Central Office Operation Part3

8/9/2019 Central Office Operation Part3

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Central Office Operation:

Switching Systems

Engr. Edelito A. Handig


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Introduction

The original 1876 A.G.Bell installationswere point-to-point hard wired.

If there are 5 subscribers, how manylines are required?

If 10 subscribers?

No. of lines = n(n-1)/2


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Introduction

The two basic technologies in theengineering of a telephone network:

Switching

Transmission


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Introduction

Switching is


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Switching

User Requirements:

1. Capability of communicating with all

other subscribers.

2. Connection time should be small.

3. Completion of a call should be high.

4. Guaranteed conversation privacy.

5. System must be available at any time.


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Basic Function ofaSwitch

1. Interconnection

2. Control

3. Alerting

4. Attending

5. Information receiving

6. Information transmitting

7. Busy testing

8. Supervisory


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Switching Control

1. Manual Control

2. Progressive Control

3. Common Control 4. Stored Program Control


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Basis ofSwitching Control

It is the Telephone Number

Important functions:

1. It routes the call2. It activates the necessary

equipment for proper call charging.


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Manual Control System

Early telephone switchboards were

operated manually using a jack for each

line and two plugs on a long flexible wire,

called a cord pair, for making theconnection.


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Manual Control System


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Progressive Control System

Definition:

A call is progressively carried out to the

desired terminal under the direct control

of the subscribers initiated pulses.


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Step by Step or Strowger Switch

It is a mechanism substitute for the

biased operator that could complete a

connection under direct control of thecalling party.


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Step by Step or Strowger Switch


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Strowger Switch Operation


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The Strowger switching system has thefollowing significant limitations:

1. Possible to blocked a call, even

though the called line is free.2. Not possible to use tone dialing

(DTMF) telephones directly.

3. Low reliability.

4. Hard to maintain.

5. Difficult to make switching changes.


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Common Control System

Definition:

It is defined as providing a means of

control of the interconnecting switch

network, first identifying the input andoutput of the terminals of the network that

are free and then establishing a path

between them.

This implies a busy-test of the path

before setting the path up.


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Crossbar Switch

As the name implies, it depends on the

crossing or intersection of two points to

make a connection.The crossbar system utilizes a switching

matrix, which is externally managed by

common control, to route telephone calls.


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Crossbar Switch


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Crossbar Switch Operation

It takes in the dialed digits, and then

sets up the path through the switching

matrix according to hard-wired rules.Control signals from transmission lines

are detected and used to control the matrix

to connect the proper lines for the path

from the calling telephone to the called

telephone. The crossbar moved and

electrical contacts were made mechanically.


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Crossbar Switch advantages:1. It provides for variations in the handling

of local and long-distance calls, for

choosing an alternate route for a call incase the first route chosen is busy, and

2. for trying the call again automatically in

case of blocking or faults in the switchingpath.


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ElectromechanicalVersion

It is a relay type of mechanism that has

vertical and horizontal selector bars

operated by electromagnets that closerelay contacts to provide the matrix

interconnection.


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ReedRelay Version

The reed relay is a small, glass-

encapsulated, electromechanical switching

device. They open or close depending onthe polarity of the electrical impulses

input.


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Stored Program Control System

DefinitionThe switching procedures are

controlled by means of a computer and

program software that monitors all centraloffice activities, sees what service needs

are, and initiates action to fulfill the need.


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There are four basic functional elementsof an SPC switching system:

1. Switching matrix

2. Call-store (memory),3. Program store (memory), and

4. Central processor


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Advantages:

1. Rerouting and reallocation of trunks

2. Traffic statistics

3. Renumbering lines

4. Changes in subscriber class

5.E

xchange status6. Fault finding

7. Charge records


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T

hank You.

Documents

Central Office Operation Part3