Method of signalling on trunks over a T1 line.
The reason T1s were developed was the need for more trunks, connecting central offices together or connecting cities ("long-haul lines"). Rather than run additional phone lines, it was decided to move existing analog trunks onto digital equipment, and thereby run 12 times as many trunks on the same number of wires. A T1 requires four wires, and carries 24 trunks across the same distances. This saved the phone company a heckofalot of money.
In doing this, the physical properties of an actual trunk are missing. With analog trunks, to signal the equipment at the far end that a trunk was going to be used, equipment would loop the line or ground one of the wires (depending on the type of trunk), and do the opposite to return the trunk to idle. With a digital trunk, another method was needed to signal between ends.
To do this, voice equipment steals the eighth bit of each channel on every sixth frame (see Super Frame and Extended Super Frame). Voice is not very sensitive to losing this data, so it doesn't cause much degradation of voice quality. However, when carrying a modem signal, the difference is noticable. With full 64kbps, a voice channel has a signal to noise ratio of 37 dB. At 56kbps, a voice channel has a signal to noise ratio of 31 dB. Although only one bit out of 48 is robbed, there is no way to know which frames will be robbed by the various T1 connections in a phone conversation, so frequency range will be somewhere between 31 and 48 dB.
With Super Frame framing, this amounts to two bits, named A and B. With Extended Super Frame, the same stream is divided into four bits, named A, B, C, and D. C and D are very rarely used in trunking, although there is a set of standards for their use.