Exercise 1-4. Pulse Dialing

Transcription

1 Exercise 1-4 Pulse Dialing When you have completed this exercise, you will be able to demonstrate pulse dialing, an older signaling technique to transmit telephone numbers to central offices using a series of current pulses. Tone dialing is the newer faster technique that allows telephone users to transmit telephone numbers to central offices. Pulse dialing is the older slower technique used for transferring telephone numbers to central offices. Although tone dialing is now used almost universally, pulse dialing is still found in some areas where local offices with older switching equipment are in operation. Also, most modern central offices can accept pulse dialing signals. Pulse dialing originates from older analog telephone sets provided with a rotary dial. Figure 1-14 shows a simplified diagram of a rotary-dial telephone set (with handset off-hook) connected to a central office. On rotary-dial telephone sets, a springloaded dial is rotated and released for each digit of the telephone number. As the dial returns to the rest position, a switch in the dial mechanism briefly interrupts the DC current flowing through the telephone line (loop current) several successive ROTARY-DIAL TELEPHONE SET TRANSMITTER ROTARY DIAL HANDSET CRADLE TIP (T) LOOP CURRENT CENTRAL OFFICE RECEIVER HYBRID CIRCUIT TELEPHONE LINE (LOCAL LOOP) LINE INTERFACE 48 V SWITCHHOOK (OFF-HOOK) (R) RING BATTERY FEED CIRCUIT

2 times to transmit the dialed digit to the central office as a series of short current pulses. The number of momentary loop current interruptions that are produced depends on the digit dialed: one interruption for the digit 1, two interruptions for the digit 2, and so on up to the digit 0 which results in 10 interruptions. In the central office, each series of loop current interruptions is detected and translated into the corresponding digit to recover the dialed telephone number and perform the connection requested. Note that the star (a) and octothorpe (#) symbols do not exist in the pulse dialing system. Figure 1-15 shows an example of the loop current when the digits 1, 2, and 4 are dialed successively, as well as the corresponding voltage across the tip (T) and ring (R) connections of the telephone line. The loop current interruptions that occur when the rotary dial returns to the rest position produce current pulses at a rate of 10 pulses per second. This sets the current pulse period (dial pulse period) to 100 ms (see A in Figure 1-15). In North America, the loop current is interrupted during an interval of 60 ms every dial pulse period, resulting in a current interruption ratio (break ratio) of 60% and 40-ms current pulses (see B and C in Figure 1-15). In other countries, the break ratio is usually around 67% (67-ms loop current interruption), resulting in current pulses having a duration of about 33 ms. An interdigit interval, during which the loop current is not interrupted (see D in Figure 1-15), is provided to allow the central office to distinguish the series of current pulses associated with each digit dialed. The duration of the inter-digit interval is always greater than 300 ms. In North America, the nominal duration of the inter-digit interval is about 700 ms. DIALED NUMBER "1" "2" "4" A LOOP CURRENT 0 B C D TIME 0 VOLTAGE ACROSS THE TELEPHONE LINE -48 V TIME A: DIAL PULSE PERIOD (100 ms) B: LOOP CURRENT INTERRUPTION (NORTH AMERICA: 60 ms, OTHER COUNTRIES: 67 ms) C: CURRENT PULSE DURATION (NORTH AMERICA: 40 ms, OTHER COUNTRIES: 33 ms) D: INTER-DIGIT INTERVAL (>300 ms, NORTH AMERICA: 700 ms)

3 The loop current interruptions produced by pulse dialing have a direct effect on the voltage across the telephone line. This voltage is equal to that provided by the battery feed circuit (48 V in many countries) whenever the loop current is interrupted. It has a lower value when the loop current flows because of currentlimiting resistors in the line interface circuit. Many electronic-type analog telephone sets not only include tone dialing, but pulse dialing too. This is even an essential feature when such telephones are connected to central offices where older switching equipment, not compatible with tone dialing, is used. On electronic-type analog telephone sets, the keypad is connected to a pulse generator when the pulse dialing mode is selected, as shown in Figure Whenever a number key is depressed, the pulse generator produces the corresponding number of pulses (at a rate of 10 pulses per second). These pulses control an electronic switch that momentarily interrupts the loop current in a way similar to the switch in the dial mechanism of a rotary-dial telephone set. Note that no dialing pulses are generated when either the star (a) key or the cross-hatch (#) key is depressed. CENTRAL OFFICE ELECTRONIC-TYPE ANALOG TELEPHONE SET (USED IN PULSE DIALING MODE) HANDSET CRADLE AND PLUNGER BATTERY FEED POWER CIRCUIT ELECTRONIC SWITCH TIP (T) TELEPHONE LINE (LOCAL LOOP) KEYPAD PULSE GENERATOR DC LOOP CURRENT LINE INTERFACE SIGNALING CIRCUIT SWITCHHOOK (OFF-HOOK) (R) RING CALL PROCESSOR

4 In the first part of the exercise, you will set up a central office with the Telephony Training System (TTS). In the second part of the exercise, you will dial different digits and observe the waveforms of the pulse dialing signals that are output to the telephone line. This will allow you to determine how pulse dialing operates. You will also observe the waveform of the pulse dialing signal that is produced when a series of numbers is dialed. This will allow you to measure various parameters such as the dial pulse period, the duration of the current interruptions, and the inter-digit interval. Refer to Appendix A of this manual to obtain the list of equipment required to perform this exercise. * 1. Make sure that the Reconfigurable Training Module, Model 9431, is connected to the TTS Power Supply, Model Make sure that there is a network connection between the Reconfigurable Training Module and the host computer. Install the Dual Analog Line Interface, Model 9475, into one of the analog/digital (A/D) slots of the Reconfigurable Training Module. Connect two analog telephone sets to the Dual Analog Line Interface. Make sure that the pulse dialing mode is selected on the analog telephone sets. Connect the AC/DC power converter supplied with each analog telephone set to one of the AC power outlets on the TTS Power Supply. Connect the DC power output jack of each AC/DC power converter to the DC power input connector on either one of the analog telephone sets. The analog telephone set requires an auxiliary DC power source for the digital display to be operative.

5 * 2. Turn on the host computer. Turn on the TTS Power Supply then the Reconfigurable Training Module. * 3. On the host computer, start the Telephony Training System software, then download the CO program to the Reconfigurable Training Module. The CO program configures the Reconfigurable Training Module so that it operates as a central office. If the host computer is unable to download the CO program to the Reconfigurable Training Module, it may not be using the proper IP address. Have your instructor or the LAN administrator check if the host computer uses the proper IP address to communicate with the Reconfigurable Training Module. * 4. On the host computer, go to the Call Processor and disable the telephone number detection to make the central office insensitive to dialing signals. This will allow you to dial as many digits as you wish and observe the resulting pulse dialing signals, with no response from the central office that can disturb your observations. * 5. On the host computer, zoom in on ANALOG LINE INTERFACE A, connect Oscilloscope Probe 1 to TP1 (voltage across the telephone line connected to ANALOG LINE INTERFACE A), and start the Oscilloscope. Probe 1 is associated with channel 1 of the Oscilloscope. * 6. Make the following settings on the Oscilloscope: Channel 1 Mode Normal Sensitivity V/div Input Coupling DC Time Base ms/div Display Refresh Continuous Observe that the DC voltage across the telephone line is close to the nominal value of 48 V (about 45 V in the Telephony Training System) because the loop current is zero (the handset of telephone set A is onhook). Lift off the handset of telephone set A. Observe that the DC voltage across the telephone line decreases greatly (to approximately 7 V) because DC current is flowing through the telephone line (this will be explained in

6 detail in Unit 2 of this manual). Replace the handset of telephone set A on the cradle. The voltage across the telephone line indicates whether or not DC current flows through the line. This voltage will be used in this exercise to study pulse dialing. * 7. Lift off the handset of telephone set A. Press different number keys on the keypad of telephone set A while listening to the handset earpiece and observing the signal on the Oscilloscope screen. Pressing the star (a) key causes the analog telephone set to switch to the tone dialing mode. If this occurs, simply hang up then lift off the handset again to return to the pulse dialing mode. Describe what happens whenever a number key on the telephone set is depressed. * 8. On the host computer, make the following settings on the Oscilloscope: Time Base s/div Trigger Source Ch 1 Level V Slope Negative () Display Refresh Manual * 9. Refresh the Oscilloscope display and immediately press number key 6 on telephone set A. The waveform of the resulting voltage across the telephone line should be recorded by the Oscilloscope. Figure 1-17 shows an example of the voltage waveform you should observe on the Oscilloscope screen.

7 Using the voltage waveform displayed on the Oscilloscope screen (voltage across the telephone line), describe what happens when the digit 6 is dialed. * 10. Refresh the Oscilloscope display and immediately press a number key on telephone set A. The waveform of the resulting voltage across the telephone line should be recorded by the Oscilloscope. Determine the number of times the loop current has been interrupted. Note the number of current interruptions as well as the number key that you depressed. * 11. Repeat the previous step a few times with different number keys. If a printer is available, you can print the waveforms of the voltage across the telephone line that you obtained for different number keys.

8 Briefly describe pulse dialing from the results you obtained so far. * 12. On the host computer, set the Oscilloscope time base to 50 ms/div. Refresh the Oscilloscope display and immediately press number key 3 on telephone set A. The waveform of the resulting voltage across the telephone line should be recorded by the Oscilloscope. Measure the dial pulse period and the duration of the current interruptions using this voltage waveform. Record your results in the following blank spaces. Dial Pulse Period: ms Duration of Current Interruptions: ms What is the duration of the current pulses resulting from the current interruptions? Duration of Current Pulses: ms * 13. On the host computer, set the Oscilloscope time base to 0.2 s/div. Refresh the Oscilloscope display and immediately dial digits 2 and 3 quickly on telephone set A. The waveform of the resulting voltage across the telephone line should have been recorded by the Oscilloscope. Measure the inter-digit interval using this voltage waveform. Record your result in the following blank space. Inter-Digit Interval: ms * 14. On the host computer, close the Telephony Training System software. Turn off the TTS Power Supply as well as the host computer (if it is no longer required). Disconnect the AC/DC power converters from the TTS Power Supply and the analog telephone sets. Disconnect the analog telephone sets from the Dual Analog Line Interface.

9 Remove the Dual Analog Line Interface from the Reconfigurable Training Module. In this exercise, you learned that pulse dialing is a means of transmitting telephone numbers to the central office. You observed pulse dialing waveforms (waveforms of the voltage across the telephone line), and saw that pulse dialing momentarily interrupts the loop current several successive times to transmit a dialed digit to the central office as a series of short current pulses. You verified that the number of momentary loop current interruptions that are produced depends on the dialed digit. You measured the dial pulse period, the duration of the current interruptions, the duration of the current pulses, and the inter-digit interval. 1. What is pulse dialing used for? 2. Briefly describe pulse dialing. 3. By how much time are digits dialed with a pulse dialing telephone set separated from each other? 4. What is the duration of the loop current interruptions produced by a pulse dialing telephone set?

10 5. Briefly explain why tone dialing is preferred to pulse dialing?