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The portion of a PBX or end-office switch that provides transit switching. Single stages in multiple stage switches. In most of the foregoing applications, it is not necessary that the inlets be connectable to every outlet. In situations involving large groups of outlets, considerable savings in total cross-points can be achieved if each inlet can access only a limited number of outlets. When such a situation occurs “limited availability” is said to exist. By overlapping the available outlet groups for various inlet grou a technique called “grading” is established. An example of a graded switching matrix is shown in Figure 5. 3. Notice that if outlet connections are judiciously chosen, the adverse effect of limited availability is minimized. For example, if inlets 1 and 8 in Figure 5. 3 request a connection to the outlet group, outlets 1 and 3 should be chosen instead of outlets 1 and 4 to avoid future blocking for inlet 2. Graded switching structures were often used for access to large trunk groups in electromechanical switches where cross-points were expensive and individual switching modules were limited in size.
Intragroup switching. as for oop-to-loop switching, requires each loop to be connectable to every other loop. Thus full availability from all inlets to all outlets of the switching matrix is required. Figure 5. 4 shows two matrix structures that can be used to fully interconnect two-wire lines. The dashed lines indicate that corresponding inlets and outlets of twowire switching matrices are actually connected together to provide bidirectional transmission on two-wire circuits.
For purposes of describing switching matrices, however, it is convenient to consider the inlets and outlets of two-wire switching matrices as being istinct. Both structures in Figure 5. 4 allow any connection to be established by selecting a single cross-point. The square matrix, which is also called a two-sided matrix, allows any particular connection to be established in two ways. For example, if input link i is to be connected to input link j, the selected cross- point can be at the intersection of inlet i and outlet j”or at the intersection of inlet j; and outlet i.
For simplicity these cross-points are referred to as (i,j) and (j, i), respectively. In a typical implementation cross-point (ij) is used when input requests ervice, and crosspoint (j, i) is used when input j requests service. In the triangular matrix of Figure 5. 4, the redundant cross-points are eliminated. The cross-point reduction does not come without complications, however. Before setting up a connection between switch input and switch input j, the switch control element must determine which is larger: or j.
If i is larger, cross-point (i, j) is selected. If i is smaller, cross-point (j, i) must be selected. With computer controlled switching, the line number comparison does not represent a significant imposition. Switching machines for four-wire circuits require separate connections for the go and return branches ofa circuit. Thus two separate connections must be established for each service request. Figure 5. 5 depicts a square matrix structure used to provide both connections. The structure is identical to the square matrix shown in Figure 5. for two- wire switching. The difference corresponding inlets and outlets are not connected to a common two-wire input. All Of the inlets Of the four-wire switch are connected to the wire pair carrying the incoming direction of transmission, and all of the outlets are connected to he outgoing pairs. When setting up a connection between four-wire circuits i and j, the matrix in Figure 5. 5 must select both crosspoints (i,j) and (j,i). In actual operation these two crosspoints may be selected in unison and implemented as a common module.
Multiple Stage Switching In the switching Structures described to this point, an inlet is connected directly to an outlet through a single crosspoint. For this reason, these switching structures are referred to as “single stage” switches Single stage switches have the property that each individual crosspoint can only be used to interconnect one particular inlet/outlet pair. Since the number of inlet/ outlet pairs is equal to N(N – 1)/2 for a triangular array, or N(N – 1) for a square array, the number of crosspoints required for a large switch is prohibitive.
Furthermore, the large number of crosspoints on each inlet and outlet line imply a large amount of capacitive loading on the message paths. Another fundamental deficiency of single stage switches is that one specific crosspoint is needed for each specific connection. If that crosspoint fails, the associated connection cannot be established. To increase the utilization efficiency of the crosspoints and thereby reduce he total number, it is necessary that any particular crosspoint be usable for more than one potential connection.
If crosspoints are to be shared, however, it is also necessary that more than one path be available for any potential connection so that blocking does not occur. The alternate paths serve to eliminate or reduce blocking and also to provide protection against failures. The sharing of crosspoints for potential paths through the switch is accomplished by multiple stage switching. A block diagram of one particular form of a multiple stage switch is shown in Figure 5. 6. The switch of Figure 5. is a three-stage switch in which the inlets and outlets are partitioned into subgroups of n inlets and n outlets each.
The inlets of each subgroup are serviced by a rectangular array of crosspoints. The inlet arrays (first stage) are n x k arrays where each one of the k outputs is connected to one of the k center stage arrays. The interstage connections are often called junctors. The third stage consists of k x n rectangular arrays that provide connections from each center stage array to the groups of n outlets.