Data Communications Networking Devices - 4th Ed.pdf

362 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTFigure 4.10Typical envelope delay curvesits frequency. The direct measurement of phase delay is not practical due to therequirement to have an absolute phase reference and keep track of phase changesover multiples of 360 . This resulted in the use of the slope of the phase plottedagainst frequency, which is known as envelope delay.Mathematically, envelope delay is the ®rst derivative of phase delay. The shapeof the envelope delay curve obtained by measuring delays at different frequenciesre¯ects the degree of change in the slope of the phase versus frequency curve. Thisdelay change varies based upon the transmission distance. Figure 4.10 illustratestwo typical delay curves for signals transmitted on a telephone channel, with thesteeper curve representing the envelope delay on a longer distance circuit than the¯atter curve.To illustrate the potential effect of envelope delay upon communications, assumea modem transmits one of two tones f 1 ) to represent a binary zero and the secondtone f 2 ) to represent a binary one. This is one of the earliest methods used formodulation, a technique referred to as frequency shift keying or FSK. If theenvelope delay curve is not symmetrical, consider what can happen if tone f 1 istransmitted, followed by tone f 2 . Due to different delays associated with differentfrequencies there now exists the possibility that the delay in f 1 being received couldresult in that tone reaching the receiving modem at the same time that tone f 2 ,which represents a different binary value, is received. This could cause onereceived signal to be superimposed upon the second signal by time, resulting in onetone distorting the other tone.Although all communications circuits will exhibit a degree of delay, it isimportant to ¯atten the delay time across the passband to minimize the potential forone tone of a signal to be superimposed on another tone by time. Some modems aredesigned with delay equalizers which introduce a delay approximately inverseto that exhibited by the telephone channel. Through the use of a delay equalizerthe delay time associated with frequencies within the passband can be maderelatively ¯at as illustrated in Figure 4.11. Doing so reduces the potential of onetone interfering with another, a condition formally referred to as intersymbolinterference.