ck to the digital bit stream used by the attached data terminal equipment. Digital to analog con- version is necessary to establish communications compatibility with the electrical characteristics of a telephone line, which are suitable to voice communications, but hostile to digital transmission. The term modem is an acronym for modulation/demodulation. Digital signals are converted to analog form through a process called modulation, which uses the digital signal to alter a fixed signal called the carrier. The modulated carrier signal contains the digital data, which is extracted from the carrier by the receiving modem through a process called demodulation. Simple modulation techniques are used for low data rates, but as the data rate is increased, more com- plex methods are required. Frequency shift keying (FSK) is the simplest form of modulation used by low speed modems. This commonly used frequency modu- lation technique transmits digital data via two unique frequencies or tones (Fig. 2-2), one assigned to binary ones (marks), the other as- signed to zeros (spaces). Phase shift keying (PSK) is a phase mod- ulation technique commonly used by medium and high speed New York Denver Digital Analog Digital Modem Modem Terminal Computer Fig. 2-1. Role of the modem. 18 TELECOMMUNICATIONS Modem computer 1 0 1 1 0 1 1 0 Modem computer Modem computer 180° phase shift Fig. 2-2. Modulation techniques. In frequency shift keying (a), digital signal (left) received by modem is shifted in frequency to represent binary 1 and binary 0. In amplitude modulation (b), one carrier signal amplitude (A1) represents binary 1, and another (A2), binary 0. In phase modulation (c), whenever 180° phase shift is encountered, receiving modem assigns value of binary 0 and at all other times, value of binary 1 is assumed. modems. The phase of the modem's carrier signal is shifted by a fixed multiple with respect to a known reference (clock frequency) for assigned digital bit combinations. For example, the Bell System 201 modems use a four-phase