TSS-S3
Transcript of TSS-S3
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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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