 PSK technique to achieve a data rate of 2400 bps. The four combinations of bit pairs (dibits) are encoded as 90-degree phase shifts. Similarly, three bits (tribits) can be transmitted with each 45-de- gree phase shift using an eight-phase PSK technique, to achieve a data rate of 4800 bps. Higher data rates require even more sophis- ticated and complex modulation techniques to compensate for the limitation imposed by voiceband facilities. Quadrature amplitude modulation (QAM) is commonly used to achieve data rates of 9600 bps or higher within a 3 kHz bandwidth. QAM modulation combines MODEMS AND MULTIPLEXERS 19 amplitude and phase modulation to encode multiple data bits with each signal change. Multiple bit encoding techniques yield a higher data rate in bits per second (bps) than the actual signaling rate in bauds. Baud is an old telephone term defined as changes per second. Baud rate is equiv- alent to bits per second only when a signal change represents a single bit; baud rate is not equivalent to bits per second with multiple bit encoding, where each signal change represents multiple bits. 2.3 MODEM APPLICATIONS Modem vendors produce modems for various applications. Each is designed to meet specific operating criteria. Application categories include dedicated point-to-point or multipoint communication net- works, the public switched telephone network (DDD), and private facilities. Modems designed for dedicated facilities fall into two cat- egories: voiceband (or voice-grade) and wideband. Voiceband mo- dems communicate at rates up to 19.2 k bps with a 3 kHz bandwidth, such as type 3002 service provided by the telephone company. Wide- band modems communicate at rates of 19.2 k bps and above and require broadband facilities such as the Bell System series 8000 wide- band service or equivalent. Modems designed for multipoint communications reduce the training time required for modem synchronization as compared with point-to-point modems. Fast training time significantly reduces the time delays, which can be subst