In the late 1930's and throughout 1940's, AT & T introduced various versions of the crossbar switches. This crossbar switch basically consists line link frames trunk, link frames and common control equipments. With crossbar switchies and common control equipments, the crossbar exchange achieves full access and nonblocking capabilities. Active elements called crosspoints are placed between input and output lines. In common control switching systems, the switching and the control operations are separated. This permits a particular group of common control circuits to route connections through the switching network for many calls at the same time on a shared basis. The unique features of the crossbar switches are
(i) Common control allows the customer and the switch to share the common equipments used to process the call.
(ii) Wire logic computer allows specific routine functions of call processing to be handwired into the switch.
(iii) Flexible concentration ratios allows the system designer to select the appropriate ratio for a specific switch based on customer mix in a specific location.
(iv) Crossbar switches are easier to maintain because the switch have significantly fewer moving parts than strowger switching system.
Basic principle. The fundamental concept of crossbar switching is that it uses common control networks. The common control networks enables the exchange to perform event monitoring, call processing, charging, operation and maintenance. The common control also facilitates uniform numbering of subscribers in a multiexchange area like big cities and routing of calls from one exchange to another via some intermediate exchanges. The common control method of switching overcomes the disadvantages of step-by-step switching. The common control makes no call processing until it receives entire number. It receives all the number, stores, and then establishes connection.
Crossbar switching matrix. The basic crossbar matrix requires atleast M x N sets of contacts and M + N or less activactors to select one of the contacts. Fig. 4.10 illustrates the 3 x 4 crossbar switching. It contains an array of horizontal and vertical wires (shown as a solid line). Both wires are connected to initially separated contact points of switches. Horizontal and vertical bars (shown as dotted lines) are mechanically connected to these contact points and attached to the electromagnets.
-Vertica l magnets (hold magnets or bridge magnets) -
Vertical bars Fig. 4.10. 3 x 4 crossbar switch.
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 continues to hold. When the electromagnets are deenergized both horizontal bar and vertical bars are released from the contact. In order to prevent the catching of different crosspoint in the same circuit, a procedure is followed to establish a connection. Accordingly, horizontal bar is energised first and then vertical bar is energised to make contact or in reverse. But while removing contact horizontal bar is deenergized first and then the vertical bar is deenergized.
The crossbar switch is known as a non-blocking crossbar configuration. It requires N2 switching elements for N subscribers. Thus for 100 subscribers, 10000 crosspoint switches are required. Hence, crossbar is economic only for small private exchanges requiring small switches.
For connecting and releasing the subscriber, the select magnet and bridge magnet should be energised and deenergised respectively. External switch must decide which magnet to operate. This is called marker. A marker can control many swtiches and serve many registers. Thus, even a large exchange needs few markers. In Ericsson ARF system, groups of 1000 subscribers are served by a line switch network controlled by the two markers.
Diagonal crosspoint matrix. A diagonal matrix for 5 subscriber is shown in Fig. 4.11. The number of crosspoints are reduced to N (N-1)/2, where N is the number of subscribers. It is also called triangular matrix or two way matrix.
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