The fundamental operation of a switch is to setup and release connection between subscribers. It involves direct connection between subscriber loops at an end office or between station loops at a PBX. The switches are hardware and/or software devices capable of creating temporary connections between two or more subscribers. In this section, the space division switching is described and in section 5.6, time division switching is explained.
In space division switching, the paths in the circuit are separated from each other spatially. It was originally designed for analog networks, but is used currently in both digital and analog switching. A crosspoint switch is referred to as a space division switch because it moves a bit stream from one circuit/bus to another. For large group of outlets, considerable savings in total crosspoints can be achieved if each inlet can access only a limited number of outlets. Such situation is called limited availability.
By overlapping the available outlet groups for various inlet groups, a technique called "grading" as established. Rectangular crosspoint array is an example of grading. For longer trunk groups, large crosspoints were expensive and not used now-a-days. The number of crosspoints required are M x N, where M is number of inlets and N is number of outlets.
It is inefficient to build complete exchanges in single stages. Single stage can only be used to interconnect one particular inlet outlet pair. Also the number of crosspoints grows as the square of the inputs for grading, N (N-1)/2 for a triangular array and N (N-1) for a square array. Also the large number of crosspoints on each inlet and outlet line imply a large amount of capacitive loading on the message paths. Therefore, it is usual to build exchanges in two or three stages to reduce the number of crosspoints and to provide alternative paths. The sharing of crosspoints for potential paths through the switch is accomplished by multiple stage swtiching.
Fig. 5.11 shows the three stage switching structure to accomodate 128 input and 128 output terminals with 16 first stage and last stage.
Digital Switching Systems 99 Third stage
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