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.


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Channel Associated Signaling — (CAS), also known as per trunk signaling (PTS), is a form of digital communication signaling. As with most telecommunication signaling methods, it uses routing information to direct the payload of voice or data to its destination. With CAS… …   Wikipedia

  • A and B signaling —    Procedure most often used in T1 transmission facilities operated by U.S. telephone companies, in which one bit, robbed from each of the 24 subchannels in every sixth frame, is used for carrying dial and control information …   IT glossary of terms, acronyms and abbreviations

  • T-carrier — In telecommunications, T carrier, sometimes abbreviated as T CXR , is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and Korea …   Wikipedia

  • Primärmultiplexanschluss — Der Primärmultiplexanschluss (PMxAs), englisch Primary Rate Interface (PRI), ist eine Schnittstellendefinition im ISDN, die im Wesentlichen von Unternehmen zum Anschluss von Telefonanlagen an das ISDN verwendet wird. Eine andere Anschlussart im… …   Deutsch Wikipedia

  • Integrated Services Digital Network — ISDN redirects here. For other uses, see ISDN (disambiguation). Integrated Services Digital Network (ISDN) is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the… …   Wikipedia

  • Telephone exchange — A telephone operator manually connecting calls with cord pairs at a telephone switchboard. In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls. A central… …   Wikipedia

  • RBS — Royal Bank of Scotland (Business » Firms) Royal Bank of Scotland (Business » Accounting) * Rutherford Back Scattering (Academic & Science » Physics) * Research for Better Schools (Community » Educational) * Robbed Bit Signaling (Computing »… …   Abbreviations dictionary

  • Digital Signal 0 — (DS0) is a basic digital signaling rate of 64 kbit/s, corresponding to the capacity of one voice frequency equivalent channel.[1] The DS0 rate, and its equivalents E0 and J0, form the basis for the digital multiplex transmission hierarchy in… …   Wikipedia

  • Modified AMI code — Modified AMI codes are Alternate Mark Inversion (AMI) line codes in which bipolar violations may be deliberately inserted to maintain system synchronization. There are several types of modified AMI codes, used in various T carrier and E carrier… …   Wikipedia

  • Business telephone system — Early model 464G Western Electric key telephone set A business telephone system is any of a range of a multiline telephone systems typically used in business environments, encompassing systems ranging from small key systems to large scale private …   Wikipedia


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.