ns Basics Modem Each bit results in a change in wave frequency Each bit results in a change in wave amplitude Each bit results in a change in phase Figure 5.4 Signal modulation the modem can detect, the greater the bit rate it can produce. Clever modulation schemes that vary phase, amplitude, and frequency at the same time are possible today, and they can offer higher bit rates because multiple bits can be encoded at the same instant. Modulation schemes vary in their spectral efficiency, which is a measure of the number of digital bits that can be encoded in a single cycle of a waveform. A bit is a unit of information, while a baud is a unit of signaling speed-the number of times a signal on a communications circuit changes. The ITU-T now recommends that the term baud rate be replaced by the term symbol rate. Each signal, or symbol, can contain multiple bits, based on the modulation scheme and how many variables can be encoded onto one waveform. In the simplest of examples, where there is one bit per baud, the bit rate equals the baud or symbol rate. However, with con- temporary techniques, which enable multiple bits per symbol (such as 64-QAM, which encodes 8 bits per symbol), the bps rate is much higher than the baud or symbol rate. To get more bits per Hertz, many modulation techniques provide more voltage levels. To encode k bits in the same symbol time, 2k voltage levels required. It becomes more difficult for the receiver to discriminate among many voltage levels with consistent precision as the speed increases. So it becomes a challenge to discriminate at a very high data rate. (Chapter 13, "Wireless Commu- nications Basics," talks more about spectrum reuse.) Data Flow Categories of Modulation Schemes There are two main categories of modulation schemes: Single-carrier-In the single-carrier modulation scheme, a single channel occupies the entire bandwidth. Multicarrier-The multicarrier modulation scheme uses and aggregates a certain amount of bandwidth and then divides it into subbands. Each sub- band is en