aw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. First Things First First Things First technique in which a zero is represented by the presence of positive volt- age, while a one is represented as zero voltage. This is called a unipolar signaling scheme. Figure 1-20 shows a different technique, in which a zero is represented as positive voltage, while a one is represented as neg- ative voltage. This is called a nonreturn to zero (NRZI) signaling scheme, because zero voltage has no meaning in this technique. Finally, Figure 1-21 demonstrates a bipolar signaling system. In this technique, the presence of voltage represents a one, but notice that every other one is opposite in polarity from the one that precedes it and the one that follows it. Zeroes, meanwhile, are represented as zero voltage. This technique, called alternate mark inversion (AMI), is commonly used in T- and E- Carrier systems for reasons that will be discussed later. There are other techniques in use, but these are the most common. Figure 1-19 Unipolar signaling scheme Figure 1-20 NRZI signaling scheme Figure 1-21 Bipolar signaling scheme Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. First Things First Chapter 1 Clearly, both analog and digital signaling schemes can be used to rep- resent digital data depending upon the nature of the underlying trans- mission system. It is important to keep the difference between data and signaling clearly separate. Data is the information that is being trans- ported, and it can be either analog or digital in nature. For example, music is a purely analog signal because its values constantly vary over time. It can be represented, however, through both analog and digital sig- naling techniques. The zeroes and ones that spew forth from a computer are clearly digital information, but they, t