l, a technique known as HA, LF HA, HF LA, LF LA, HF Figure 1-22 Di-bit encoding 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 Figure 1-23 Di-bit values Frequency Amplitude Figure 1-24 Quadrature amplitude modulation (QAM) quad-bit encoding. This scheme, sometimes called quadrature amplitude modulation (QAM) (pronounced "Kwam"), permits a single signal to rep- resent four bits, which means that there is a 4:1 ratio between the bit rate and the signaling rate. Thus, it is possible to achieve higher bit rates in the bandwidth-limited telephony local loop by using multibit encoding techniques such as QAM. The first "high bit rate modems" (9,600 bits- per-second) used this technique or a variation of it to overcome the 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 First Things First design limitations of the network. In fact, these multibit schemes are also used by the cable industry to achieve the high bit rates they need to operate their multimedia broadband networks. There is one other limitation that must be mentioned: noise. Look at Figure 1-25. Here we have a typical QAM graph, but now we have added noise in the form of additional points on the graph that have no implied value. When a receiver sees them, however, how does it know which points are noise and which are data? Similarly, the oscilloscope trace shown in Figure 1-26 of a high-speed transmission would be dif- ficult to interpret if there were noise spikes intermingled with the data. There is, therefore, a well-known relationship between the noise level in a circuit and the maximum bit rate that is achievable over that cir- cuit, a relationship that was