E3125/1/1 INTRODUCTION TO TELEPHONY SYSTEM INTRODUCTION TO TELEPHONY SYSTEM OBJECTIVES General Objective : To understand the concept of telephony system, the types of the telephone exchanges and the switching concept. Specific Objectives : At the end of the unit you will be able to: explain the need of the telephony networking system. UNIT 1
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E3125/1/1INTRODUCTION TO TELEPHONY SYSTEM
INTRODUCTION TO TELEPHONY SYSTEM
OBJECTIVES
General Objective : To understand the concept of telephony system, the types of the
telephone exchanges and the switching concept.
Specific Objectives : At the end of the unit you will be able to:
explain the need of the telephony networking system.
sketch the telephone network for local exchange and telephone
network hierarchy.
define PSTN and MTX telephone exchange.
explain switching concept including matrix switching, step by
step, common controller and SPC.
draw the SPC block diagram and to understand the advantages
and disadvantages of the system.
UNIT 1
E3125/1/2INTRODUCTION TO TELEPHONY SYSTEM
1.0 Introduction
The Telephony Communication Principle
There are 3 main parts as shown in figure 1.1
a) Transmitting transducer
b) Receiving transducer
c) One pair wire system
INPUTINPUT
We use telephony system in our everyday life. The equipment involves telephone sets at home, the internet, e-mails, our mobile phones and telefaxes. Telephony system makes our life much more easier.
E3125/1/3INTRODUCTION TO TELEPHONY SYSTEM
Conversation
Energy
Produces
the conversation
energy again
Figure 1.1 Telephony Communication Principle
1.1 The Telephone
The telephone is one of the simplest devices we have in our house. It is so very
simple because the telephone connection to our house has not changed in
nearly a century.
The telephone only contains three parts and they are all simple as shown in figure
1.2.
A switch to connect and disconnect the phone from the network. This
switch is generally called the hook switch. It connects when you lift the
handset.
A speaker - It is generally in a small size, 8-ohm speaker of some sort.
A microphone - In the past, telephone microphones have been as simple
as carbon granules compressed between two thin metal plates. Sound
waves from our voice compress and decompress the granules, changing
the resistance of the granules and modulating the current flowing through
In order to allow more long-distance calls to be transmitted, the frequencies
transmitted are limited to a bandwidth of about 3000 hertz. All of the frequencies in
our voice below 400 hertz and above 3,400 hertz are eliminated. That's why
someone's voice on a phone has a distinctive sound.
1.3 Digital Telephone
The digital button is the latest technique of dialing. It uses the button to give signal
for every one digit. The diagram in figure 1.3 shows the Dual Tone Multi
Frequency (DTMF) type of dialing. The button on the phone is connected to a set of
oscillators which produces a pair of tone on the local line whenever a button is being
pressed. The tone will be detected at the main distributor and the digit will be
confirmed. The detector circuits in the main distributor will confirm the tone within
33ms.
E3125/1/5INTRODUCTION TO TELEPHONY SYSTEM
1209Hz 1336Hz 1477Hz
Figure 1.3 Dual Tone Multi Frequency (DTMF)
1.3.1 Progress Tones
The various types of tones generated by the exchange to guide the users are :
Dial Tone (DT). This is a 33 c/s continuous note and is applied to the line
after the subscriber has lifted his handset and the switching equipment has
allocated him an available outlet for this call to proceed. There would have
been a physical limit on the number of calls an exchange could handle so if
all equipment was already in use, the subscriber would not get a dial tone.
Busy Tone (BT). A higher pitched note of 400 c/s interrupts to give a
cadence of 0.75 seconds on, 0.75 seconds off. Busy tone indicates either that
the called subscriber is already off-hook (busy) or that the route to the called
subscriber is congested. In later systems, a slightly different cadence was
introduced in order to distinguish between these two scenarios. A busy tone
is made up of a 480-hertz and a 620-hertz tone, with a cycle of one and a half
second on and one and a half second off.
1
4
7 8 9
5 6
32
#0*
697Hz
770Hz
852Hz
941Hz
E3125/1/6INTRODUCTION TO TELEPHONY SYSTEM
Number Unobtainable Tone (NUT). Identical pitch to the busy tone but
continuous. This tone is used to indicate that a number is out of service,
faulty or that a spare line has been dialed.
Ring Tone (RT). A tone of 133c/s which interrupts in the same cadence as
the ring current which rings the telephone bell at the called party's end : 0.4
seconds on, 0.2 seconds off.
E3125/1/7INTRODUCTION TO TELEPHONY SYSTEM
Example 1.1
Do you know the human’s voice frequency ?
Solution to Example 1.1
The answer is between 300 – 3400 Hz.
Telephony System surely makes my job easier.Yahoo !!!
E3125/1/8INTRODUCTION TO TELEPHONY SYSTEM
Activity 1A
TEST OUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE
NEXT INPUT…!
1.1 State THREE main parts of the telephone system and draw the block
diagram.
1.2 List and explain FOUR types of the telephone supervisory (progress) tones .
Telephony system makes the world becomes smaller through the usage of the internet. This won’t be achieved without the advancement in telephony technology.
E3125/1/9INTRODUCTION TO TELEPHONY SYSTEM
Feedback To Activity 1A
1.1 There are 3 main parts as shown in figure 1.1
a) Transmitting transducer
b) Receiving transducer
c) One pair wire system
1.2 The various types of tones generated by the exchange to guide the users are :
Dial Tone (DT). This is a 33 c/s continuous note and is applied to the line after the
subscriber has lifted his handset and the switching equipment has allocated him an
available outlet for this call to proceed. There would have been a physical limit on
the number of calls an exchange could handle so if all equipment was already in use,
the subscriber would not get a dial tone.
Busy Tone (BT). A higher pitched note of 400 c/s interrupts to give a cadence of
0.75 seconds on, 0.75 seconds off. Busy tone indicates either that the called
TransmittingTransducer
ReceivingTransducer
ReceivingTransducer
Transmitting Transducer
Produces
the conversation energy again
Produces
the conversation energy again
Conversation EnergyConversation Energy
Figure 1.1 Telephony Communication Principle
E3125/1/10INTRODUCTION TO TELEPHONY SYSTEM
subscriber is already off-hook (busy) or that the route to the called subscriber is
congested. In later systems, a slightly different cadence was introduced in order to
distinguish between these two scenarios. A busy tone is made up of a 480-hertz and
a 620-hertz tone, with a cycle of one and a half second on and one and a half second
off.
Number Unobtainable Tone (NUT). Identical pitch to the busy tone but continuous.
This tone is used to indicate that a number is out of service, faulty or that a spare line
has been dialed.
Ring Tone (RT). A tone of 133c/s which interrupts in the same cadence as the ring
current which rings the telephone bell at the called party's end : 0.4 seconds on, 0.2
seconds off.
E3125/1/11INTRODUCTION TO TELEPHONY SYSTEM
1.4 The Telephone Exchange
The telephone exchange is the place where all the telephone lines are connected so
that the communication using the human’s voice could be achieved. The device in
the exchange will act as a switch which contacts whenever it is directed.
1.4.1 The Telephone Connection Between Two Subscribers.
The connection process is shown in figure 1.3.
1. When Subscriber A picks up the telephone set, Local Exchange will
detect a call request signal from Subscriber A..
2. Local Exchange will send a dial tone to Subscriber A.
3. Subscriber A starts to dial the numbers. The numbers received will be
tested by the Local Exchange.
4. If the numbers are valid, Local Exchange will test whether the line is
available or not.
5. If the line is available, Local Exchange will do the connection. Local
Exchange will send a ringing tone to Subscriber A and the signal to
ring the Subscriber B’s telephone. Subscriber B will pick up the
telephone set, thus making the dialing tone stop.
INPUTINPUT
E3125/1/12INTRODUCTION TO TELEPHONY SYSTEM
6. If either one of the subscribers hangs up the phone , communication
will be disconnected and the Local Exchange will clear the line.
1.5 Telephone Network For Local Exchange
The telephone network starts from the house. A pair of copper wires runs from a box
at the road to a box at our house. Figure 1.4 shows the connection from the
exchange to the customer’s house. From there, the pair of wires is connected to the
phone jack in our house.
Along the road runs a thick cable packed with 100 or more copper pairs. Depending
on where we are located, this thick cable will run directly to the phone company's
switch in our area or district and it will run to a box that acts as a digital
concentrator (cabinet)
The concentrator digitizes the voice at a sample rate of 8,000 samples per second
and 8-bit resolution. It then combines the voice with dozens of others and sends them
1
2
3
4
5 5
6 6
SU
BS
CR
IBE
R A
SA
SU
BS
CR
IBE
R B
LO
CA
L E
XC
HA
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Figure 1.3 Telephone communication between subscribers