CROSSBAR EXCHANGE
Crossbar exchange
Between 1930 and 1940 AT&T introduces a new switch called cross bar
Basic principles
1.The fundamental concept of crossbar switching is that it uses common control networks. This
common control circuits handle(route) several calls simultaneously, through the switching network, at
the same time, on a shared basics
2. The common control networks, at the cross-bar exchange performs all functions related to, setup,
monitor and disconnect the call, besides billing.
3. The common control also facilitates uniform numbering of subscribers, in a large area like a big city
with multiple exchanges
4. The common control methods of switching eliminates, the disadvantages of step-by-step switching
by receiving the entire number (all the digits) before processing the call
Crossbar switching Matrix
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The basic crossbar matrix requires at least M x N sets of contacts and M + N or less
activators, to select one of the contacts.
Look at 3 x 3 crossbar switch.
It has an array of 3 horizontal and 3 vertical wires (shown as a solid line)
Both H and V wires are connected to individual contact points and are normally kept
separated (open)
There are two types of bars known as horizontal and vertical bars (shown as dotted
lines) are mechanically connected to these contact points and are also attached to
electromagnets as shown in the figure.
HOW SWITCHING IS DONE?
The horizontal and the vertical wires both, can be subscribers in a very small exchange or
The horizontal wires can subscribers and vertical wires can trunks in a local exchange
The horizontal and the vertical wires both can be trunks in a higher group exchange.
Let us say that the switch is in a very small exchange and there are three subscribers A, B, and C
connected to the horizontal wires and D, E and F connected to the vertical wires.
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If A wants to talk to E magnets 4 and 2 are energized (always horizontal magnets first)
If B wants to talk to D magnets 5 and 1 are energized
One common control circuit can service all these 3 calls simultaneously.
A maximum of only 3 calls are, in this switch with 9 cross points.
When both horizontal and vertical bars connected to the electromagnet are activated, the
contact of the intersection of the two bars will close together. Thus, the contact is made and
continuous to hold.
It requires N squares switching elements for N subscribers. Thus for 10 subscribers, 100
cross point switches are required. Hence, crossbar is economically only for small private
exchanges requiring small amount of traffic.
Features:
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A common control unit allows the subscribers and the crossbar switch , to share the
common equipment’s to process the call.
Crossbar switches are easier to maintain, because the switch have significantly fewer
moving parts, than strowger switching system