A T-1 trunk operates at 1.544 Mbps divided into 24 time slots with each time slot operating at 64 kbps [digital signal level 1 (DS-1) rate]. A single 8-bit sample from each of 24 voice channels comprises a D4 frame of 24 time slots on a T-1 trunk. Twelve D4 frames make up a D4 superframe.
Signaling information is carried on a T-1 trunk by two signaling bits, an A-bit and a B-bit. Each time slot in the sixth frame of a D4 superframe has the least significant bit replaced with A-bit signaling information. Likewise, each time slot in the twelfth frame of the D4 superframe has the least significant bit replaced with B-bit signaling information. This method of replacing the least significant bit with signaling information is called robbed bit signaling. Thus, a T-1 robbed bit trunk carries all signaling within the voice time slot (channel) itself.
Dialing, if not done using DTMF or MF tones, is accomplished by alternating the A and B signaling bits between 0 and 1 to mimic rotary dial pulses. Signaling bits represent the state of the M lead on the E&M interface of the calling party. When the called party answers, the M lead returns continuous 1s. When a party hangs up, their signal bits revert to 0s to indicate on-hook. Some telcos invert these signaling bits so that 0 = off-hook and 1 = on-hook.
New telco services may require the use of more than the four signaling states provided by the A and B bits. An extended superframe (ESF) adopted by AT&T provides two additional signaling bits, the C-bit in frame 18 and the D-bit in frame 24.
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