Download Analog Access to the Telephone Network, 1

Exercise
1-1
Telephone Ringing
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When you have completed this exercise, you will be able to demonstrate how a
central office makes a telephone set ring. You will also be familiar with the operation
of a telephone electronic ringer circuit.
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A telephone set rings and a distinct tone or bell sound is heard. The ringing alerts
the called party that a call is waiting. The ringing is a result of the central office of
the local telephone company applying an AC ringing voltage to the called party's
telephone set as shown in Figure 1-5.
CENTRAL OFFICE
OF TELEPHONE COMPANY
DIAL PULSES OR DIALING
TONES TRANSMITTED TO
CENTRAL OFFICE TO
REQUEST CONNECTION
RING
GENERATOR
AC RINGING VOLTAGE
APPLIED TO CALLED
PARTY'S TELEPHONE
SET
TIP (T)
HANDSET
CRADLE
TIP (T)
CALLING
PARTY'S
TELEPHONE
SET
DIALS
TELEPHONE
NUMBER OF
CALLED
PARTY
CALLED PARTY'S TELEPHONE SET
LINE
INTERFACE
LINE
INTERFACE
RINGER
CIRCUIT
RING (R)
RING (R)
TELEPHONE
LINE
(LOCAL LOOP)
TELEPHONE
LINE
(LOCAL LOOP)
DETECTS SERVICE REQUEST,
SENDS AC RINGING VOLTAGE TO
CALLED PARTY'S TELEPHONE SET
(IF NOT BUSY), AND EVENTUALLY
INTERCONNECTS THE TELEPHONE SETS
DIALING
AND
SPEECH
CIRCUIT
SWITCHHOOK (ON-HOOK)
RINGS WHEN AC RINGING VOLTAGE
IS DETECTED BY THE RINGER CIRCUIT
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The call is placed when the calling party makes a connection request to the central
office of the local telephone company by dialing a telephone number. If the called
telephone set is not busy, the central office applies an AC ringing voltage to this
Telephone Ringing
telephone set by connecting the output of a ring generator (AC source consisting of
an AC generator or an inverter) to the corresponding telephone line via the line
interface associated with that telephone line. The AC ringing voltage activates a
ringer circuit in the called party's telephone set which starts to ring. Ringing
continues until the telephone call is answered or the calling party hangs up.
The AC ringing voltage is switched on and off at regular time intervals to produce
a ringing cadence, as shown in Figure 1-6. In North America and many other
countries, the ringing voltage is applied (on) for 2 seconds followed by a 4-second
pause. In the United Kingdom, the ringing voltage is applied for 0.4 second, then off
for 0.2 second, and then back on for 0.4 second, and repeated after a 2-second
pause.
2 s ON
AMPLITUDE
4-s PAUSE
TIME
(a) AC ringing voltage (North America and many countries)
ON
ON
0.4 s
ON
2-s PAUSE
0.4 s
OFF
0.2 s
ON
0.4 s
2-s PAUSE
ON
0.4 s
AMPLITUDE
0.4 s
OFF
0.2 s
TIME
(b) AC ringing voltage (United Kingdom)
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The frequency and RMS voltage values of the AC ringing voltage varies from one
country to another. Typical values in North America are 20 Hz and 86 V. In Europe,
values for the AC ringing voltage cover a large range of frequencies (16 to 50 Hz)
and voltages (40 to 130 V).
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The ringer circuit in today's electronic telephone sets uses smaller-sized
components than in older telephone sets. The large bell in old telephone sets has
been replaced with an electronic circuit driving a sound transducer. Figure 1-7 is a
block diagram of an electronic telephone ringer circuit.
Telephone Ringing
VOLTAGE
REGULATOR
TIP (T)
DC POWER TO
RINGER CIRCUITRY
FULL-WAVE
RECTIFIER
RING (R)
ATTENUATOR
LOW-PASS
FILTER
+
−
FIXED-VOLTAGE
SOURCE
SOUND
TRANSDUCER
VOLTAGE
COMPARATOR
MULTI-TONE
GENERATOR
RINGINGTHRESHOLD
VOLTAGE
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The AC ringing voltage from the telephone company is applied to the ringer circuit
via the tip and ring connections of the telephone set and a coupling capacitor. The
ringing voltage is first rectified by a full-wave rectifier. The rectified ringing voltage
is applied to a voltage regulator which provides DC power to the electronic ringer
circuitry. The rectified ringing voltage is also attenuated and passed through a lowpass filter to obtain a DC voltage proportional to the amplitude of the AC ringing
voltage. When the filtered ringing voltage exceeds a certain threshold voltage
provided by a fixed-voltage source, the voltage comparator output changes state.
This enables a multi-tone generator that drives a sound transducer, and the
telephone set rings.
Comparing the rectified and filtered ringing voltage with a fixed voltage prevents
voltage spikes on the telephone line from causing undesired telephone ringing,
without affecting normal telephone ringing triggered by the AC ringing voltage.
Voltage spikes on the telephone line can occur during pulse dialing or can be
induced by external phenomenons such as lightning. The value of the ringing
threshold voltage varies from one telephone set to another and is a compromise
between AC ringing voltage detection and voltage spike rejection. Increasing the
threshold voltage provides a better protection against undesired telephone ringing,
but increases the AC ringing voltage required to make the telephone set ring. Note
that the low-pass filter, fixed-voltage source, and voltage comparator form what is
usually called an "anti-tinkle" circuit, because of the "tinkle" sound made when
dialing a rotary dial phone.
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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 observe the waveform of the AC ringing
voltage produced in the central office. You will also observe how the central office
makes a telephone set ring.
Telephone Ringing
In the last part of the exercise, you will evaluate the threshold voltage at which the
analog telephone sets in the Telephony Training System ring.
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Refer to Appendix A of this manual to obtain the list of equipment required to
perform this exercise.
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*
1. Make sure that the Reconfigurable Training Module, Model 9431, is
connected to the TTS Power Supply, Model 9408.
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 tone dialing mode is selected on the analog telephone sets.
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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.
1RWH The analog telephone set requires an auxiliary DC power
source for the digital display to be operative.
*
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.
Telephone Ringing
1RWH 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.
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*
4. On the host computer, zoom in on ANALOG LINE INTERFACE A, connect
Oscilloscope Probe 1 to TP2 (AC ringing voltage), and start the
Oscilloscope.
1RWH Probe 1 is associated with channel 1 of the Oscilloscope.
*
5. Make the following settings on the Oscilloscope:
Channel 1
Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Normal
Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 V/div
Input Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . AC
Time Base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 ms/div
Trigger
Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ch 1
Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 V
Slope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Positive (+)
Display Refresh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Manual
*
6. Refresh the Oscilloscope display. The AC ringing voltage waveform should
be displayed on the Oscilloscope screen.
What is the waveform of the AC ringing voltage?
*
*
7. Record the RMS value and frequency of the AC ringing voltage that are
indicated on the Oscilloscope.
AC Ringing Voltage RMS Value:
V
AC Ringing Voltage Frequency:
Hz
8. On the host computer, connect Oscilloscope Probe 2 to TP1 (voltage
across the telephone line connected to ANALOG LINE INTERFACE A).
1RWH Probe 2 is associated with channel 2 of the Oscilloscope.
Telephone Ringing
*
9. Make the following settings on the Oscilloscope:
Channel 2
Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Normal
Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 V/div
Input Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . AC
Time Base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 ms/div
Display Refresh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Continuous
1RWH If the UK ringing cadence is selected and/or the
AC ringing voltage frequency is set to 50 Hz, set the time base
to 2 ms/div.
* 10. Lift off the handset of the telephone set connected to ANALOG LINE
INTERFACE B and dial the number of the telephone set connected to
ANALOG LINE INTERFACE A. This will make the telephone set connected
to ANALOG LINE INTERFACE A ring. Let it ring a few times while
observing the signal at TP1 on the Oscilloscope screen, and hang up.
1RWH In the rest of this manual, the telephone sets connected
to ANALOG LINE INTERFACEs A and B will be referred to as
telephone sets A and B, respectively.
1RWH The rate at which the Oscilloscope display is refreshed
depends on the speed of the host computer running the
Telephony Training System software. Closing other applications
that are running on the host computer releases system
resources, and thereby, should improve the Oscilloscope display
refresh rate.
Describe what happens when a telephone set rings.
* 11. Repeat the previous step, but this time answer the call while the telephone
set is ringing, then hang up both telephone sets.
Describe what happens when you answer a telephone call.
Telephone Ringing
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* 12. Make the following settings on the Oscilloscope:
Display Refresh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Manual
* 13. On the host computer, set the output voltage of the RING GENERATOR
(AC ringing voltage) to 45 V.
1RWH The RING GENERATOR is located in the Signaling Circuit
of the central office.
* 14. Make the following settings on the Oscilloscope:
Channel-2 Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 V/div
Display Refresh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Continuous
Observe that the value of the AC ringing voltage has decreased.
Lift off the handset of telephone set B and dial the number of telephone
set A. This will make this telephone set ring. Do not answer the call.
On the host computer, increase the resistance of the telephone line
connected to ANALOG LINE INTERFACE A by steps until telephone set A
stops ringing. While doing this, observe the signal at TP1 (voltage across
the telephone line connected to ANALOG LINE INTERFACE A).
Note that LINE MONITOR 1, which measures the voltage across the
telephone line, is connected to the telephone side of the telephone line as
shown in Figure 1-8. Thus, the signal observed at TP1 is the voltage across
the Tip and Ring terminals of telephone set A. Resistors are used in
ANALOG LINE INTERFACE A to simulate a telephone line of a certain
length. The resistance of these resistors is variable to allow different line
lengths to be simulated.
Briefly explain why telephone set A stops ringing when the telephone line
resistance is increased to a certain value.
Telephone Ringing
ANALOG LINE INTERFACE A
TELEPHONE
LINE
VARIABLE
RESISTORS
TIP
(T)
TELEPHONE
SET A
RING
RELAY
OV
TO SLIC
(R)
RING
LINE
MONITOR
1
TP 1
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* 15. Measure the amplitude of the AC ringing voltage (signal at TP1) displayed
on the Oscilloscope. This value is approximately equal to the ringing
threshold voltage of telephone set A.
Ringing Threshold Voltage:
V
Replace the handset of telephone set B on the cradle.
On the host computer, set the Oscilloscope display refresh to Manual.
* 16. 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.
Remove the Dual Analog Line Interface from the Reconfigurable Training
Module.
Telephone Ringing
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In this exercise, you learned that the central office applies AC voltage to an analog
telephone set (via the line interface and telephone line) to make it ring. You
observed that the AC ringing voltage is switched on and off at regular intervals so
as to produce a ringing cadence. You saw that the frequency, RMS value, and
on/off intervals of the AC ringing voltage vary from one country to another.
You learned that modern analog telephone sets use an electronic ringer circuit. This
circuit measures the AC ringing voltage amplitude, and drives a sound transducer
to produce a ringing sound when the measured voltage amplitude exceeds a certain
threshold voltage. You saw that comparing the AC voltage amplitude to a threshold
voltage prevents voltage spikes on the telephone line from causing undesired
telephone ringing.
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1. How does a central office make an analog telephone set ring?
2. What are the frequency and RMS voltage values of the AC ringing voltage in
North America?
3. What is the ringing cadence in North America and many other countries?
4. Why do electronic ringer circuits measure the amplitude of the AC ringing
voltage and compare it to a fixed threshold voltage?
Telephone Ringing
5. What is the threshold voltage of the electronic ringer circuit in the analog
telephone sets of the Telephony Training System?