Chapter 2

The Telecommunication Network Overview

• The basic purpose of a telecommunications network is to transmit user information in any form to another user of the network

• users of public networks, for example, a telephone network, are called subscribers

• Three technologies are needed for communication through the network: transmission, switching and signaling

Transmission

• Transmission is the process of transporting information between end points of a system or a network

• uses four basic media for information transfer from one point to another: copper cables, optical fiber cables, radio waves and free space optics (IR).

Switching• In principle, all telephones could still be

connected to each other by cables as they were in the very beginning of the history of telephony.

• as the number of telephones grew, operators soon noticed that it was necessary to switch signals from one wire to another---only a few cable connections were needed between exchanges because the number of simultaneously ongoing calls is much smaller than the number of telephones

Signaling• Signaling is the mechanism that allows

network entities to establish, maintain, and terminate sessions in a network

• Signaling is carried out with the help of specific signals or messages that indicate to the other end what is requested of it by this connection

• Signaling is naturally needed between exchanges as well because most calls have to be connected via more than just one exchange

• Off-hook condition: The exchange notices that the subscriber has raised the telephone hook and gives a dial tone to the subscriber.

• Dial: The subscriber dials digits and they are received by the exchange.

• On-hook condition: The exchange notices that the subscriber has finished the call (subscriber loop is disconnected), clears the connection, and stops billing.

Operation of a Conventional Telephone

• The telephone is a familiar end instrument in telecommunication system

• The telephone is basically a transducer• The transmitter telephone converts sound

energy into electrical energy• The receiver telephone converts electrical

energy into sound waves

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Handset• Transmitter/Microphone• It consists of a box containing a powder of small

carbon granules• One side of the enclosure is flexible and is

mechanically attached to a diaphragm on which sound wave impinges

• The diaphragm causes the carbon granules to compress or allow them to expand

• Consequently, the resistance of the carbon granules decreases or increases in the box

• The carbon granules conduct electricity and the resistance offered by them is dependent upon the density with which they are packed

• If a voltage is applied to the microphone, the current in the circuit varies according to the vibrations of the diaphragm

• The varying electrical signal is similar to the varying sound signal

• Microphone functions like amplitude modulator

Why carbon microphones in telephones?

• Carbon mikes were the first microphones and consisted of a small button of carbon powder connected to a metal diaphragm

• When sound flexed the diaphragm, the carbon grains changed their electrical resistance

• When a voltage source is applied between the microphone wires , a variable current is generated

• This is how the first telephones were constructed and many telephones to this day still use the idea

Microphone as amplitude modulators

• When the sound waves impinges on the diaphragm, the instantaneous resistance is given by,

• ri = rq- rm sin wt• Where, ri = instantaneous resistance

rq = quiescent resistance (resistance when there is no speech signal)rm= maximum variation in resistance offered by the carbon granules, rm < rq

W=2f(The negative sign indicates the decrease in resistance

when the carbon granules are compressed and vice versa.)

• At ideal condition, the instantaneous current in the microphone is given by:

i =

i =

Where, m = rm/rq; m<1

Approximation

• Using binomial theorem and ignoring higher order terms,

• I = iq (1 + m sin wt)

[This equation resembles the amplitude modulation equation.]

Receiver/ Earphone

• The varying signal from hand set A ( caller ) is coupled by wires to a receiver of hand set B (called subscriber)

• The receiver is an electromagnet with accompanying magnetic diaphragm

• The elctromagnet usually have two coils of about 100 turns with nominal resistance of about 400 ohms.

• The receiver diaphragm must always be displaced in one direction from its unstressed position.

• It must be positioned with an air gap between it and the poles of the electromagnet.

• The diaphragm is made of cobalt iron and it is slightly conical shaped near the ear for uniform pressure distribution and hence the sound.

Earphone as sound detector• The variations in current through the coils

wound on the electromagnet results a change in flux.

• This instantaneous flux linking the poles of the elctromagnet and the diaphragm is given by:

Where: = instantaneous flux = flux due to quiescent current = maximum amplitude of flux variation

• The instantaneous force exerted on the diaphragm is proportional to the square of the instantaneous flux linking the path.

F = K ( + sin wt)2