Computer Busses

15

Modems

15.1 Introduction

Modems (MOdulator/DEModulator) connect digital equipment to a telephone line. It connects digital equipment to a speech bandwidth-limited communications channel. Typically, modems are used on telephone lines, which have a bandwidth of between 400 Hz and 3.4 kHz. If digital pulses were applied directly to these lines, they would end up severely distorted.

Modem speeds range from 300 bps to 56 kbps. A modem normally transmits about 10 bits per character (each character has 8 bits). Thus, the maximum rate of characters for a high-speed modem is 2880 characters per second. This chapter contains approximately 15 000 characters and thus to transmit the text in this chapter would take approximately 5 seconds. Text, itself, is ...