Question

Explain crossbar exchange, with all call processing steps and diagrams

Answer 1

The basic idea 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 known as coordinate switching as the switching contacts are arranged in a x-y-plane. A diagrammatic representation of a cross point switching matrix is shown in Fig. 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 bars shown as dotted lines. The bars, in turn, are attached to a set of electromagnets.

 

When an electromagnetic 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 bars to close. This happens because the contact points move towards each other. For example, if electromagnets M2 and M3’ are energized, a contact is established at the cross point 6 such that the subscriber B is connected to the subscriber C.