coded by using a single-carrier technique, and bitstreams from the subbands are bonded together at the receiver. Therefore, no bits need to be placed on portions of the frequency band that may be subject to noise and might result in distortion. Multicarrier techniques became popular as a result of developments in digital signal processing (DSP). Table 5.2 lists some of the most commonly used modulation schemes, and the fol- lowing sections describe them in more detail. Table 5.2 Single-Carrier and Multicarrier Modulation Schemes Scheme Description Single-Carrier Used with ISDN, IDSL, and HDSL. 64-QAM Used with North American and European digital cable for forward (i.e., downstream) channels. 256-QAM Used with North American digital cable for forward (i.e., downstream) channels. 16-QAM Used with North America digital cable for reverse (i.e., upstream) channels. Used in North American digital cable for reverse (i.e., upstream) chan- nels, as well as in direct broadcast satellite. Used in older ADSL deployments. Multicarrier Used in European digital over-the-air broadcast, including 802.11a, 802.11g, 802.11n, 802.16x, 802.20x, and Super 3G; it is the basis of 4G and 5G visions. Used with xDSL and is a preferred technique because it provides good quality. Chapter 5 Data Communications Basics Single-Carrier Modulation Schemes There are a number of single-carrier schemes: 2 Binary 1 Quaternary (2B1Q)-2B1Q is used in ISDN, HDSL, and IDSL. 2B1Q uses four levels of amplitude (voltage) to encode 2 bits per Hertz (bits/Hz). It is well understood, relatively inexpensive, and robust in the face of telephone plant interference. Quadrature Amplitude Modulation (QAM)-QAM modulates both the amplitude and phase, yielding a higher spectral efficiency than 2B1Q and thus providing more bits per second using the same channel. The number of levels of amplitude and the number of phase angles are a function of line quality. Cleaner lines translate into more spectral efficiency or more bits per Hertz. Various levels of QAM exist, referred