CHAPTER 1

Learning Outcome
CLO1-> Describe clearly the basic principle of a telephone network using the block diagram and the characteristic of telephone switching system using the concept of circuit-switch exchange.


Introduction to Telephone Network

1.     Basic principle of telephone network.
2.     Public switching telephone network.

Basic principle of telephone network.

•    Telephone and telephony system
•     Subscriber loop.
•     Tip & Ring 
•     Basic function of telephone set.
•     Telephone set compenent.

Public switching telephone network.

•     What is PSTN
•     Function of PSTN
•     Major component of telephone network.
•     Function of exchange.
•     Type important of exchange.
•     Type of exchange.
•     Telephony network architecture.
•     Basic all procedures.
•     Private branch exchange. (PBX)

TELEPHONY

* In the world "TELEPHONY" , TELE means at a distant end and to phone is to speak. Thus telephony means speaking to a distant end by using telephone.

TELEPHONE SYSTEMS

* Is principally line communication systems in which signal are transmitted on the line and received from the line.

TYPE OF TELEPHONE SYSTEM.

*Analog telephone system.

*Digital telephone systems.

*Cellular telephone systems.


ANALOG TELEPHONE SYSTEM
*The analog systems was the first telephone systems established worldwide.
*Currently, telephone systems in a lot of countries are still completely analog to telephony.
*Two physically conductors, forming telephone line
-run from the subscriber's premises to telephone exchange (the connection to called party is made by exchange)
*Then the caller has access, through telephone exchange.

DIGITAL TELEPHONE SYSTEM
*Most telephone calls today are really digital telephone calls.
*In digital telephone system, the two ends of the call are analog, and the middle section is digital.
-conversions from analog to digital (A/D), and back to analog (D/A)
-They are made in such a way that it is essentially impossible to determine that they were made at all.


SUBSCRIBER LOOP
*Also referred to as a LOCAL LOOP.
*Is the physical link or circuit that connect from the customer premises to the central office (local exchange)
                                                                             local loop=subscriberloop.

*A basic telephone set requires only two wires (one pair) from the telephone company to operate.

*The pair of wires connecting a subscriber to the closest telephone office is called LOCAL LOOP.

*One wire on the local loop is callled the Tip and the other the Ring.

*-48V was selected to minimiza electrolytic corrosion on the loop wires.

*-48V is used for supervisory signalling and to provide talk battery for microphone in the telephone set.

Tip & Ring

*Tip

*Is the ground side,

*Usually a green wire in the home of a single party customer.

Ring

*Is the battery (negative) side of a phone circuit.

*carries -48V of DC voltage when in an "idle" or "on hook" state.

*Usually a RED wire in the home of a single party customer.

**more reading at: http://en.wikipedia.org/wiki/Tip_and_ring

Basic function of telephone set

*Notify the subscriber when there are an incoming call.

*Provide signal to telephone network

*Convert speech energy to electrical energy in both side.

*Send the telephone number destination over the local loop.

*Regulate the amplitude of a speech signal of the calling person output of the line.

*Ensure that a small amount of the transmit signal is feedback to the speaker.

*Provide an open circuit and closed circuit to local loop.

*Provide call process signal between local exchange and subscriber.

Ringer Circuit

*An electromagnetic bell.

*Placed directly across the tip and ring of the local loop.

*Purpose: to alert the destination party of incoming calls.

*Audible tone must be loud enough to be heard from a reasonable distance.

ON/OFF Hook Circuit

*Sometimes called a switch hook.

*A simple single-throw, double-pole (STDP)

switch placed across the tip and ring.

*Mechanically connected to the telephone handset.

*Telephone is in use (off hook), switch is closed.

Equalizer Circuit

*Combination of passive component.

*Used to regulate the amplitude and frequency response of the voice signal.

*Help solve an important transmission problem:

-interdependence of transmitting and receiving efficiencies .

-Wide range of transmitter current caused by a variety of local loop cables with different resistance.

Speaker

*The receiver of the telephone 

*Convert electrical signal received from the local loop through the hybrid network.

*Typically enclosed in the handset of the telephone along with the microphone.

Microphone

*Transmitter

*Convert acoustical signal in the form of sound pressure wave to electrical signals.

*Connected to the local loop through the hybid network.

*Converts:-

-Acoustical energy-mechanical energy-electrical energy

Hybrid Network

*Special balanced transformer

*To convert a two-wire circuit into a four-wire circuit and vice versa

*Separated the transmitted signal from the received signals.

*To allow a small portion of the transmit signal to be returned to the receiver in the form of side tone