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Categorizing switching

SPC: Stored program control


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Common Control System


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Principle of Common Control

Common control system was first introduced in crossbarexchanges. The common control can be traced in director system

facilitate the uniform numbering of subscribers in multi-exchangearea.

Uniform numbering is that to call a particular subscriber ,the

same number is dialed ,no matter from which exchange

the call is originated.

Consider multi-exchange network shown, it is not

fully connected network .

If a subscriber in exchange A wants to call a subscriber F , the callis routed at least three exchanges.

Two routes are possible

A - B - C - J - F and A - I - H - G - F


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Let 1457 be subscriber to be called in exchange F from exchange

. The called subs can be reached by dialing either of following

sequence .

For route A - B- C- J- F 01-04-03-01 1457

For route AIHGF 02-05-01-02 1457

If routing is done by exchange and uniform numbering scheme is

presented, the numbering may consist of1. An exchange identifier.

2. Subscriber line identifier within the exchange.

The exchange must have capable of receiving and storing the

dialed digits ,translating the exchange identifier in the routingdigits and transmitting the routing and subs line identifier digits t

switching network.

This function is perform by director system in Strowger Exchange


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DirectorIt involved a device (the director) which received dialled digits and automaticallytranslated them to route calls between exchanges in the city; in modern parlance adirector incorporated a register-translator and a digit store. Directors were applied tostep-by-step switching equipment;

Each subscriber was given a seven digit number where the first three digitscorresponded to the local exchange name, and were chosen to give the name ameaningful mnemonic. This was done by linking each number on the telephone dial to letters.12 ABC

3 DEF4 GHI5 JKL6 MN7 PRS

8 TUV9 WXY0 OQThus a subscriber inWimbledon could be allocated the numberWIMbledon 1234; thefirst three letters, written in capitals, indicated the code to be dialled. The actual trains ofpulses from the subscriber's dial would, of course, be 946 1234.

As the code (946 in this example) was the same from any telephone in the Londondirector area this uniformit is an exam le of a linked numberin scheme.
http://en.wikipedia.org/wiki/Mnemonichttp://en.wikipedia.org/wiki/Rotary_dialhttp://en.wikipedia.org/wiki/Wimbledon,_Londonhttp://en.wikipedia.org/wiki/Linked_numbering_schemehttp://en.wikipedia.org/wiki/Linked_numbering_schemehttp://en.wikipedia.org/wiki/Wimbledon,_Londonhttp://en.wikipedia.org/wiki/Rotary_dialhttp://en.wikipedia.org/wiki/Mnemonic


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As soon as the translated digits are transmitted ,the director is free to process an other call and is not

involve in maintaining the circuit for conversation. Call processing is independent of switching

network.

Functional diagram of common control system is

shown in , the control functions in switching systemplaced in four broad categories.

1. Event Monitoring

2. Call Processing 3. Charging

4. Operation and Maintenance.


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Common Control System: Event Monitoring Event occurring outside the exchange at the line units ,trunks ,

junctors and inter exchange signaling sender/receiver units are allmonitoring by the control system.

Typical events include call request and call release signals atline units.

The occurrence of events are signaled by operating relayswhich initiate control action.

The control subsystem may operates relays in the junctors senders/receivers and line units and command these units to performcertain functions.

Event monitoring may be distributed .

The line unit may initiate control action on occurrence of certainline events.

When subs goes off hook .the event is sensed , the calling

location is determined and dial tone is extended and register finderis activated to find free register .


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Identity of calling line is used to determine the category andclass of service to which subscriber belongs.

Call Processing A register is chosen which send out dial tone to

concerned subscriber .

As soon as initial digits (usually 2 to 5 ) which identifythe exchange are received in the register , they are passed on toinitial translator for processing ,similarly the register continue toreceive the remaining digits .

The initial translator determine the route for call

through network and decide whether call should putthrough or not.

It also determine charging method and rates applicable tosubscriber.


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Decision such as service information of subscriber as follow :

1. Call barring : A subscriber may be barred from making

certain calls e.g. STD or ISD barring.

2. Call Priority: When exchange or network is overloaded,

only calls from subscribers identified as priority calls may be

put through. 3. Call Charging : It is possible to define different charging

rules for different subscribers in same exchange.

4. Origin based Charging : Routing or destination of

certain calls may depend on geographical location of callingsubscribers.

5. No dialing calls: These calls are routed to

predetermined number without calling party to dial e.g. hot line

connection.


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Common Control System If the call is destined to a subscribers within the same

exchange, the digits are processed by final translator .

The translation to directory number to equipmentnumber take place at that stage .

The final translator may determine line unit to whichcall must be connected and category of called line,

for example

there may be no charge for emergency numbers or faultrepair service line.

Administration of Telephone Exchange

involves activities such as new subs line and trunk intoservice .

Modifying subscriber service entitlement and changingrouting plans based on network status.

Control subsystem may facilitates such administrativefunctions.


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Administration of Telephone Exchange involves

activities such as new subs line and trunk into service . Modifying subscriber service entitlement and

changing routing plans based on network status.

Control subsystem may facilitates such administrativefunctions.

Maintenance Activities

May includes supervision and proper functioning of

exchange equipment , subscribers lines and trunks.

It should be possible for maintenance personnel toaccess any line or trunk for performing tests and making

measurements of different line parameters.


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Crossbar Switch

More faster and sophisticated.

Lattice of crossed bars that make and break theswitch.

Electromechanically activated.

Magnets cause vertical and horizontal bars to crosseach other and make contacts at coordinates determinedby the number being called.

Each switch typically has either 100 or 200 cross points.

The lattice structure of cross bar switch has 10horizontal select bars and either 10 or 20 vertical holdbars.

The horizontal and vertical hold bars are activated by

magnets.


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Crossbar Switch

A crossbar switch (also known as cross point

switch or matrix switch ) is switch connectingmultiple inputs in to multiple outputs.

Crossbar switch was invented in 1917 by G.ABatulander.

The crossbar switch retain a set of contacts ateach crosspoint.

These are operated through horizontal and verticalbar magnets at the side of each switch.

The switch with N inlets and N outlets needs 2Nmagnets .

The magnets which operates horizontal bars calledselect magnets and which operates vertical bars are

called hold magnets or bridge magnets.


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Cross bar switching

The strowger switching system has been the basis oftelephone switching for almost 70 years since itsintroduction in 1889.

The major disadvantage of strowger system is itsdependence on moving parts and contacts that aresubject to wear and tear.

Mechanical systems require regular maintenance andadjustments and for this purpose they must be locatedin places that are easily and speedily accessible by skilledtechnicians.


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Cross bar switching

As the telephone network spread to remoteareas, it became necessary to devise switchingsystems that would require less maintenance and

little readjustment after installation.

Efforts in this direction led to the invention ofcrossbar switching systems.


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Principles of crossbar switching

The basic idea of crossbar switching system is to

provide a matrix of n x m sets of contacts with onlyn + m activators or less to select one of the n x msets of contacts.

This form of switching is also known as thecoordinate switching as the switching contacts arearranged in a xy-plane.

A diagrammatic representation of a cross pointswitching matrix is 3X3 Crossbar.


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Crossbar

horizontals

Crosspointdetail

verticals


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M1 M3M2

M3

M2

M1

A

C

B

wires

3x3 cross bar switching

11 22 3

4 5 6

7 8 9

A B C

Principles of crossbar s itching


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Principles of crossbar switching There is an array of horizontal and vertical wires shown by solid lines. A set of vertical and horizontal contact points are connected to these

wires.

The contact points form pairs, each pair consisting of a bank of three orfour horizontal and a corresponding bank of vertical contact points. A contact point pair acts as a cross point switch and remains separated

or open when not in use. The contact points are mechanically mounted (and electrically insulated)

on a set of horizontal and vertical bar shown as dotted lines. The bars in turn, are attached to a set of electromagnets. When an electromagnet, say in the horizontal direction, is energized, the

bar attached to it slightly rotates in such a way that the contact pointsattached to the bar move closer to its facing contact points but do not

actually make any contact. Now, if an electromagnet in the vertical direction is energized, the

corresponding bar rotates causing the contact points at the intersectionof the two bar to close.


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AA AB AC AD AE AF

BA BB BC BD BE BF

CA CB CC CD CE CF

DA DB DC DD DE DF

EA EB EC ED EE EF

FA FB FC FD FE FF

B

A

C

D

E

FA B C D E F

6x6 crossbar matrixoutlets

inlets

L t id th t bli h t f th f ll i


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Let us consider the establishment of the following

connections in sequence: A to C and B to E.

First the horizontal bar A is energized.

Then the vertical bar C is energizedThe crosspoint AC is latched and the conversation

between A and C can now proceed.

Suppose we now energize the horizontal bar of B to

establish the connection B-E, the crosspoint BC maylatch and B will be brought into the circuit of A-C. This is

prevented by an energizing sequence for latching the

crosspoints.

A crosspoint latches only if the horizontal bar isenergized first and then the vertical bar.

I d t t bli h th ti B E th ti l b


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In order to establish the connection B-E, the vertical bar

E need to be energized after the horizontal bar is

energized.

In this case the crosspoint AE may latch as the

horizontal bar A has already been energized for

establishing the the connection A-C.

This should also be avoided and is done by de-

energizing the horizontal bar A after the crosspoint is

latched and making a suitable arrangement such that

the latch is maintained even though the energisation inthe horizontal direction is withdrawn.

The crosspoint remains latched as long as the vertical

bar E remains energized.


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Procedure for establishing a connection in a

crossbar switch

1. Energize horizontal bar

2. Energize vertical bar

3. De-energize horizontal barOr

1. Energize vertical bar

2. Energize horizontal bar3. De-energize vertical bar

Crossbar Switch configuration


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Crossbar Switch configuration In a non blocking crossbar configuration, there are N2 switching elements

for N subscribers. When all the subscribers are engaged, only N/2 switches are actually used

to establish connections

A 1000 subscriber exchange would require 1 million crosspoint switches, Therefore ways and means have to be found to reduce the number of

switch contacts for a given number of subscribers.



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In the switching matrix, different switch points areused to establish a connection between 2 given

subscribers, depending upon who initiates the call. When subscriber C wishes to call B , crosspoint CB isenergized.

When B initiates the call to C, the switch BC is used.

In this case the crosspoint matrix reduces to adiagonal matrix with N2/2 switches.

By designing suitable control mechanism, only 1switch may be used to establish a connection betweentwo subscribers, irrespective of which one of theminitiates the call.


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Diagonal crosspoint matrix for 4

Subscribers

M tri for 4 s bscribers


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Matrix for 4 subscribers.

A diagonal connection matrix for 4 subscribers.

The crosspoints in the diagonal connect the inlets and outletof the same subscriber, which is not relevant.

Hence these are eliminated.

The number of crosspoints then reduces to N(N-1)/2

N(N-1)/2 is the number of links in a fully connectednetwork.

Energize horizontal bar first and then the vertical bar.

It is a non blocking configuration.



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N(N-1)/2 can be even very large number to handle practically.

The cross bar hardware may be reduced by connecting two subscribers to a singlbar and letting the bar turn both clockwise and anticlockwise directions and thusclosing two different crosspoints contacts.

With such an arrangement the number of crossbar reduces, but the number ofcrosspoint switches remains the same.

In blocking crossbar switches, the number of vertical bars is less than the

number of subscribers and determines the number of simultaneous calls that cabe put through the switch.

Consider the 8 x 3 Blocking switch .

Let a connection be required to be established between the subscribers A and B.

First the horizontal bar A is energized. Then one of the free vertical bar, say P, is

energized. The crosspoint AP latches. Now if we energize the horizontal bar B,BP will not be latched as the P vertical is energized before B was energized.

In order to be able to connect A to B, we need another vertical crossbar whichshould electrically correspond to the vertical bar P.

In this case , the bar P is associated with the same electrical wire as the bar P.


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P Q R P Q R

A

B

C

DE

F

G

H



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Crossbar Switch configuration When P is energized after B, the cross point BP is latched and a connection

between A and B is established. The sequence to be followed in establishingthe A-B circuit may be summarized as:

Energize horizontal A

Energize free vertical P

De- Energize horizontal A

Energize horizontal B

Energize vertical P

De- Energize horizontal B We thus see that in a blocking configuration we need to operate fourcrossbars to establish a connection.

The number of switches required is 2NK where N is the number ofsubscribers and K is the number of simultaneous circuits that can besupported.

Another alternative is to follow a different sequence of energisation such thata contact is established with the use of only one vertical crossbar instead oftwo as described above

Energize horizontal A and B

Energize vertical P

De- Energize horizontal A and B

Crossbar S itch config ration


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Both blocking and non blocking type crossbar switches can supporttransfer lines.

This is done by introducing additional vertical crossbars and crosspointswitches.

The switch shown in 1st case is a nonblocking locally and has twotransfer lines.

In 2nd case blocking both locally and externally .

The no: of crosspoints in first case N(N+L)

The no: of crosspoints in 2nd case N(2K+L)

N is no of subscribers , L is number of transfer lines, and K is thenumber of simultaneous calls that can be supported locally.


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Crosspoint technology

The hardware ofcrossbar consists ofcrosspoints switches.

Cost no of crosspointswitches

Two technologies areprevalent for crosspoint

design.

Crosspoints

Electromechanical Electronic

El h i l i


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Electromechanical crosspoints

Extensively used

Switching time 1-10ms Two types miniswitches and reed relays

Miniswitches are made up of precious metal like palladium(electrically quieter contacts) .Switching time is 8-10 ms

Reed relay eliminate mechanical motion of bars in a crossbarsystem, hence more operating life of the system. It comprisesof a pair of contacts made of a magnetic material sealed in aglass tube. Switching time is less than 1 ms.


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Space Division Switches

smaller connected crossbar switches.

Theses are called space division switches.

For example, if we had 16 lines, we couldhave four crossbar switches each taking 4lines.

The output of the crossbar switches can

themselves be fed into crossbar switches.


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Pros and Cons of Space Division

Switches

Because the space division switches use manysmaller crossbar switches,

if one fails it can be easily replaced withoutdisrupting all the calls.

it is possible for a Space Division Switch tobe jammed

i.e. a lot of calls had to go through one crossbarswitch, all its input or output lines may be usedup.

Settingk=2n-1will ensure this will not happen


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Cross point technology

crosspoint

Electromecha

nicalelectronic

Reed relay miniswitch

Electricallylatched

Magneticallylatched

bipolar Field effect

Thyristors transistor FET MOS

CMOS

Western Electric 100 Point six wire Type B crossbar


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Western Electric 100 Point six-wire Type B crossbar

switch

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