. The longer the line or the faster the pulse rate, the more difficult it is to interpret the received signal. This signal degradation is the reason for the closely spaced regenerative repeaters used in digital data transmission facilities. When noise and distortion threaten to destroy the integrity of the pulse stream, the pulses are detected and regen- erated. If the regeneration process is repeated properly, the received signal will be an exact replica of the transmitted signal. It is possible to transmit pulses over short distances using privately owned cable or wire pairs. This is baseband transmission and usually requires line THE MEDIUM AND THE MARKET 7 Sampled values Converted in binary code in volts 10110101 11000000 10100110 10000000 00011000 00101010 11011000000 Fig. 1-4. In a digital network analog signals must be converted into digital bits. drivers and receivers on each end of the line. Longer distance com- munication must use the digital transmission facilities of the com- mon carriers. In analog transmission, a continuous range of signal amplitudes or frequencies is sent over the communications line. Linear ampli- fiers maintain signal quality. The voice telephone network supplied by the common carriers uses analog transmission facilities to service most data communications users. To interface the analog voice chan- nels to digital terminals and computers, a modulator-demodulator (modem) is used. In a modem, digital information modulates a car- 8 TELECOMMUNICATIONS rier signal, which passes through the telephone network just as does a voice signal. At the receiving end, the signal is demodulated back into digital form. 1.2.4 Asynchronous and Synchronous Transmission Asynchronous data are typically produced by low-speed terminals with rates of less than 1200 baud. In asynchronous systems (Fig. 1- - 5a), the transmission line is in a mark (binary 1) condition in its idle state. As each character is transmitted, it is preceded by a start bit, or transition from mark to space (binary 0), which indicat