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
1ISDN 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 are the same bits that were introduced in Section 2.4.3 and later applied in Chapter 6, and to a lesser extent in Chapter 7. In Chapter 6, Digital Networks, the primary purpose of those bits was to signal the distant end value of the voltage level of an analog channel at some moment in time. Here we will be assembling bit groupings that will represent letters of the alphabet, numerical digits 0 through 9, punctuation, graphic symbols, or just operational bit sequences that are necessary to make the data network operate with little or no ostensible outward meaning to us.
From old-time telegraphy the terminology has migrated to data communications. A mark is a binary 1 and a space is a binary 0. A space or 0 is represented by a positive-going voltage, and a mark or 1 is represented by a negative-going voltage. (Now I am getting confused. When I was growing up in the industry, a 1 or mark was a positive-going voltage, and so forth.)
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