ur- wire lines. Full-duplex operation on two-wire lines requires a differ- ent pair of frequencies for sending and receiving. The maximum number of channels that can be supported on a voice-grade channel using the FDM technique is limited by the data rate of each channel. Bandwidth is proportional to data rate; higher data rates require greater bandwidth, reducing the available band- width for additional channels. The guard bands between channels further reduce the available bandwidth. The FDM technique is, therefore, a practical solution to combining transmissions from low- speed devices such as teleprinters that do not exceed 1200 baud. The FDM technique (Fig. 2-5) offers several advantages. No modem is MODEMS AND MULTIPLEXERS 25 3-k Hz voice grade line Guard bands 2400 Hz < 1800 Hz < 1200 Hz 600 Hz Terminals Terminals 3-k Hz voice grade line Fig. 2-5. With frequency division multiplexing, division depends on desired frequency of transmission, e.g., 75-Baud channel requires less spectrum space than a 50-baud channel. required for the composite link since the multiplexer composite, or aggregate, consists of modulated analog signals, not digital bit streams. Full-duplex operation can be achieved on a two-wire line although only half the bandwidth of a four-wire line is available. Individual channels can be dropped at remote sites to support a sin- gle device. Voice and data can be mixed and the FDM technique is the most economical. Time-division multiplexing (Fig. 2-6) is a digital technique. It in- terleaves bits (bit TDM) or characters (character TDM), one from each attached channel, and transmits them at high-speed down a tele- phone line equipped with high-speed synchronous modems. At the other end, another multiplexer demultiplexes the "bit train," or "character frame," presenting one bit or character to each low-speed channel just as they originated. The statistical-multiplexer, sometimes referred to as a data con- centrator, uses a relatively new technique. Like the TDM it is digital but utilizes microcomp