Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Telephony Chapter 3 In the Belly of the Beast When a customer makes a telephone call, a complex sequence of events takes place that ultimately leads to the creation of a temporary end-to- end, service-rich path. Let's follow a typical phone call through the net- work. Cristina in South Burlington is going to call Adam in Shelburne. This is a high-level explanation; we'll add more detail later. When Cristina picks up the telephone in her home, the act of lifting the handset1 closes a circuit, which allows current to flow from the switch -in the local central office that serves her neighborhood-to her tele- phone. The switch electronically attaches an oscillator to the circuit called a dial tone generator, which creates the customary sound that we all listen for when we wish to place a call.² The dial tone serves to notify the caller that the switch is ready to receive the dialed digits. Cristina now dials Adam's telephone number by pressing the appro- priate buttons on the phone. Each button generates a pair of tones (listen carefully-you can hear them) that are slightly dissonant. This is done to prevent the possibility of a human voice randomly generating a fre- quency that could cause a misdial. The tone pairs, shown in Figure 3-10, are carefully selected SO that they cannot be naturally generated by the human voice. This technique is called dual tone, multifrequency (DTMF). It is not, however, the only way. You may have noticed that there is a switch on many phones with two positions labeled "TONE" and "PULSE." When the switch is set to the TONE position, the phone generates DTMF. When it is set on PULSE, it generates the series of clicks that old dial telephones made when they were used to place a call. 3 When the dial was rotated-let's say to the number 3-it caused a switch contact to open and close rapidly three times, sending a series of three electrical pulses (or more correctly, interruptions) to the 1It 