CMC ICT3207 Call Flow

Simple telephone Communication

 In the simplest form of a

telephone circuit, there is a one way comm. involving two entities, one receiving

(listening) & the other transmitting (talking)

 The microphone & the

earphone are the transducer elements of the telephone comm system

 Most commonly used

Simple telephone communication

 Carbon microphones do not produce high fidelity

signals, but give out strong electrical signals at

acceptable quality levels for telephone conversation.

 In CMs, a certain quantity of small carbon granules is

placed in a box.

 They conduct electricity and the resistance offered by

them depends upon the density with which they are packed.

 One side of the box cover is flexible & is mechanically

attached to a diaphragm

 When sound waves impinge on the diaphragm, it

Simple telephone communication

 If a voltage is applied to the microphone, they

current in the circuit varies according to the vibrations of the diaphragm

 The instantaneous resistance of the microphone is,

r_i=r₀-r

sinwt

r₀= quiescent resistance of the mic when

there is no speech signal

r = maximum variation in resistance

offered by the carbon granules, r<r₀

r_i= instantaneous resistance

-ve sign indicates when the carbon granules are

Simple telephone communication

 Ignoring impedances, the instantaneous current is given by,

i=V/(r₀-rsinwt)=I₀(1-msinwt)-1

where, I₀=V/r₀=quiescent current in the mic. m=r/r₀, m<1

Using binomial theorem,

i=I₀(1+msinwt+m2sin2_{wt+……….)}

Neglecting higher order terms, as m is sufficiently Small, i=I₀(1+msinwt)

 Thus, CGM acts as a modulator of the direct current I₀

which is analogous to the carrier wave in AM systems & m is equivalent to modulation index. Higher order terms

represent higher order harmonic distortions, so m should be kept sufficiently low.

Simple telephone communication



The inductor acts as a high

impedance element for voice

frequency signals but permits

the dc from the battery to flow

to the microphone & the

receiver



The voice frequency signals

The earphone

 The earphone is usually an electromagnet with a

magnetic diaphragm positioned such that there is an air gap between it and the poles of the EM.

 When the EM is energized by passing a current, a

force is exerted on the diaphragm.

 The voice frequency current from the mic causes

variation in the force exerted by the EM, thus

vibrating the diaphragm and producing sound waves.

 For faithful reproduction of the signals, the

diaphragm should be displaced in one direction from its unstressed position

 The quiescent current provides this bias. In some

The earphone

 The instantaneous flux linking the poles & the

diaphragm is given by, Ф_i= Ф₀+Ф

sinwt

where,

Ф = maximum amplitude of flux

variation, Ф< Ф₀

Ф_i= instantaneous flux

This equation assumes that the vibrations of the

The earphone

 The instantaneous force exerted on the diaphragm

is proportional to the square of the instantaneous flux linking the path. So,

F=K(Ф₀+ Ф

sinwt

=K(_Ф

02+Ф2

sin

wt

sinwt

When(Ф/Ф₀)<<1, neglecting second order term,

F=KФ₀2_(1+K

1I0

sinwt

Where, I₀

sinwt

A half-duplex telephone circuit

 The form of comm where the

information transfer takes place both ways but not

simultaneously is known as half-duplex comm.

 Speech of A is heard by B as

well as in A’s own earphone.

 This audio signal heard at

the generating end is called sidetone.

 A certain amount of sidetone

Half-duplex telephone circuit

 Human speech & hearing system is a feedback

system in which the volume of speech is

automatically adjusted, based on the sidetone heard by the ear.

 If no sidetone is present, a person tends to shout,

and if too much of sidetone is present, there is a tendency to reduce the speech to a very low level.

 In the previous circuit, the entire speech intensity is

heard as sidetone, which is not desirable.

 Lets see a circuit where a small level of sidetone

Half-duplex telephone circuit with sidetone coupling

 Z_b is equal to the

impedance seen by the circuit to the right of AA’_.

 The speech signal from M

divides equally between P & Q

 Since they are in opposite

direction, so a small voltage is induced as sidetone

 But the speech from other

Basics of a switching system

 Major component of a SS

or exchange is the set of input and output circuits called inlets & outlets

 Primary function of a SS is

to establish an electrical path between a given inlet-outlet pair

 The hardware used for

establishing such a

connection is called the switching matrix or the

Basics of a switching system

 When N=M, the

switching network is called a symmetric network

 The inlets/outlets may be

connected to local subscriber lines or to trunks from/to other exchanges

 In this fig, four types of

Basics of a switching system

 When all the inlets/outlets are

connected to the subscriber lines, the logical connection appears as shown in the fig.

 In this case, the output lines are folded back to the input, hence it is called a folded network.

 In a folded network with N subscribers, there can be a

maximum of N/2 simultaneous calls  If the switching network is

Basics of a SS

 In a NBN, the subscriber is not denied a connection for want of switching resources, i.e., a calling subscriber will always be able to establish a connection to the called subscriber as long as he is free.

 In reality, it rarely happens that all the possible conversations takes place simultaneously.

 As a result, it is economical to design a switching network that has as many simultaneous switching paths as the

average number of conversations expected.

 In that case, there is a certain probability that a subscriber is denied connection for want of free switching paths.

 In a blocking network, the number of simultaneous switching paths is less than the maximum no. of simultaneous

Basics of a SS

 A good design generally ensures a low blocking probability  In a switching network , all the inlet/outlet connections may

be used for inter exchange transmission. In such a case it is known as transit exchange

 A switching network of this kind is known as nonfolded network

Control subsystem of the SS

 Though the switching networks provides the

switching path, it is the control subsystem that actually establishes the path.

 SN doesn’t distinguishes between inlets/outlets that

are connected to the subscribers or to the trunks.

 Control subsystem establishes a connection based

on the signalling information received on the inlet lines and sends out signalling information to the subscriber and other exchanges connected to the outgoing trunks

 A SS provides for three different forms of signalling:

Control subsystem of the SS

 A SS is composed of

elements that

perform switching, control and signalling functions

 Sub. Lines are

terminated at the SLI circuits & trunks at the TI circuits.

 There are some

service lines used for maintenance and

Control subsystem of the SS

 Junctor circuits

implement a folded

connection for the subs. & the service circuits.

 Line scanning units sense Signaling Information (SI) from the respective lines & distributor units sends SI on the respective lines  Console permits

interaction with the SS for maintenance and

Basics of a SS

 In some SS, the control subsystem may be an

integral part of the switching network itself which is known as direct control SS.

 Those systems in which the control subsystem is

outside the switching network are known as common control switching systems

 Strowger exchanges are usually direct control

systems, whereas crossbar and electronic exchanges are common control systems

 All SPC systems are common control systems

 Common control is also known as indirect control or

Manual SS

 Microphone requires to be energized in order to

produce electrical signals corresponding to the speech waveform.

 Early systems in two categories:

 LB exchanges/magneto exchange

A subscriber needed to rotate a handle to generate the required alternating current to operate indicators at the exchange

 CB exchanges

Manual SS

 The system consists of one or more switchboards manned by operators

 SL are terminated on jacks  One jack for every subscriber  Off-hook signal operates the

lamp relay & the indicator corresponding the subscriber lights up.

 The operator establishes contact with the subscriber by connecting the head set to the subscriber line via the

Manual SS

 On being told the called subscriber number, the

operator verifies whether he is free, & if so sends a ringing current.

 If the called party is busy, the calling subscriber is

informed

 When the called party answers, his indicator lamp

lights up

 Then the operator establishes a connection by

plugging in the cord pair to the called party jack.

 The operator has full control of the connection.

 He enables the signalling systems, perform

Manual SS

 A user experiences blocking on account of the unavailability of

the switching circuits or the control system circuit.

 The subscriber switchboards at the exchange may be of two

types:

 Single termination/ nonmultiple switchboards

Manual SS

 The need for two operators per call is avoided in the multitermination switchboard scheme.

 A single operator can establish a call between any two subscribers

 Two drawbacks:

 Total no. of connection in the system increases considerably reducing

the reliability

 Terminating all the subscribers in all the boards, such that the

Manual SS

 The problem is solved by

terminating half the

number of subscribers in alternate switchboards in a cyclic manner and