Robbed bit signaling
Encyclopedia
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 we can analyze that 5 out of 6 frames have 8 bit resolution equal to 64 kbit/s (8 Bits x 8,000 Samples per Second = 64 kbit/s) and 1 out of every 6 frames has a 7 bit resolution (7 Bits x 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 signals the data will render errors when 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 system
s, 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.
Robbed-bit signaling can have an effect on audio quality under certain circumstances, though. When a voice call is connected to a quiet termination, as can happen when on hold in a PBX which does not have music on hold
or comfort noise
enabled, and certain types of framing circuits are used, the noise due to robbed-bit signaling can be heard in the handset as a faint 1333 Hz tone, this frequency being a result of low-bit corruption at a rate of 8000 Hz / 6 = 1333 Hz. This is normally not a very noticeable problem, but if the audio path contains in-line speech compression
, such as G.729
, the tone can be amplified and modulated by the compression algorithm to the point of annoyance to the user.
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.
Line signaling
Line signaling is a class of telecommunications signaling protocols. Line signaling is responsible for off-hook, ringing signal, answer, ground start, on-hook unidirectional supervision messaging in each direction from calling party to called party and vice versa...
services on many T1 digital carrier circuits using Channel Associated Signaling
Channel Associated Signaling
Channel Associated Signaling , also known as per-trunk signaling , 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 signaling, this routing information is...
(CAS).
The T1 carrier
Digital Signal 1
Digital signal 1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside North America, Japan, and South Korea...
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.
| T1 Carrier Designation | DS Designation | Speed |
|---|---|---|
| FT1 | DS0 | 64 kbit/s |
| T1 | DS1 | 1.544 Mbit/s |
| T2 | DS2 | 6.312 Mbit/s |
| T3 | DS3 | 44.376 Mbit/s |
| T4 | DS4 | 274.176 Mbit/s |
Intuitively we can analyze that 5 out of 6 frames have 8 bit resolution equal to 64 kbit/s (8 Bits x 8,000 Samples per Second = 64 kbit/s) and 1 out of every 6 frames has a 7 bit resolution (7 Bits x 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 signals the data will render errors when 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
AT&T
AT&T Inc. is an American multinational telecommunications corporation headquartered in Whitacre Tower, Dallas, Texas, United States. It is the largest provider of mobile telephony and fixed telephony in the United States, and is also a provider of broadband and subscription television services...
was moving from analog trunks onto digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
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
Frequency-division multiplexing
Frequency-division multiplexing is a form of signal multiplexing which involves assigning non-overlapping frequency ranges to different signals or to each "user" of a medium.- Telephone :...
.
As in other carrier system
Carrier system
In telecommunication, a carrier system is a multichannel telecommunications system in which a number of individual channels are multiplexed for transmission...
s, 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
Ground start
In telephony, a ground start or GST is a method of signaling from a terminal or subscriber local loop to a telephone exchange, in which method a cable pair is temporarily grounded to request dial tone...
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
Super Frame
Super Frame is an older framing standard for T1s. Also called D4 or D3/D4 framing. In the 1970s it replaced the original T1/D1 framing scheme of the 1960s in which the framing bit simply alternated....
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
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
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.
Robbed-bit signaling can have an effect on audio quality under certain circumstances, though. When a voice call is connected to a quiet termination, as can happen when on hold in a PBX which does not have music on hold
Music on hold
Music on hold is the business practice of playing recorded music to fill the silence that would be heard by telephone callers who have been placed on hold...
or comfort noise
Comfort noise
Comfort noise is synthetic background noise used in radio and wireless communications to fill the artificial silence in a transmission resulting from voice activity detection or from the audio clarity of modern digital lines....
enabled, and certain types of framing circuits are used, the noise due to robbed-bit signaling can be heard in the handset as a faint 1333 Hz tone, this frequency being a result of low-bit corruption at a rate of 8000 Hz / 6 = 1333 Hz. This is normally not a very noticeable problem, but if the audio path contains in-line speech compression
Speech compression
Speech compression may mean different things:*Speech encoding refers to compression for transmission or storage, possibly to an unintelligible state, with decompression used prior to playback....
, such as G.729
G.729
G.729 is an audio data compression algorithm for voice that compresses digital voice in packets of 10 milliseconds duration. It is officially described as Coding of speech at 8 kbit/s using conjugate-structure algebraic code-excited linear prediction .Because of its low bandwidth requirements,...
, the tone can be amplified and modulated by the compression algorithm to the point of annoyance to the user.
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
Loop start
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 referred to as 'off-hook'. When idle, or 'on-hook', the loop is at 48V DC...
, ground start
Ground start
In telephony, a ground start or GST is a method of signaling from a terminal or subscriber local loop to a telephone exchange, in which method a cable pair is temporarily grounded to request dial tone...
, and E&M.
Unlike T1 systems, most telephone systems in the world use E1
E-carrier
In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...
systems that transparently pass all 8 bits of every sample. Those systems use a separate out-of-band
Out-of-band
The term out-of-band has different uses in communications and telecommunication. In case of out-of-band control signaling, signaling bits are sent in special order in a dedicated signaling frame...
channel to carry the signalling information.