12.1 INTRODUCTION

The rapidly increasing need for computer communications has been met primarily by higher-speed data transmission over the widespread network of voice-bandwidth channels developed for voice communications. A modem (see Chapter 7) is required to carry digital signals over these analog passband (nominally 300 to 3000 Hz) channels by translating binary data to voice-frequency signals and back (Fig. 12.1). The thrust toward common-carrier digital transmission facilities has also resulted in application of modem technology to line-of-sight (LOS) terrestrial radio and satellite transmission and recently to subscriber loops for integrated services digital networks (ISDN, see Chapter 2).

Figure 12.1: Data transmission system.

Analog channels deliver corrupted and transformed versions of their input waveforms. Corruption of the waveform usually statistical may be additive and/or multiplicative because of possible background thermal noise, impulse noise, and fades. Transformations performed by the channel are frequency translation, nonlinear or harmonic distortion, and time dispersion.

In telephone lines, time dispersion results when the channel frequency response deviates from the ideal of constant amplitude and linear phase (constant delay). Equalization, which dates back to the use of loading coils to improve the characteristics of twisted pair telephone cables for voice transmission, compensates for these nonideal characteristics by filtering.

A synchronous modem transmitter collects an integral number of bits of data at a time and encodes them into symbols for transmission at the signaling rate. In pulse amplitude modulation (PAM), each signal is a...