Analog Access to the
Telephone Network
Analog Access to the Telephone Network
UNIT 1 THE TELEPHONE SET
EXERCISE 1-1 TELEPHONE RINGING
ANSWERS TO PROCEDURE QUESTIONS
* 6. The AC ringing voltage is a sine wave.
* 7. AC Ringing Voltage RMS Value: 86 V, can also be set to 45 or 75 V in the Telephony Training System.
AC Ringing Voltage Frequency: 16.7 Hz, 20 Hz (North America), 25 Hz or 50 Hz (UK), depending on country.
* 10. The AC ringing voltage is applied momentarily across the telephone line at regular intervals. The telephone set rings whenever the AC ringing voltage is applied across the telephone line, thereby producing a certain ringing cadence.
* 11. The AC ringing voltage is immediately removed from the telephone line to stop telephone ringing.
* 14. Telephone set A stops ringing because the increase of the telephone line resistance makes the amplitude of the AC ringing voltage across the Tip and Ring terminals decrease below the ringing threshold voltage.
* 15. Ringing Threshold Voltage: 40 V
ANSWERS TO REVIEW QUESTIONS
1. To make an analog telephone set ring, the central office applies an AC ringing voltage to the corresponding telephone line via the analog line interface associated with that line.
2. In North America, the frequency and RMS voltage values of the AC ringing voltage are 20 Hz and 86 V, respectively.
3. The telephone rings during 2-s intervals separated by 4-s pauses.
4. Electronic ringer circuits compare the amplitude of the AC ringing voltage to a fixed threshold voltage to prevent telephone ringing triggered by undesired voltage spikes on the telephone line (such as those induced by lightning for example).
5. The threshold voltage of the electronic ringer circuit in the analog telephone sets of the Telephony Training System is approximately 40 V.
Analog Access to the Telephone Network
EXERCISE 1-2 THE TELEPHONE SWITCHHOOK AND HANDSET
ANSWERS TO PROCEDURE STEP QUESTIONS
* 6. DC Voltage Across the Telephone Line: 45 V
Note: In the Lab-Volt Telephony Training System, the DC voltage across the telephone line may be a little less than the nominal value of 48 V.
The DC voltage across the telephone line comes from a battery feed circuit in the central office.
* 7. No current is flowing through the telephone line because the handset of telephone set A is on the cradle, thereby leaving the switchhook contacts open. Therefore, the telephone set is like an open circuit.
* 8. DC Loop Current: 36 mA (handset off the cradle)
Lifting off the telephone handset from the cradle closes the switchhook contacts. This connects the dialing and speech circuits of the telephone set to the telephone line. These circuits require DC power to operate, hence the DC current flowing through the telephone line.
* 10. The signal at TP3 of ANALOG LINE INTERFACE A represents an electrical equivalent of the voice of the person who is talking into the handset of telephone set A.
* 12. The signal at TP3 of ANALOG LINE INTERFACE A represents an electrical equivalent of the voice of the person who is talking into the handset of telephone set B.
The handset microphone converts speech sound waves into an electrical signal. Conversely, the handset earpiece converts an electrical signal into sound waves. The speech circuit performs 2W/4W conversion, that is, it directs the voice signal to be transmitted from the handset microphone to the telephone line, and the received voice signal from the telephone line to the handset earpiece.
* 13. The voice sound level in the handset earpiece does not change significantly as the telephone line resistance (length) increases. This is because the telephone speech circuit is able to increase its line side impedance, thereby
Analog Access to the Telephone Network
ANSWERS TO REVIEW QUESTIONS
1. A switchhook is a DPST switch that closes when the telephone handset is lifted off the cradle.
2. The telephone switchhook connects the dialing and speech circuits to the telephone line when the handset is off-hook. It is also a means to notify the central office of service requests.
3. The speech circuit properly routes the electrical voice signals on the two wires of the telephone line to the four wires of the handset (this process is referred to as two-wire to four-wire conversion). It also adjusts the level of the voice signals so as to compensate for different telephone line lengths.
4. Loop length equalization is a process that adjusts the line side impedance of the speech circuit so that it is inversely proportional to the DC current flowing in the telephone line. This makes the speech circuit impedance virtually proportional to that of the telephone line, thereby maintaining the voice signals at a nearly constant level.
5. The portion of the voice signal of a telephone user that is sent back to the handset earpiece is a sidetone. It gives feedback to the telephone user to help judge the proper volume at which to speak.
EXERCISE 1-3 TONE DIALING
ANSWERS TO PROCEDURE STEP QUESTIONS
* 7. A DTMF dialing tone is output to the telephone line whenever a key is depressed on the telephone set. These tones can be heard through the handset earpiece.
The waveform of the DTMF dialing signals resembles a distorted sine wave that fluctuates.
* 9. The frequency spectrum of each DTMF dialing signal contains two frequency components.
Analog Access to the Telephone Network
Table 1-2. Frequency components of the DTMF signal associated with each key of a telephone set using tone dialing.
* 11.
Figure 1-13. Frequencies associated with the rows and columns of the telephone keypad.
Analog Access to the Telephone Network
ANSWERS TO REVIEW QUESTIONS
1. DTMF tone dialing is a means of transmitting telephone numbers to the central office using dual-tone audio signals.
2. DTMF tone dialing assigns a specific frequency to each row and column of the telephone keypad. When a key is depressed, a dual-tone audio signal consisting of the frequencies associated with the corresponding row and column of the keypad is output to the telephone line.
3. The frequencies of the DTMF dialing signal associated with number key "8" of a telephone set are 852 and 1336 Hz.
4. The frequencies associated with the rows of the keypad on a telephone set that produces DTMF dialing signals are 697 (upper row), 770, 852, and 941 Hz (lower row).
5. The frequencies associated with the columns of the keypad on a telephone set that produces DTMF dialing signals are 1209 (leftmost column), 1336, and 1477 Hz (rightmost column).
EXERCISE 1-4 PULSE DIALING
ANSWERS TO PROCEDURE STEP QUESTIONS
* 7. Whenever a number key is depressed on the telephone set, the loop current is briefly interrupted a certain number of times to create a series of current pulses. These pulses can be heard through the handset earpiece.
* 9. When the digit 6 is dialed, the loop current is interrupted briefly six successive times since the voltage across the telephone line briefly decreases to about 45 V six successive times. This produces a series of current pulses that will be interpreted as the digit 6 in the central office.
* 11. Whenever a digit is dialed on the telephone set, the loop current is interrupted briefly. The number of current interruptions 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.
* 12. Dial Pulse Period: 100 ms
Duration of Current Interruptions: 60 ms Duration of Current Pulses: 40 ms
Analog Access to the Telephone Network
* 13. Inter-Digit Interval: 820 ms
ANSWERS TO REVIEW QUESTIONS
1. Pulse dialing is a means of transmitting telephone numbers to the central office through a series of momentary loop current interruptions.
2. 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.
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.
3. Digits dialed with a pulse dialing telephone set are separated by time intervals of at least 300 ms (nominal duration of 700 ms in North America) during which the loop current is not interrupted.
4. In North America, the duration of the loop current interruptions is 60 ms. In other countries, the duration is about 67 ms.
5. Tone dialing is preferred to pulse dialing because it allows telephone numbers to be dialed more rapidly.