Q. Explain crossbar exchange, with all call processing steps and diagrams.

Ans:

The basic concept of crossbar switching 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 called coordinate switching as switching contacts are arranged in a x-y-plane. A diagrammatic representation of a cross point switching matrix is displayed in Figure. There is an array of vertical and horizontal wires displayed by solid lines. A set of vertical and horizontal contact points are connected to these wires. Contact points form pairs, each pair comprising a bank of three or four horizontal and a corresponding bank of vertical contact points. A contact point pair acts just like a cross point switch and remains separated or open when not in use. Contact points are mechanically mounted (and electrically insulated) on a set of vertical and horizontal bars displayed as dotted lines. The bars, in turn, are attached to a set of electromagnets.

When an electromagnetic energized, bar attached to it slightly rotates in such a way  that  contact  points attached  to  bar move  closer  to  its  facing contact points  but  don't  actually  make  any  contact.  Now  if an  electromagnet  in  the vertical  direction  is  energized,  corresponding  bar  rotates  causing  the  contact points at the intersection of the two bars to close. This happens as the contact point's move towards each other. For example if electromagnets M2 and M3' are energized, a contact is established at cross point 6 such that subscriber B is connected to the subscriber C.