Data Communications Networking Devices - 4th Ed.pdf

4.2 MODEMS _____________________________________________________________________ 367Table 4.2 Common phase-angle values used in multilevel,phase-level keyingBitstransmittedPossible phase-angle values degrees)dibit00 0 45 9001 90 135 010 180 225 27011 270 315 180tribit000 0 22.5 45001 45 67.5 0010 90 112.5 90011 135 157.5 135100 180 202.5 180101 225 247.5 225110 270 292.5 270111 315 337.5 315Combined modulation techniquesSince the most practical method to overcome the Nyquist limit is obtained byplacing additional bits into each signal change, modem designers have combinedmodulation techniques to obtain very high-speed data transmission over voicegradecircuits. One combined modulation technique commonly used involvesboth amplitude and phase modulation. This technique is known as quadratureamplitude modulation QAM) and results in four bits being placed into each signalchange, with the signal operating at 2400 baud), causing the data rate to become9600 bps.The ®rst implementation of QAM involved a combination of phase andamplitude modulation, in which 12 values of phase and three values of amplitudeare employed to produce 16 possible signal states as illustrated in Figure 4.15. Oneof the earliest modems to use QAM in the United States was the Bell System 209,which modulated a 1650 Hz carrier at a 2400 baud rate to effect data transmission at9600 bps. Today, most 9600 bps modems manufactured adhere to the ITU V.29standard. The V.29 modem uses a carrier of 1700 Hz which is varied in both phaseand amplitude, resulting in 16 combinations of eight phase angles and fouramplitudes. Under the V.29 standard, fallback data rates of 7200 and 4800 bps arespeci®ed.In addition to combining two modulation techniques, QAM also differs from thepreviously discussed modulation methods by its use of two carrier signals. Figure4.16 illustrates a simpli®ed block diagram of a modem's transmitter employingQAM. The encoder operates upon four bits from the serial data stream and causesboth an in-phase IP) cosine carrier and a sine wave that serves as the quadraturecomponent QC) of the signal to be modulated. The IP and QC signals are thensummed and result in the transmitted signal being changed in both amplitude and