 line is broadcasting data to a selected group of terminals simultaneously. You may also improve the cost-effectiveness of your data com- munications system through alterations in the connecting links and by examining a variety of other services available from the telephone company, specialized carriers, and private companies. MODEMS AND MULTIPLEXERS 2.1 INTRODUCTION Modems and multiplexers are essential components of data com- munications networks. Both are needed to support communications between a central site processor and remote terminals or network nodes. Modems are necessary to support digital communications over long distances. They convert digital signals from data terminal equipment to an analog form for transmission over analog com- munications facilities, and reconvert the analog signal to a digital form at the other end. Multiplexers combine transmissions from sev- eral sources on one line, eliminating the cost of several lines and modems between two points to support independent transmissions. Modems and multiplexers are available from a multitude of vendors. The products offered by these vendors differ in application, perfor- mance, and price. Selecting the appropriate modems and multiplex- ers to satisfy user requirements is often a compromising task requir- ing considerable time and effort. Network managers and planners need a sound understanding of data communications technology and an awareness of the available products and their specifica- tions to design a cost-effective network using modems and multi- plexers that meet current and future needs. This chapter provides a basic understanding of modem and multiplexer technology, explains essential operating parameters, and describes general application areas. MODEMS AND MULTIPLEXERS 17 2.2 THE ROLE OF MODEMS A modem converts a digital bit stream (Fig. 2-1) from a computer or terminal to an analog signal which it transmits over a commu- nications medium, typically a telephone line. A modem at the other end of the line converts the analog signal ba