Robbed-bit signaling

In communication systems, robbed-bit signaling is a scheme to provide maintenance and line signaling services on many T1 digital carrier circuits using channel-associated signaling (CAS).

The T1 carrier circuit is a type of dedicated circuit currently employed in North America and Japan. The T1 circuit is divided into 24 channels, each carrying 8,000 samples per second, each 8 bits long. The Super Frame (SF) consist of 12 frames of 24 channels. The DS1 designation consist of 24 frames called, Extended Super Frame (ESF). In either designation, these channels are multiplexed together and sample at 8000 bit/s. In the DS0 designation, each of the channels induces eight bits into the multiplex output stream, ten are utilized entirely for voice/data and two are utilized partially for voice. Hence, each of the two partial channels yields 7 x 8000 bit/s = 56 kbit/s for voice data and the remaining channels yields 8 x 8000 bit/s = 64 kbit/s.

Intuitively, 5 out of 6 frames have 8 bit resolution equal to 64 kbit/s (8 bits × 8,000 samples per second = 64 kbit/s) and 1 out of every 6 frames has a 7 bit resolution (7 bits × 8,000 samples per second = 56 kbit/s). The distortion effect on voice and data signals is negligible when a modem is used for modulation. However, for a 64 kbit/s digital signal the data will render errors when a data signal is transmitted. If such is the case the robbed-bit signaling should be turned off.

The robbed-bit signal scheme is used in the super frame circuit (SF). It takes the least significant bit of every sixth channel and utilizes it to convey on or off hook, and busy signal status on telephone lines. The first bit of every six is called A bit, the second bit is called B bit.

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.

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