Robbed bit signaling
Encyclopedia : R : RO : ROB : Robbed bit signaling
Robbed Bit Signalling (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. This permitted AT&T to run 12 times as many calls over the same number of physical wires, saving money and improving call quality.
In doing this, 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 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). Voice is not very sensitive to losing this data, so it doesn't cause much degradation of voice quality, however, when carrying data the difference is noticeable, reducing available bandwidth 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. 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 the signal to noise ratio will be somewhere between 31 and 37 dB.
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.
From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.