The interface with the subscriber's line is at present the costliest part of all digital exchanges, largely because a 10000 lines exchange has to have 10000 of these units, each with all the features needed to inter work with various types of subscribers' lines. It is customary to describe these features as the BORSCHT functions, based on the initials of the key words.

  1. Battery feed to line – there is normally no active power source at the subscribers' telephone; all the power, needed to drive the microphone and keypad, is fed out from the exchange along the subscriber's line.
  2. Overvoltage protection – solid-state devices are very sensitive to high voltages, and rapid-action protective devices have to be provided in each ...

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