pter 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.) 10.3 REMOVING AMBIGUITY: BINARY CONVENTION To remove ambiguity of the various ways we can express a 1 and a 0, CCITT in Rec. V.1 (Ref. 2) states clearly how to represent a 1 and a 0. This is summarized in Table 10.1, with several additions from other sources. Table 10.1 defines the sense of transmission SO that the mark and space, the 1 and 0, respectively, will not be inverted. Inversion can take place by just changing the voltage polarity. We call it reversing the sense. Some data engineers often refer to such a table as a "table of mark-space convention." 10.4 CODING Written information must be coded before it can be transmitted over a data network. One bit carries very little information. There are only those two possibilities: the 1 and the 0. It serves good use for supervisory signaling where a telephone line could only be in one of two states. It is either idle or busy. As a minimum we would like to transmit every letter of the alphabet and the 10 basic decimal digits plus some control characters, such as a space and hard/soft return, and some punctuation. Suppose we join two bits together for transmission. This generates four possible bit sequences ²: Table