acous- tic coupler is a device with two built-in rubber cups into which a standard telephone handset fits. After dialing the telephone num- ber of the distant end device, the communications link is made by placing the telephone handset into the coupler to form an acous- tic connection, rather than an electrical one, between the modem and the telephone line. This device was developed in the "pre- Carterfone Decision" days when the Bell System was concerned about any foreign devices being hardwired into the telephone lines. Acoustic couplers are asynchronous modems. Let's look at how a dial-up modem works. The basic com- ponents of a modem are a transmitter, a receiver and a power supply. The transmitter includes circuitry for, among other things, modulation. The telephone line carries a signal in the form of a sine wave. The three parts of the sine wave which can be manipulated or modulated by the modem to represent data are frequency, ampli- tude and phase. Modulation techniques have been developed around each of these components. The higher the speed of the modem, the more complex the scheme needed to impress infor- mation on the sine wave. The more complex circuitry leads to greater cost. Here are the modulation techniques: Frequency Modulation is used for low-speed asynchronous transmission. The number of waves per unit of space on the 238 The Telecom Handbook CHAPTER 11 circuit is varied while the height or amplitude of the waves is kept constant. The one bit is represented by two waves and the zero bit is represented by four waves. (Remember, the computer can only understand ones and zeroes. The bit is the one or the zero.) Amplitude Modulation varies the height or amplitude of the waves while keeping the frequency constant. The waves rep- resenting the one bit are taller than the waves representing the zero bit. This technique is often used to transmit data be- tween 300 bps and 1200 bps. Amplitude modulation is seldom used in telecommunications. AM radio stations use ampli- tude modulation. In Phase Modula