5 2 Space Division Switching 1 Digital SwitchingSPACE DIVISION SWITCHING The simplest switching structure is a rectangular array of crosspoints as shown in Figure 5.2. This switching matrix can be used to connect any one of N inlets to any one of M outlets. If the inlets and outlets are connected to two-wire circuits, only one cross-point per connection is required. Rectangular cross-point arrays are designed to provide intergroup (transit) connections only, that is, from an inlet group to an outlet group. Applications for this type of an operation occur in the following: Remote concentrators.Call distributors.The portion of a PBX or end-office switch that provides transit switching. Single stages in multiple stage switches.SPACE DIVISION SWITCHINGSPACE DIVISION SWITCHING 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 groups, 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.