Crossbar switchingBy :Engr.Nuzhat Madina

Cross bar switching The strowger switching system has been the basis of telephone switching for almost 70 years since its introduction in 1889. The major disadvantage of strowger system is its dependence on moving parts and contacts that are subject to wear and tear. Mechanical systems require regular maintenance and adjustments and for this purpose they must be located in places that are easily and speedily accessible by skilled technicians.

Cross bar switchingAs the telephone network spread to remote areas, it became necessary to devise switching systems that would require less maintenance and little readjustment after installation. Efforts in this direction led to the invention of crossbar switching systems.

Principles of crossbar switchingThe basic idea of crossbar switching system is to provide a matrix of n x m sets of contacts with only n + m activators or less to select one of the n x m sets of contacts. This form of switching is also known as the coordinate switching as the switching contacts are arranged in a xy-plane. A diagrammatic representation of a cross point switching matrix is shown in fig 3.6

M1 wires A 1 1 B 4 C 7

M2

M3

22

3

M1 6

5

M2

8

9 M3

A

B

C

Fig.3.6 3x3 cross bar switching

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 or four 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.

Principles of crossbar switchingWhen an electromagnet, say in the horizontal direction, is energized, the bar attached to it slightly rotates in such a way that the contact points attached 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 intersection of the two bar to close.

AA

AB

AC

AD

AE

AF

ABA BB BC BD BE BF

BCA CB CC CD CE CF

C inletsDA DB DC DD DE DF

DEA EB EC ED EE EF

EFA FB FC FD FE FF

F A B C D outlets Fig 3.7 6x6 crossbar matrix E F

Procedure for establishing a connection in a cross bar switchEnergize horizontal bar Energize vertical bar De-energize horizontal bar

Energize vertical bar Energize horizontal bar De-energize vertical bar

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. By designing suitable control mechanism, only 1 switch may be used to establish a connection between two subscribers, irrespective of which one of them initiates the call. In this case the crosspoint matrix reduces to a diagonal matrix with N2/2 switches. A diagonal connection matrix for 4 subscribers is shown in fig 3,8. The crosspoints in the diagonal connect the inlets and outlet of the same subscriber. This is irrelevant . Hence these are eliminated. The number of crosspoints then reduces to N(N-1)/2 It is a fully connected network. Energize horizotal bar first and then the vertical bar. It is a non blocking configuration.

Crossbar Switch configuration 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 single bar and letting the bar turn both clockwise and anticlockwise directions and thus closing two different crosspoints contacts. With such an arrangement the number of crossbar reduces, but the number of crosspoint switches remains the same. In blocking crossbar switches, the number of vertical bars is less than the number of number of subscribers and determines the number of simultaneous calls that can be put through the switch Consider the 8 x 3 switch shown in figure 3.9. 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 which should electrically correspond to the vertical bar P. In this case , the bar P is associated with the same electrical wire as the bar P.

P A B C D E F G H

Q

R

P

Q

R

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 establishing the 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 four crossbars to establish a connection. The number of switches required is 2NK where N is the number of subscribers and K is the number of simultaneous circuits that can be supported. Another alternative is to follow a different sequence of energisation such that a contact is established with the use of only one vertical crossbar instead of two as described above Energize horizontal A and B Energize vertical P De- Energize horizontal A and B

Crossbar Switch configuration Both blocking and non blocking type crossbar switches can support transfer lines. This is done by introducing additional vertical crossbars and crosspoint switches as shown in fig 3.10 The switch shown in figure 3.10(a) is a nonblocking locally and has two transfer lines 3.10(b) 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 the number of simultaneous calls that can be supported locally.

Crosspoint technology The hardware of crossbar consists of crosspoints switches. Cost no of crosspoint switches Two technologies are prevalent for crosspoint design.

Crosspoints

Electromechanical

Electronic

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 crossbar system, hence more operating life of the system. It comprises of a pair of contacts made of a magnetic material sealed in a glass tube. Switching time is less than 1 ms.

Cross point technologycrosspoint

Electromecha nical

electronic

Reed relay

miniswitch

bipolar

Field effect

Electrically latched

Magnetically latched

Thyristors

transistor

FET

MOS

CMOS