PURPOSE: To facilitate the increase the unit channels by installing the space switch groups for interface use between unit channels to the link between the optional switches in the unit channel.

CONSTITUTION: The unit channel, consisting of time switch T and space switch H, is coupled to digital circiut multiplying part M and distributing part D. At the same time, space switch group G is provided for the unit channel interface use between space switches. Increase parts T-l∼T-n plus H-l∼H-n are installed in accordance with the steps of increase. Switch group G of the basic hardware is decided by the selected unit channel scale and imaginary end station scale. As a result, each icrease unit can be completely made into a unit.