A signaling method in a T1 circuit that uses bits from each of the 24 DS-0 channels. When channels carry switched voice traffic, each channel bank must be able to alert the other when a call is being placed on that channel. T-carrier systems grab one bit from each sixth voice sample to do this, which works out to 1,333 bps from the 64 Kbps channel. When a channel is seized from one end, the signaling bit is changed from 0 to 1 to alert the far end.
The other signaling option is an ISDN PRI circuit where all signals travel in a separate, shared "D" channel carried in the 24th time slot. See T1
and ones density