f the PC not only spurred data communications forward, it added a completely new direction: distributed processing. No longer are we tied to the main frame computer; it has taken on a secondary role in the major scheme of things. Another major impetus in this direction is of course the INTERNET. The IEEE (Ref. 1) defines data communications (data transmission) as "The movement of encoded information by means of communication techniques." The objective of this chapter is to introduce the reader to the technology of the movement of encoded infor- mation. Encoded information includes alphanumeric data that may broadly encompass messages that have direct meaning to the human user. It also includes the movement of strictly binary sequences that have meaning to a machine, but no direct meaning to a human being. Data communications evolved from automatic telegraphy, which was prevalent from the 1920s through the 1960s. We start the chapter with information coding or how we can express our alphabet and numeric symbols electrically without ambiguity. Data network performance is then covered with a review of the familiar BER. We next move on to the organization of data for transmission and introduce protocols including electrical and logical interfaces. Enterprise networks covering LAN and WAN technology, frame relay, and ISDN¹ are treated in Chapters 11, 12, and 13. The asynchronous transfer mode (ATM) is discussed in Chapter 20. The principal objective of this chapter is to stress concepts and to leave specific details to other texts. 10.2 THE BIT - A REVIEW The bit is often called the most elemental unit of information. The IEEE (Ref. 1) defines it as a contraction of binary digit, a unit of information represented by either a 1 or a 0. These ISDN stands for Integrated Services Digital Networks. Fundamentals of Telecommunications, Second Edition, by Roger L. Freeman ISBN 0-471-71045-8 Copyright © 2005 by Roger L. Freeman DATA COMMUNICATIONS are the same bits that were introduced in Section 2.4.3 and later applied in Cha