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. See also Signalling (telecommunications Channel Associated Signaling (CAS also known as Per-Trunk Signaling (PTS is a form of digital communication signaling
RBS was developed at the time that AT&T was moving from analog trunks onto digital equipment. Before proposing a merge request please see Talk and see if the merger you propose has recently been made and An analog or analogue signal is any continuous signal for which the time varying feature (variable of the signal is a representation of some other A digital system uses discrete (discontinuous values usually but not always Symbolized Numerically (hence called "digital" to represent information for 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. Frequency-division multiplexing ( FDM) is a form of signal Multiplexing where multiple Baseband signals are Modulated on different frequency
As in other carrier systems, the physical properties of an actual trunk wire are missing. In Telecommunication, a carrier system (loosely a synonym with carrier) is a Multichannel telecommunications System in which a number of individual 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. In Telephony, a ground start or GST is a method of signaling from a terminal or subscriber Local loop to a Telephone exchange, in 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. Super Frame is an older framing standard for T1s Also called D4 or D3/D4 framing In Telecommunication, an Extended Super Frame ( ESF) is a T1 framing standard, sometimes called D5 framing invented in the 1980s 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. Digital Signal 0 ( DS0) is a basic Digital signaling rate of 64 Kbit/s, corresponding to the capacity of one Voice-frequency -equivalent 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). Signal-to-noise ratio (often abbreviated SNR or S/N) is an Electrical engineering concept also used in other fields (such as scientific Measurements The decibel ( dB) is a logarithmic unit of measurement that expresses the magnitude of a physical quantity (usually power or intensity relative to 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. In Telecommunications, a loop start is a supervisory signal given by a Telephone or PBX in response to the completion of the loop circuit, commonly In Telephony, a ground start or GST is a method of signaling from a terminal or subscriber Local loop to a Telephone exchange, in E&M is a type of supervisory line signaling that uses separate leads called the "E" lead and "M" lead traditionally used in the North American telecommunications
Unlike T1 systems, most telephone systems in the world use E1 systems that transparently pass all 8 bits of every sample. In digital Telecommunications where a single physical wire pair can be used to carry many simultaneous voice conversations worldwide standards have been created and deployed Those systems use a separate out-of-band channel to carry the signalling information. Out-of-band is a technical term with different uses in Communications and Telecommunication.
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