T1 trunks use what is called “robbed bit signaling” to carry signaling messages for voice applications. The least significant bit of all time slots in every 6th frame is robbed as the signaling bits. In SF format, A, B bits can define up to 4 signaling states. In application level, there are two signaling schemes–CAS and CCS. In CAS (channel associated signaling), each channel's signaling is carried by its own time slot every 6th frame. In CCS (Common channel signaling), use a dedicated channel, usually time slot 24, to carry the signaling messages for the entire trunk. In ESF format, twice as many robbed bits from 6th, 12th 18th and 24th frames can be utilized for signaling, defining up to 16 signaling states. The four bits are called A, B, C, D bit, respectively. Sometimes this type of signaling is called ABCD Signaling. In ESF format, besides for frame alignment, F-bit is utilized for two other purposes–Data Link and CRC bits. Data link bits are from every other frame of 24 ESF frames, totaling 12 bits. This creates a 4 kbps data link called Facility Data Link (FDL) used for delivering maintenance information and supervisory control for the trunk. CRC bits are from 2nd, 6th, 10th, 14th, 18th and 22nd frame of ESF frames. Since there are in total 6 of these bits, it is denoted as CRC-6, which provides a mechanism to monitor the transmission quality of the DS1 facility.
Before a call is setup, suppose both phones are in on-off state. Both multiplexers send on-hook signaling ABCD = 0000 to the switch through T1 lines. In the figure above, Step , Multiplexer A will send the signaling ABCD = 0101 to the switch through T1 line that Phone A goes off-hook from on-hook state; In Step , the switch sends the signaling ABCD = 1111 to Multiplexer B for it to ring Phone B. In Step , if Phone B was already off-hook while Phone A is dialing its number, the switch would send a busy signaling ABCD = 0111 to Multiplexer A for it to send a busy tone to Phone A. SLC-96 (TR-08) is a signaling scheme proprietary to AT&T aiming at enhancing the efficiency of T1 line utilization when T1 facility was precious resources. It is similar to D4/SF framing with respect to signaling. There is an A bit that appears in the least significant bit position of the time slots in Frame #6 and the B bit is in the least significant bit position of the time slots in Frame #12, and so on. A and B bits permit nine possible signaling states via a three-level (tri-level) logic system:
1. Continuously zero;
2. Continuously one;
3. Alternating between zero and one. For example, 1/0 or 0/1 in alternating signaling frames.