e3125_telecommunication_unit1.doc

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


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.


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

the microphone.

TransmittingTransducer

ReceivingTransducer

ReceivingTransducer

Transmitting Transducer

http://www.howstuffworks.com/question309.htm


Figure 1.2 Parts of the telephone set.

1.2 Telephone Bandwidth

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.


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


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.


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 !!!


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.


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


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.


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


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

NG

E

Figure 1.3 Telephone communication between subscribers




all down a single wire (usually a coax cable) to the phone company office. Either

way, the line connects into a line card at the switch so we can hear the dial tone

when we pick up our phone.

Figure 1.4 The Connection from the Exchange to the Customer House

Main Distribution Frame (MDF)

The Main Distribution Frame is situated at the telephone exchange. It is used to

connect the customer (phone user) with the main switching system after passing

through the cabinet. It comprises of all the customers and data which are situated in

one area or district.

The duties of MDF :

a) To test the phone line.

b) To execute the jumper process.

c) To check the problem phone line.

d) To cut the phone line manually.

e) To connect the phone line manually.

Cabinet

The cabinet is a place to distribute lines to the DP Box before reaching a customer’s

house as portrayed in figure 1.5. Generally, the cabinet is the end for the main cable

where it starts at the Main Distribution Frame and passes through the cabinet.

Exchange Cabinet

Upper Cable

IB Cable

DP Box

House

Connection Underground CableMan - hole


Connection from the cabinet to the main distributor normally uses the ‘lead’ cable of

different sizes where each of it has 200 pairs and to be distributed to about 20 DP

Box. From the cabinet to the DP Box, it uses the plastic cable. In the cabinet, there

are 1800 pairs of terminals.

Figure 1.5 Cabinet

The Cabinet’s functions are as follows :

a. To save the usage of cable and replacing cable would be easier.

b. To avoid the joint to be opened and this would save cost and time.

c. To act as a place to perform maintenance checking.

d. To upgrade the arrangement of lines.

Distribution Point Box (DP)

The DP Box’s function is to make maintenance work and underground cabling

easier. Usually, the DP Box has 10 pairs but only 8 pairs are used and the others

kept for testing work from DP Box to the main distributor.

Upper Cable

The upper cable is used to make connection with the customers using the DP Box.

The two wire cable normally is used.


Underground Cable

The underground cable is connected to the cabinet from the DP through man-holes.

The underground cable will end at the telephone exchange.

1.6 The National Telephone Network Hierarchy

Figure 1.6 The Telephone Network Hierarchy

MSC – Main Switching Center ( for international call )

ZSC – Zone Switching Center ( for call between states in Malaysia )

DSC – District Switching Center ( for call between different districts )

LE – Local Exchange

S – Phone User / Subscriber

The National Telephone Network Hierarchy is the network that links all the exchanges in

one country.

MSC

ZSC ZSC

DSC DSC DSC DSC

LE LE LE LELE LE LELE

S S S S S S S S S S S S S S S S


Example 1.2

What does DTMF stand for ?


DTMF (Dual-tone Multi frequency) is a type of dialing system which uses a pair of

audio tone to create signals representing the numbers to be dialed. It is also named as

Touch Tone.


Activity 1B


NEXT INPUT…!

1.3 Sketch and explain the connection from the exchange to the customer ‘s

house.

1.4 Describe with the aid of a suitable diagram the route of your call if you want

to make a call from your polytechnic to another polytechnic nearest to you.


Feedback To Activity 1B

1.3 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

all down a single wire (usually a coax cable) to the phone company office. Either

way, the line connects into a line card at the switch so we can hear the dial tone

when we pick up our phone.

Figure 1.4 The Connection from the Exchange to the Customer House

Exchange Cabinet

Upper Cable

IB Cable

DP Box

House

ConnectionUnderground Cable

Man - hole




1.4 If you are at Politeknik Ungku Omar and you want to call a friend at

Politeknik Seberang Perai, the route of the call would be as follows:

PUOPUO LE LE DSCDSC ZSC ZSC

PSP PSP LE LE DSC DSC ZSC ZSC


1.7 Switching Concept

Switch is used to provide a path for the call. The switch performs three main

functions to process a call:

a) To identify the subscriber’s location

b) To set up the communication path

c) To supervise the call

By using the jack position, the customers occupying the switchboard can be easily

identified. With the introduction of electromechanical switches, customers are given

telephone numbers. The customer's cable pair is terminated and cross-connected to

the office equipment at the main distributing frame. Office equipment terminated on

the MDF represents a physical location in the switch and a specific telephone

number.

INPUTINPUT

Do you wonder how the telephone exchange works?


1.8 Public Switching Telephone Network (PSTN)

Public Switching Telephone Network (PSTN) is the world's collection of

interconnected voice-oriented public telephone networks. The PSTN consists of a

digital backbone of switched circuits together with the analog local loop wiring still

found in many residences. The PSTN provides the most popular basis for creating

wide area networks (WANs) through both leased lines and dial-up lines between

local and remote networks. PSTN is often used in wide area networking and because

of its ubiquitous nature local loop connections exist almost everywhere in the world.

In relation to the Internet, the PSTN actually furnishes much of the Internet's long-

distance infrastructure. Because Internet service providers ISPs pay the long-distance

providers for access to their infrastructure and share the circuits among many users

through packet-switching, Internet users avoid having to pay usage tolls to anyone

other than their ISPs.

The process of signal transmission needs switching network such as concentrator,

distributor, expander and multiplexer. Combination of all these networks will form a

complete PSTN.

Concentrator

Figure 1.7 shows the concentrator concept in PSTN network.The concentrator unit is

controlled directly by the local exchange. All the subscribers can be connected

simultaneously at different channels. This will reduce the cost of installation and

maintenance.


Distributor

Distributor is a network in the middle of the concentrator and the expander as

shown in figure 1.8. This unit will connect thousands of subsribers in stages . For

example, for every 1000 incoming lines, it will allow 100 outgoings to the distributor

at one time.

Expander

Concentrator Subscribers

DistributorDistributor

Figure 1.7 The traffic concentration in PSTN network.

Local line 1

Local line 1000Local line 1000

Con

cent

rato

r

Distributor

Exp

ande

r

Local line 1

Outgoing channelIncoming channel

Figure 1.8 PSTN Switching NetworkFigure 1.8 PSTN Switching Network


This unit will expand the actual number of lines giong into the concentrator. The

switching process for the expander and the concentrator relies on the number of lines

being activated simultaneously.

Multiplexer

The function of the unit is to send signals from multiple sources through the single

channel as potrayed in figure 1.9. This will reduce the number of lines needed. Some

of the multiplexing techniques that are widely used are the Time Division

Multiplexing (TDM) and Frequency Division Multiplexing (FDM).

Demultiplexing is the technique used to regain the input signals at the receiver.

1.9 Types Of Switching

Generally, there are THREE types of switching : The Crossbar Switch, Step by step

Switch and Stored Program Control (SPC).

1.9.1 Crossbar Switch (Matrix)

It consists of a vertical and horizontal lines. Switches are needed to connect

the combination of both lines. The number of crossing can be found with the

method by multiplying the input and output of channel and total them up. The

number of crossing will be the multiplication of the number of input and

output channels. For figure 1.10, the number of crossing is 16. To connect

input channel 2 and the output channel 4, the “d” switch will be activated.

This will allow subscriber input 2 and output 4 to communicate.

MU

X

Information signal 1Information signal 2

Information signal 10 D

EM

UX

Information signal 1Information signal 2

Information signal 10

Figure 1.9 Multiplexing and Demultiplexing TechniquesFigure 1.9 Multiplexing and Demultiplexing Techniques


1.9.2 Step-by-Step Switch

Almon Strowger invented the first "automatic" switch in 1890. It was the step

by step electromechanical switch. It was first installed in 1892. The name

Step-by-Step came from the way the path is set up step-by step as each digit

was dialed as shown in figure 1.11. The fundamental process was that

telephone calls progressed through switches in steps, each step being made in

response to the dialing of a digit.

The problems of the Step-by-Step switch were:

Power : Required a large amount of electricity to move switch components

for call processing

Floor Space: Major switch components were made of brass, copper and

magnets which were so heavy that required steel reinforcement in the floors

where they were installed

High Maintenance: The large number of moving parts in this switch

required constant dusting and oiling to ensure connections could be made

inside the switch

Noise: The contacts were chattering as a physical path is set up through the

switch for call processing.

1 a b c d 1 a b c d

2 e f g h2 e f g h

Input 3 i j k lInput 3 i j k l

4 m n o p4 m n o p

1 2 3 4 1 2 3 4Output Output

Figure 1.10 The Matrix SwitchingFigure 1.10 The Matrix Switching


1.9.3 Stored Program Control (SPC)

The Stored Program Control (SPC) is a method of switching using computer

to process a call automatically. The computer will operate based on a certain

program which can be changed whenever it is needed.

Figure 1.12 shows the block diagram of the SPC system.

Memory

To store the user’s activities status in the form of software..

Control Director

The program that controls and directs the whole system.

File

To store the information.

DTMF/decodec

To decode the signal produced during the dialing process.

Scanner

To detect the telephone status (on/off hook) and signal from the line.

Driver

To on and off the switch.

To allow the tones signal to pass through.

Central Processing Unit (CPU)

To control and execute the program.

Tone Generator / Ringer

To generate the tones.

Figure 1.11 The Selector SwitchFigure 1.11 The Selector Switch


SPC Exchange Switching Process

# 1

# 2

# 1

# M

Tone Generator DTMF/ Decadic

Scanner Driver

CPURinger

Memory Control Director

File

# n

1 2 n

Line Unit Switching Unit Saluran

Figure 1.12 SPC Switching Block Diagram.Figure 1.12 SPC Switching Block Diagram.


i. If caller A picks up the telephone set (off hook) , the line will be

activated and this will be automatically detected by the scanner.

ii. The scanner will identify the location and condition of caller A and

inform the CPU.

iii. Once the feedback is received, the CPU will check the status of A and

store it in the memory.

iv. The CPU will then connect A and the switching section via the driver.

The dial tone will be sent from the generator to A.

v. When A starts dialing, the dial tone will be stopped by the DTMF

(Dual Tone Multi Frequency) .

vi. The frequency representing the dialed number will be stored in the

memory.

vii. The CPU will identify types of call and the destination.

viii. The driver will drive the switching equipment to create a path

between A and B.

ix. The CPU will instruct the ringer to produce the ringing tone for

receiver B. The ringing tone will be sent to caller A simultaneously.

x. When B picks up the set, the scanner will detect the answering signal

and send it to the CPU. The CPU will direct the ringer to stop the

ringing tone and starts the timer .

xi. After the communication, if anyone of the users hangs up, the scanner

will detect the stop signal and send it to the CPU.

xii. The CPU will store the user’s bill.

The Advantages of the SPC


a) The monitoring and maintenance will be easier by inspecting the

program.

b) The signaling from the exchange can be operated more efficiently.

Any increase in the number of lines will not need the upgrading of the

signaling equipment.

c) The maintenance cost is low and the speed is faster.

The Disadvantages of the SPC.

a) Any single minor breakdown will cease the operation of the

exchange.

b) Will need proper air-conditioned room.


Example 1.3

Elaborate what is meant by the Public Switching Telephone Network (PSTN)


PSTN (Public Switching Telephone Network) is the world's collection of

interconnected voice-oriented public telephone networks. The PSTN consists of a

digital backbone of switched circuits together with the analog local loop wiring still

found in many residences. The PSTN provides the most popular basis for creating

wide area networks (WANs) through both leased lines and dial-up lines between

local and remote networks. PSTN is often used in wide area networking because its

ubiquitous nature local loop connections exist almost everywhere in the world.


Activity 1C


NEXT INPUT…!

1.5 True or false. The PSTN provides the most popular basis for creating wide

area networks (WANs) through both leased lines and dial-up lines between

local and remote networks.

1.6 State THREE types of telephone switching and mention the main differences

between them.

1.7 Define THREE functions of the switch in the telephone exchange.


Feedback To Activity 1C

1.5 True

1.6 Matrix (electromechanical), Step by Step (auto electromechanical) and Stored

Program Control (fully computerized).

1.7 a) Identify the subscriber’s location

b) Set up the communication path

c) Supervise the call


KEY FACTS

1. In the older telephone, the use of a rotary dialing mechanism produces what is

known as pulse dialing.

2. For the newer telephone, it uses a dialing system known as touchtone. It is also

referred to as the dual tone multi frequency (DTMF) .

3. Do you know that telephones are connected directly by twisted-pair cable to a

local exchange. Each exchange (identified by the first three digits of a phone

number) can serve up to 10000 lines.


SELF-ASSESSMENT

You are approaching success. Try all the questions in this self-assessment section

and check your answers with those given in the Feedback on Self-Assessment given

on the next page. If you face any problems, discuss it with your lecturer. Good luck.

Question 1-1

a. What is the name of the building or facility to which every telephone is

connected?

Question 1-2

a. Briefly define the terms MDF, DTMF and MSC.

b. List FOUR possible functions of the cabinet.

c. Explain the process of the telephone connection between two subscribers.

d. What is the difference between the upper cable and the underground cable.

Question 1-3

a. Explain briefly the disadvantages of a step-by-step exchange.

b. PSTN (Public Switching Telephone Network) is the collection of interconnected

voice-oriented public telephone networks. Name the FOUR important switching

networks in the PSTN and explain.

c. Based on the diagram in figure 1.10, name the type of the telephone switching

system. Explain the connection that is involved in order to activate “j” switch.


d. The Stored Program Control (SPC) is a method of switching using computer

to process a call automatically. Name all the empty blocks in figure 1.12.

Figure 1.12

# 1

# 2

# 1

# M

# n

1 2 n

Line Unit Switching Line Unit

1 a b c d 1 a b c d

2 e f g h2 e f g h

Input 3 i j k lInput 3 i j k l

4 m n o p4 m n o p

1 2 3 4 1 2 3 4Output Output

Figure 1.10 The Matrix SwitchingFigure 1.10 The Matrix Switching


Feedback To Self-Assessment

Have you tried the questions????? If “YES”, check your answers now.

Answer 1-1

a. The telephone exchange.

Answer 1-2

a. MDF – Main Distribution Frame.

DTMF- Dual Tone Multi Frequency

MSC – Main Switching Center.

b. The Cabinet’s functions are as follows :

i. To save the usage of cable and replacing cable would be easier.

ii. To avoid the joint to be opened and this would save cost and time.

iii. To act as a place to perform maintenance checking.

iv. To upgrade the arrangement of lines.

c.

1

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6 6

SU

BS

CR

IBE

R A

SA S

UB

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RIB

ER

B

LO

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XC

HA

NG

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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.

6. If either one of the subscribers hangs up the phone , communication

will be disconnected and the Local Exchange will clear the line.

d. Upper Cable :The upper cable is used to make connection with the

customers using the DP Box. The two wire cable is normally used.

Underground Cable : The underground cable is connected to the cabinet

from the DP through man-holes. The underground cable will end at the

telephone exchange.

Answer 1-3

a. The disadvantages of the Step-by-Step switch were:

Power : Required a large amount of electricity to move switch components

for call processing

Floor Space: Major switch components were made of brass, copper and

magnets which were so heavy that required steel reinforcement in the floors

where they were installed


High Maintenance: The large number of moving parts in this switch

required constant dusting and oiling to ensure connections could be made

inside the switch

Noise: The contacts were chattering as a physical path was set up through the

switch for call processing.

b. Concentrator: The concentrator unit is controlled directly by the local exchange.

All the subscribers can be connected simultaneously at the different channels.

This will reduce the cost of installation and maintenance.

Distributor : Distributor is a network in the middle of the concentrator

and the expander as shown in figure 1.8. This unit will connect thousands of

subsribers in stages .

Expander : This unit will expand the actual number of lines giong into the

concentrator. The switching process for the expander and the concentrator

relies on the number of lines being activated simultaneously.

Multiplexer : The function of the unit is to send signals from multiple

sources through the single channel. This will reduce the number of lines

needed. Some of the multiplexing techniques that are widely used are the

Time Division Multiplexing (TDM) and Frequency Division Multiplexing

(FDM). Demultiplexing is the technique to regain the input signals at the

receiver.

Combination of all these networks will form a complete PSTN.

c. To activate “j” switch, the input channel 3 and output channel 2 must be connected.

d.


# 1

# 2

# 1

# M

Tone Generator DTMF/ Decadic

Scanner Driver

CPURinger

Memory Control Director

File

# n

1 2 n

Line Unit Switching Line Unit

e3125_telecommunication_unit1.doc

Documents

higher pitched note

main distribution frame

frequency division multiplexing

perform maintenance checking

required steel reinforcement

wide area networking

major switch components

move switch components