the preponderance of subscriber access lines. These are the subscriber loops described in Chapter 1. Let us now distinguish and define analog and digital transmission 2.4.2.1 Analog Transmission. Analog transmission implies continuity as contrasted with digital transmission that is concerned with discrete states. Many signals can be used in either the analog or digital sense, the means of carrying the information being the distinguishing feature. The information content of an analog signal is conveyed by the value or magnit