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Crossbar Switching

Major disadvantage of Strowger System is its dependence on

movin arts and contacts

That are subject to wear and tear

,

necessary to use such SS that would require lesser maintenance

and little readjustment after installation

This leads to the invention of Crossbar SS (CBSS)

The are desi ned usin the common control conce t

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r nc p e o ommon ontro :

Follows the Director System

It facilitate uniform numbering of subscriber in a multi-exchange area

like a big city

Provides routing of calls from one exchange to another via some

intermediate exchange

Uniform Numbering o ca a part cu ar su scr er, t e same num er s a e

No matter from which exchange the call is originating

Example

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T us, from Exc ange A, any ca e su scri er on Exc ange F

(with no. 1457) can be reached by dialing either of the

followin two number se uences:

For route A-B-C-J-F 01-04-03-01-1457For route A-I-H-G-F 02-05-01-02-1457

The difficulties are now obvious:

Identification number of a subscriber is route dependent

A user must have the knowledge of the topology of the network

Depending on from which exchange the call originates, the number

and its size vary for the same called subscriber

These difficulties can be overcome if the routing is done by

the exchange

And a uniform numbering scheme is followed

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A number may now consist of two parts: An exchange identifier, and

A subscriber line identifier, within the exchange

An exchange must have the capability of

receiving and storing the digits dialled, and

translatin the exchan e identifier into routin di its and

transmitting the routing and the subscriber line identifier digits to SN

As soon as the translated di its are transmitted, the director is

free to process another call

Call processing takes place independent of the SN

A user is assigned a logical number, independent of thephysical line number used to establish the connection

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A t e a ove are fun amenta features of a common contro

system. The control function in a SS may be placed under

four broad cate ories:

Event monitoring Call processing

Operation and maintenance

Figure

When a subscriber goes off-hook, the event is sensed, the

calling location is determined, and marked for dial tone

Identity of the calling line is used to determine line category

and the class of service to which the subscriber belongs

u se a ng or mu t - requency a ng

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- ,exchange, are received in the register

They are passed on to the initial translator for processing

Simultaneously, the register continues to receive the remaining digits

The initial translator determines the route for the call through the

networ an ec es w et er a ca s ou e put t roug or no

It also determines the charging method and the rates applicable to the

subscriber

Such decisions are based on the following class of service:

Call barring

Call priority Call charging

Ori in based routin

No dialing calls

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If a call is destined to a number in another exchange

The initial translator generates the required routing digits and passes

them to the Register sender

which transmit the information over the trunk to the external exchange

If a call is destined to a number in same exchange The digits are processed by the Final translator

which determines the line unit to which a call must be connected and the

category of the called line

of the common control subsystem

Path findin ma be carried out at the level of

the common control unit (map-in-memory)

or, the SN (map-in-network)

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ouc one a e ep one:

In a rotary dial telephone, it takes about 12 seconds to dial a

- g t num er

But the subscriber requires a faster dialing rate

Using the common control in CBSS, a higher dialing rate is possible

The rotary dial is replaced by a push button keyword

Touching a button generates a tone which is a combination

of two fre uencies

one from the lower band and other from the upper band

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es gn ons erat ons:

End-to-end signaling is a desirable feature, and is possible only

t e s gna ng s n t e vo ce requency an

so that the signaling information can be transmitted to any point in the

telephone network to which voice can be transmitted

This generates the problem oftalk-off c means t at t e vo ce s gna may e m sta en or touc tone

signals, and

unwanted control actions may occur

not er pro em s t at vo ce s gna may nter ere w t t e touc tonesignaling, if the calling subscriber talk while signaling is attempted

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-1. Choice of code

2. Band separation

3. Choice of frequencies

4. Choice of power levels

5. Si nalin duration

Since, two frequencies are mixed from a set of seven or eight

fre uencies

The scheme is known as dual tone multi-frequency (DTMF) signaling

Band separation of the two frequencies has the important

advantage is that before attempting to determine the two specific frequencies at the

receiver end band filterin can be used to se arate the fre uenc

group

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The limiters accentuate differences in levels between the

components of an incoming dual-frequency signal

If one frequency component is relatively strong, the output of the limiter

eaks with the stron er si nal, and

the weaker signal is further attenuated

If both the signals have similar strength, the limiter output is much below

the full out ut and

Neither signal dominates at the output

The selective circuit is designed to recognize a signal when itfalls within the narrow passband, and

has an amplitude more than a threshold value.

The limiter and the selective circuits together reduce theprobability of mistaking the voice signal to be touch tone signal.

Because the voice si nal usuall have multi-fre uenc com onents with

similar amplitudes, and hence the limiter does not produce a full output

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