Data Communications Networking Devices - 4th Ed.pdf

360 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTFigure 4.7Telephone channel passband creationThe rationale for the telephone channel passband is economics. Frequenciesunder 300 Hz and above 3300 Hz are essentially not required to understand atelephone conversation even though it precludes a soprano from being fullyappreciated at the other end of the telephone connection. By transmitting only 3000Hz instead of the 20 000 Hz that the human ear can hear the bandwidth required foreach call is reduced by a factor of approximately six. This bandwidth reductionenabled telephone companies to more ef®ciently employ frequency divisionmultiplexing, a technique which allows many voice calls to be simultaneouslycarried on a common circuit routed between telephone company of®ces.To construct a telephone channel passband the telephone company uses low andhigh-pass ®lters which are designed to permit either all signals up to a prede®nedfrequency or all signals under a prede®ned frequency to pass through the channel.As a result of the use of ®lters the amplitude-frequency response becomes roundedat the cut-off frequencies at which the ®lters operate and then begin to approachlarge negative values as the ®lters' attenuation becomes more pronounced. Figure4.7 illustrates how the use of ®lters results in the creation of a passband on atelephone channel.Ideally, all frequencies across the passband of a telephone channel shouldundergo the same amount of attenuation as illustrated by the straight line betweenthe cut-off frequencies shown in Figure 4.7. Unfortunately, high frequencies losetheir strength more rapidly than low frequencies, which results in attenuationincreasing as frequencies increase towards the end of the passband. In addition,attenuation increases as the edges of the operating frequencies of bandpass ®lters ona channel are approached. As a result of the two previously mentioned factors theamplitude-frequency response of a telephone channel which indicates theattenuation distortion that signals experience will resemble that illustrated inFigure 4.8.To minimize the effect of attenuation distortion some modems include anattenuation equalizer. This type of equalizer introduces a variable gain at frequencieswithin the passband which compensate for the differences in attenuationbetween high and low frequencies as well as the increased attenuation at the edges