TABLE OF CONTENTS A typical serial interface system for digital data conversion between DTE1 terminals and DTE2 over a telephone line includes two modems (i.e., modulation- demodulation devices), denoted with DCE1 and DCE2 symbols, as shown in Figure 6.1. The modem function in the serial interface system is to convert digital data into analog signals transmitted over telephone lines, and to convert the received analog signals into digital signals in the data receiver, as shown in Figure 6.21.
It should be mentioned that modems as separate devices connected to a computer (DTE terminal) by the RS-232C interface are now almost completely replaced by modems produced as modem boards (or cards) installed within the computer, or as devices installed on the computer motherboard. Analog telephone networks transmit signals in a frequency range from 300 to 3,400 Hz, and the frequency of signals transmitted by modems must be included in this range. Modems cooperating with telephone networks of greater transmission rate (e.g., digital networks, optical fiber telephone networks) must satisfy different requirements.
The fundamental problem concerning a modem is the choice of the mode of digital data modulation. The signal transmitted over telephone lines is a carrier (wave) in the form of sinusoidal signal:
| (6.5) |  |
Modulation consists of making one of the carrier parameters dependent on the modulation signal, which is in the case of modems a binary...
