Switching

1. INTRODUCTION TO SWITCHING
Chapters

2. Chapters
• 1 Introduction
• 2 Telephone Working
• 3 Human speech and its transmission in
Telephony
• 4 Essential Apparatus
• 5 Telephone Lines
• 6 The Dial
• 7 Push button Telephone

3. Chapters Cont.
• 8 History
• 9 Touch-tone
• 10 Features
• 11 Principal of automatic switching
• 12 Selectors
• 13 The Uniselector
• 14 Trunking Diagram
• 15 Number scheme in Automatic Exchanges
• 16 A simple 10 Line Exchange

4. 1. Introduction
Telecommunication can be subdivided into three
specialities:
External Plant (Access Network)
Switching
Transmission
Switching takes the centre stage
Switching is the connection of an input circuit with an
output circuit going to the destination requested by
the customer.

5. 2. Telephone Working
• Telephone provides a means of sending information through human
speeches when required between two persons situated at a distance
apart and in line telephony the information is send through the medium
of line conductors between them.
• When any customer speaks into the transmitter of his telephone set, his
speech is converted into oscillatory electrical current and this current is
send through lines to the other customer’s telephone set where it is
reconverted into speech sounds by the receiver. Two separate line
conductors are necessary for transmission of telephone signals for every
customer and hence there are as many pairs of line conductors as the
total number of customers. All these line conductors from different
customers’ premises are connected at a place known as the telephone
Exchange.
• Speech currents carry the major information between customers, but
other types of currents known as signalling currents are also necessary in
a telephone system to carry information between customers.

6. 3. Human Speech and its Transmission
in Telephone
• Human speeches consists of a large number of
frequency components of different values from
about 300Herzts to 3400Herzts having different
amplitudes with different phase relations
between them.
• In telephone Transmission there are three
distinct parts namely:
Conversion of Speech
Transmmision of Speech currents
Conversion of Electrical currents

7. 3.Human Speech and its Transmission
in Telephony
• The telephone receiver must be such that the
conversion from speech sounds into electrical currents
and vice versa must be perfect i.e. there should be no
frequency distortions, no amplitude distortions and no
phase distortions in the process.
• Further during transmission through lines none of
these distortions should be introduced. If none of
these distortions are introduced at any stage, then
only we can expect to get an exact reproduction of
speech sound at the receiving end. That is why
superior types of apparatus are necessary for use as
transmitters and receivers and they are designed from
these points of view.

8. Essential Apparatus
At the customer’s end:
Telephone Handset comprising of:
 Transmitter
 Receiver
 The bell

9. Essential Apparatus
At the Exchange
 MDF (Main Distribution Frame)
 SLC (Subscriber’s Line Circuit)

10. 5. Telephone Lines
In telephony two lines are necessary for sending
speech currents.
Further, in ordinary telephony each customer has
his own separate pair of lines extended from the
exchange to his premises. In very big exchange,
therefore, a very large number of lines all
connected to the exchange and this is done with
the help of telephone cables each having a large
number of pairs of insulated conductors.
Telephone cables may be laid underground (UG) or
hung in air (ADC).

11. 6. The Dial
Fig 1

12. The Dial
 The dial therefore is a form of automatic transmitter
by means of which the signals corresponding to any
digit from 0 to 9 may be transmitted to line
automatically under the control of the customer.
 The number of pulses being equal to the number
dialled, i.e. one to nine for the digits 1 to 9 and ten
for 0.
 The standard duration of a break period 66 2/3ms,
that of the make period is 33 1/3ms.

13. 7. Push-button Telephone
• The push-button telephone is a telephone
that has push-buttons or keys, and which
eventually replaced rotary dial telephones
that were first used in 1891.
• The first publicly-available push-button
telephone was released in 1963, by the Bell
System. They were first made available in the
towns of Carnegie and Greensburg,
Pennsylvania.

14. 8. History
• In the 1950s, AT&T conducted extensive
studies, and concluded that push-button
dialing was much faster than rotary dialing.
On November 18, 1963, the first electronic
push-button system, with Touch-Tone dialing,
was offered by Bell Telephones to AT&T
customers.

15. History
• The first push-button
phone, the Western
1500, had only ten
buttons. A twelve-
button model featuring
the asterisk/star (*) and
pound/hash (#) keys
was released soon
afterwards, replacing
the earlier model.
Fig 2

16. 9.Touch-tone
 Utilises Dual-tone multi-frequency signalling.
 Touch-tone technology was first made
available in 1963.
 Has additional keys to accommodate various
additional services and customer controlled
calling features.

17. 10. Features
 Last Number redial (LNR)
 Storage of commonly called numbers
 Caller Identification
 Retrieval of information and data or code and
PIN entry.
 Internet based telecommunications such as VoIP
 Mute
 Tone or Pulse Dialling
 Volume adjustments

18. 11. Principles of Automatic
Switching
In automatic telephone exchanges the switching operations are
performed under control signals send from the user’s installation.
In strowger type automatic exchanges, switching is done using
wiper-type switches, usually called selectors. These are operated as
appropriate under control either directly or indirectly of the pulses
send out from the calling customer’s dial.
When selectors are pulse operated, the drive is provided by use of
a pawl and ratchet arrangement. The ratchet is driven to by the
pawl which is operated by an electromagnet, the ratchet being
moved forward by one tooth or step for each operation of an
electromagnet armature.

19. Principles of Automatic Switching
Cont.
For this reason, automatic switching systems
operating on this principle are known as step-
by-step selection.
In a four digit number, the first digit indicates
thousands, the second digit hundreds, the
third digit tens, and the fourth digit units.

20. Principles of Automatic Switching
Cont.
Suppose, for example, in an exchange having
several thousand customers a call is to be made
to a customer having the number 3106. When
the calling customer dials 3, the thousands group
selector will operate to connect the caller’s line
through to the 3000-3999 group of customers.
The following digit 1 will cause the hundreds
group selectors to connect the caller to the 3100-
3199 group of customers and the tens and units
digits 0 and 6 will select customer 3106 out of
this group.

21. 12. Selectors
The basic selector of the step-by-step system is
known as a two motion selector. It has a set of
contact arms called wipers which are fixed to a
vertical carriage which can be raised vertically
and can also be rotated. The carriage may be
raised from 1 to 10 steps. The wipers may in
effect, therefore take up any one of 100 positions
and the arrangements allow the circuit
connected to the wipers to be connected to any
one of the 100 outlets. Generally there are three
contacts per circuit corresponding to negative,
positive and p wire in an automatic exchange.

22. 13. The Uniselector
This selector has only
rotary motion. The bank
contacts are arranged in
an arc, there being from
3 to 10 arcs, depending
on the requirements.
Commonly there are 25
contacts in each arc,
but for explanatory
purposes few may be
shown
Fig 3

23. 14. Trunking Diagram
Fig 4

24. Trunking Diagram
This is a system of straight line
diagram showing selected
mechanism and their
interconnections. Only a few
customers are used for simplicity.

25. 15. Numbering Scheme in
Automatic Exchanges
In automatic telephone systems, each customer
is given a number. In 10 line exchange, each
customer is given a single digit number; 1, 2, 3, 4,
5, 6, 7, 8, 9, and 0. In 100 line exchange, each
customer is given a two digit number from 11 to
00. In a 1000 line exchange, each customer is
given a 3 digit number from 111 to 000. If the
capacity of the exchange is more than 1000 up-to
10 000, each customer is given a 4 digit number
from 1111 to 0000.

26. 16. A simple 10 Line Exchange
Trunking Diagram
Fig 5

27. Trunking Diagram
The trunking diagram of a 10 line auto exchange
is shown in Fig. 5. each customer is given a single
digit number from 1 to 0. Each telephone is
terminated at the exchange on a relay group
which contains all the necessary apparatus for
the control of the switch mechanisms and for
various other functions such as testing, switching,
return of tones etc. The similar numbered bank
contacts of all the selectors are inter-connected
and also returned to the customer’s line.

28. Trunking Diagram
• When the customer No. 1 wants to call customer No. 3
he dials number 9 in his dial switch. The dial produces
3 pulses and these pulses operate the wiper of the
selector connected to customer No.1. The wiper
moves over to bank contact No. 3. You will notice this
bank contact No. 3 already connected to customer No.
3. Thus the connection is established.
• In bigger exchanges we use many selectors and they
are grouped as first group selectors, second group
selectors etc. But the principle of switching is the same
as explained before.