013 Fundementals of Electronic Communication 3

Access Network and Digital Telephone Exchange

Basic parts of the telephone set are:◦ Ringer◦ Hook Switch (Cradle switch)◦ Dual-Tone Multi-Frequency Pad (DTMF) or

Rotary Dial◦ Handset

Transmitter Receiver

The Simple Telephone

Ringer

Can be either a Bell or a Buzzer

Hook Switch

Earpiece

Mouthpiece

Handset Cord

Handset

Two local battery telephones interconnect by line

Change in the resistance in microphone with the speech signal generate changing electric current

Transmitter

Time varying DC Current (I)

BATT (48 Volts)

RESISTANCE CHANGE DUE TO AIR VIABRTION

R Ohms

Varying d.c. produced by carbon granule transmitter

Fixed D.C current

Handset lifted

Start of speaking Stops

speaking

Handset replaced

Varying D.C speech current

T

Current

0

D.C Polarizing current

Speech current produced by the talker appears as rapid changes in the value of the current passing through the primary winding

Similar characteristics is thereby induced in the secondary winding

( When a steady current flowing in the primary winding, no current flows in the secondary winding connected to line. )

Transmitter

Cu

rren

t

Time

Transmitter Current (Varying D.C)

Cu

rren

t

Time

Alternating Line Current

This current circulate to line

Therefore passes through the listener's telephone receiver generating sound

All the old telephones are local battery powered (each telephone contains internal battery).

Advantage is that the power requirement for transmitter operation is provided with the right power irrespective of the length of line or bad external wiring.

But now central battery system are used

Local Battery Working Telephone

Centrally located storage battery is used in place of the individual dry cells required at each telephone station of a local-battery system.

The single common battery serves all the stations of the system

Common Battery System

System, telephone instruments could be simplified

Call connection was made easier

Reduce amount of supervision required

More economical to maintain than the dry cells of the local-battery system. Save labor for telecommunication companies

Advantages of Central Battery

power at the transmitter depends on length of line or condition of external wiring ( need high quality line construction )

Switchboard equipment in common battery systems is much more complex and expensive than local battery equipment performing comparable functions

it requires a greater time for installation and maintenance

Disadvantages

Telephone Circuit

TR- Transmitter (Mic) RCV- Receiver ( Speaker ) HK-Hook Switch D-Dialer

Induction Coil

Ringer ( Bell)

Receiver- Convert the received electrical signal to sound waves . Usually receiver is a speaker

Transmitter- Convert the sound waves to electrical signal for transmitting to the other end

usually a microphone

Hook Switch- Connect and disconnect the phone from the telephone line. It is placed across the line. it is mechanical switch.

Handset lifted - off-hook state – path completed Handset kept - on-hook state – path disconnected

Telephone Circuit Cont…

Ringer ( Bell )

Use to alert the user about incoming calls.

Originally an electromagnetic bell.

Placed directly across the line prior to the Hook switch.

When ringing current arrives it will go via the bell producing a

ringing tone.

Only AC current will flow via the ringer

The ringing voltage supplied by the central office is a sine wave of

approximately 90 Vrms at a frequency of about 20 Hz .

The capacitor: ◦ act as a DC blocking capacitor.◦ provides path for AC signals◦ provides protection for ringer circuit

Dialer – Use to dial relevant number

Dialing can be accomplished in two ways:◦ Pulse dialing uses interruptions in the current loop to

dial a number

◦ Dual-Tone Multi-Frequency dialing (DTMF) uses two tones for dialing. Also known as touch dialing

If the rotary dial or key pad in a telephone is designed to send electrical pulses the dialing is known as pulse dialing.

Number of pulses depend on the dialed number.

Pulse Dialing

Dialed Number Number of pulses

1 1

2 2

3 3

4 4

5 5

6 6

7 7

8 8

9 9

0 10

Pulse Period(100 ms)

Dialing Inter-Digit

Next Digit

700 ms

Make(Circuit Closed)

Break(Circuit Open)

Off-Hook

Digit 3

T

0.67 T 0.33 T

Digit 2

Inter digit gap

The dial pad is designed to send two fixed frequencies (dual tone multi frequencies)

higher dialing speeds

Greater accuracy

DTMF dialing

1

9 8 7

6 4

3 2

# * 0

5

941

852

770

697

1477 1336 1209

4 ro

ws

3 columns

Induction coil separate the circuit of the telephone set to two circuits, Transmitting circuit and receiving circuit,

Sidetone balancing Special transformers used to convert signals from

the four wires from the transmitter and receiver into a signal suitable for a single two wire local loop.

Induction coil

If a person is speaking to a transmitter, a small part of own speech signal will flow through the own receiver. So the talker will hear his own voice.

This effect is called SIDETONE

Intensity of side tone must be approximately the same as heard through the air for comfortable listening and speaking conditions

Side Tone

Disadvantage of1. No side tone

◦ People will shout or speak too loudly and it will throw instrument into uncontrolled oscillation

◦ As telephone set appears dead it is very difficult to user to detect whether call is dropped, ended or active.

2. Excessive side tone◦ Caller lowers his voice unnecessarily

Use anti side tone circuits

Anti side tone circuit accomplish this reduction by introducing a transformer coil between transmitting and receiving circuit and by splitting transmitter signal into two path

When the divided signals, having opposite polarities, meet at the transformer, they almost entirely cancel each other in crossing to the receiver circuit

How to reduce side tone

Typical Electronics telephone mainly comprise of a multifunctional IC and other components.

Line voltage regulator is one major component of Multifunctional IC

Voltage regulator converts the DC voltage received from the local loop to a constant- level dc supply voltage to operate the electronic components in the phone

Electronics Telephone

So in early years, subscribers in a town bought telephones and lines to each person they need to speak with

They setup the connection by their own

Depending on the telephone to which the call is to be made subscriber select the proper direct line (pair of wires) to the desired end before the call is made

Initial telephone network

Number of links required to fully interconnect stations Eg 1: Fully Interconnecting three telephones

A B

C

• Number of links per station = 3-1 =2

When multiplied by number of stations• Number of links for all stations = 2 * 3

= 6

A B

C

We can replace 2 wires by a single pair wire

Number of links required to fully interconnect stations

A B

C

A B

C

• Number of links per station = 3-1 =2

When multiplied by number of stations• Number of links for all stations = 2 * 3 = 6

Since we can replace two links by one• Total number of links required = 6/2 =3

Generalizing, for n number of stations

• Number of links per station = n-1 When multiplied by number of stations• Number of links for all stations = n * (n-1)

Since we can replace two links by one• Number of links required to fully interconnect n number

of stations (l) = ½[n (n-1)]

It is clear that when more than few users wished to communicate, then the directly connected network become◦ Cumbersome ◦ Disorganized◦ Expensive◦ Impractical

To avoid this issues centralized system with capability to connect telephone calls between users

So switching is needed for telephone systems

Why switching needed?

Establish an individual connection

On demand

From desired inlet

To desired outlet

As long as needed

Switching

Establishes a circuit (or connection) between nodes before the users communicate.

Steps are,

1. User requests for the channel.2. Exchange sets up the path.3. Conversing starts. 4. After finished the path will be cleared.

Circuit Switching

Public Switched Telephone Network.

Centralized network.

Adopts circuit switching technology which means it

establish connection between nodes before

communicate.

So PSTN can be considered as an example of a Circuit-

Switch network.

PSTN

Basic Call Progress

Off-hook close the circuit and allow current to flow. Switch detect this current flow and transmit s a dial tone

Dial digits will be received by switch as pulses or tones

Switch translate pulse or tones to a port address that connects to the telephone set of the called party. This connection could go directly to the telephone set or go through another switch.

Once switch connected to the called line switch sends a 20 Hz-90V signal to this line. This rings the called party phone. Switch sends a ring back tone to the caller. If called party phone is busy switch sends a busy tone to the caller

When called party lift the handset off-hook state start at the called party side. Current starts to flow to the switch.Switch detects current flow and complete the voice connection. Now voice communication can start

In order to establish a connection between two telephones switching center need some identity on to which phone the caller is calling

This identity determines both the route for the call and the charging rate

Therefore a Telephone numbering scheme is required to allocate a unique number to each customer

ITU recommend maximum number of digits for phone number should be 15 ( excluding prefix )

Numbering Scheme

Three general parts of national telephone number

National Destination Code ( NDC) – ( mostly area code )◦ Used to identify small geographical area within the country

where the telephone belongs to. Subscriber number ( SN )

◦ Used to identify different subscribers. National prefix

◦ National prefix is not dialed in international services◦ Not a part of ITU Numbering scheme◦ Used prior to area code and usually use ‘ 0 ‘ as national prefix

Local Call Dialing Plan

Local Call – A call taken without keying an area code.

National Call – A call taken keying an area code.

International Call – A call taken keying a country code

Type of calls

Maximum number of digits – 15

National prefix is not dialed when dialing international number

Country code ( 1 to 3 digits ) should be dialed

Usually international prefix ( 00 ) is used prior to the country code

Usually + sign indicate international prefix is required

International Number

The CCITT developed a International numbering plan under which, the world is divided up into eight zones:

1 - North America 2 - Africa 3 and 4- Europe 5 - Central and South America 6 - South Pacific 7 - USSR 8 - Far East 9 - Middle East and South-East Asia .

CCITT International Numbering Plan

Each country is assigned a Country Code (CC) beginning with the zone digit

Country Zone Country Code

Sri Lanka 9 94

Italy 3 9

China 8 86

Within each zone every country has been given a single, two digit or three digit code number, depending upon the number of countries within the particular zone

Generally, three digit codes have been allocated to smaller countries having few digits in their national numbering plans

Ex:◦ Within zone 9

Sri Lanka has the code ’94’ Nepal has the code ‘977’

Call diversion The exchange can be instructed to connect calls to a different number when the customer is unavailable.

Conference calls ( three way calling )The customer can instruct the exchange to connect a third party to a call that is already in progress.

Call Back (Ring Back)when the called party is busy, this feature allows the caller to be alerted by a ring to tell that the called party is free now

Value added exchange services

Call hold The call is held. That is, the microphone is

muted and called party hears only a music

Firstly used cross bar switches

Gradually these electromechanical switches were replaced by electronic switching systems

The cross-points are electronic devices (diode)

Switching

1

2

n 1 2 3

m

Cross Point

Advances made in computer technology were incorporated and led to the development of stored program control (SPC)

Carrying out the exchange functions through programs stored in the memory of a computer is called stored program control

This enables a digital computer to be used as a central control and perform different functions with the same hardware by executing different programs

Stored Programme Control ( SPC)

computerized common control, SPC exchanges can offer a wider range of facilities than earlier switching systems

Switching matrix Is the electromagnetic cross-points such as in crossbar switch

Central Processor Does all the processing

Call store (Memory) Is the temporary storage for incoming call information ready for

use on command from the central processor Also contains availability and status information of lines, trunks,

Program store (Memory) Provides the basic instructions for the central processor

Basic Functions of SPC

Functional Block Diagram of SPC

Line Scanning-Processor will examine state of each line via scanner (Detect off-hook as a new call start) and Identify dialed digits

Update call store records

Carry out instructions necessary◦ Checking the number received◦ Finding the free path in the switch block◦ Assigning it◦ Signaling to the switch control hardware that path is assigned

These functions are performed in accordance with set of programs held in programme store, with call store recodes

SPC setting up own exchange Call

When we originate a call calling circuit is associated with SPC equipments .

Processor examine each line via scanner SPC equipment detect off-hook as new call startsWhen user dial a number via calling circuit SPC

equipment will identify itCall store in SPC equipment will be updates based on

line scanning

Telephone traffic-Aggregation of telephone calls over a group of circuits or trunks with regards to duration of calls and number of calls

Telephone Traffic

Traffic intensity

Telephone exchanges are connected by trunks

Exchange and trunks are used in switching

Traffic intensity define size of switches and capacity of transmission links ( No of trunks)

Ex- if traffic is high, higher number of trunks are needed to switch all the traffic properly

Effect on telephone traffic on switching

Studying on telephone traffic is important as it is useful to decide optimum amount of exchange equipments and number of trunks for given cost for particular satisfaction