Robbed-bit signaling

Robbed-bit signaling (RBS) is a specific type of Channel Associated Signaling in use in North America on T1 trunks, and perhaps elsewhere in the world.

RBS was developed at the time that AT&T was moving from analog trunks onto digital equipment. This permitted AT&T to run 24 digital phone lines on the same number of wires that 2 analog phone lines would have taken, saving money and improving call quality, without the high cost of frequency-division multiplexing.

As in other carrier systems, the physical properties of an actual trunk wire 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 by connecting the wires together at one end or ground start 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, signaling equipment "steals" the eighth bit of each channel on every sixth frame (see "Super Frame" and "Extended Super Frame") and replaces it with signalling information. Put another way, this means that the low-order bit on every sixth sample in every DS0 carried on the T1, in either direction, is replaced by signalling information. Simple PCM-encoded voice is not very sensitive to losing this data in a few of its lower-order bits, so it doesn't cause much degradation of voice quality; however, when carrying data, the difference is significant, reducing the available usable data rate by 12.5%. With full 64 kbit/s, a voice channel has a signal-to-noise ratio of 37 decibels (dB). At 56 kbit/s, a voice channel has a signal to noise ratio of 31 dB. As only every sixth least-significant bit is robbed, the signal to noise ratio will be somewhere between 31 and 37 dB. However, since individual T1 links are not in general synchronized to one another, a DS0 passing along several concatenated, unsynchronized, T1 spans may have its lower bit stolen in more than one frame, frequently making real-world performance closer to the lower-bound than the upper bound of signal-to-noise performance.

With "Super Frame" framing, the robbed bits are named A and B. With "Extended Super Frame", the same stream is divided into four bits, named A, B, C, and D. The meanings of these bits depend on what type of signalling is provisioned on the channel. The most common types of signaling are loop start, ground start, and E&M.

Unlike T1 systems, most telephone systems in the world use E1 systems that transparently pass all 8 bits of every sample. Those systems use a separate out-of-band channel to carry the signalling information.