Data Communications Networking Devices - 4th Ed.pdf

4.2 MODEMS _____________________________________________________________________ 365Table 4.1 previously listed the operating frequencies of the Bell System 103/113type modem. The originate-mode modem is normally connected to a terminaldevice that originates calls, whereas the answer-mode modem is normallyconnected to computer ports that answer calls occurring over the switched telephonenetwork PSTN).In Figure 4.2 the frequency assignments of modems designed to be compatiblewith Bell System 103/113 type modems is graphically illustrated. Note that 1170Hz and 2125 Hz are the channel center frequencies and two independent datachannels are derived by frequency, permitting full-duplex transmission to occurover the two-wire public switched telephone network.Phase modulationPhase modulation is the process of varying the carrier signal with respect to theorigination of its cycle as illustrated in Figure 4.14. Several forms of phasemodulation are used in modems to include single and multiple-bit phase-shiftkeying PSK) and the combination of amplitude and multiple-bit phase-shiftkeying.Figure 4.14 Phase modulation. Phase is the position of the wave form of a signal withrespect to the origination of the carrier cycle. In this illustration, the bottom wave is 180degrees out of phase with a normal sine wave illustrated at the topIn single-bit, phase-shift keying, the transmitter simply shifts the phase of thesignal to represent each bit entering the modem. Thus, a binary one might berepresented by a 90 phase change while a zero bit could be represented by a 270 phase change. Due to the variance of phase between two-phase values to representbinary ones and zeros this technique is known as two-phase modulation.Prior to discussing multiple-bit, phase-shift keying let us examine the differencebetween the data rate and signaling speed. This will enable us to understand therational for the utilization of multiple-bit, phase-shift keying, where two or morebits are grouped together and represented by one phase shift in a signal.