FOUR DIGIT STEP-BY-STEP AUTOMATIC EXCHANGE
In order to provide access to 10000 lines, a further stage of group selectors is added before the final selectors. Fig. 38.9 illustrates how a calling subscriber can be connected to other subscribers in an exchange having a four digit numbering scheme. Theoretically, a four digit numbering scheme can accomodate 10000 subscribers, but it is necessary also to provide junctions to other exchanges, lines to the operator and other enquiry services, and so on. This means that the capacity of an exchange of this type would in practice be about 6000 subscribers instead of the theoretical 10000.
Fig. 38.9 ...
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