Immediate ring-back control system for time-division telephone exchange

Abstract

An immediate ring-back control system for a time-division telephone exchange includes a time-division switching network of a time switch-space switch-time switch (T-S-T) system. An incoming time switch accommodates multi-channel interrupted ring-back tones (IRBT's) which are generated by a tone generator in a time-multiplexed manner, each of which has a different interruption phase. The interrupted ring-back tone having a phase starting from a tone-sending period is selected from the multi-channel interrupted ring-back tones (IRBT's) by an immediate ring-back control circuit provided in a time-division switch driving circuit (SDC) at the time of connection of the ring-back tone to a calling subscriber, and a channel number corresponding to the phase of the interrupted ring-back tone is stored in a control memory or a time slot memory associated with the time switch, so that the transmission of the interrupted ring-back tones to a calling subscriber can be started from a mark portion thereof.