T1 Robbed Bit Signaling?
John Finley
bit signaling? Web sites, books, application notes, whatever.
I couldn't find a FAQ. Bellcore's "Notes on the Network"
is $400, and LSSGR is > $6K.
Thanks,
John
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John Finley Engineering Consultant
jo...@finley.com Real Time Systems
(619) 689-0032 Networking Protocols
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JLS Enterprise
Rob bit signalling is used on T1 carrier lines to transition on-hook and off-hook conditions
for the 24 voice calls on the T1 line. The T1 is call a DS-1 with D4 framing. A T1 operates at
1.544 mbits and uses 8kbits for D4 framing, or every 125 microseconds a framing bit occurs to
bound your 24 channels on the T1. Each channel (called a DS0) uses 8 bits in what is called a
PCM format (pulse coded modulation)to digitally "step" the analog value of sample voice (or
analog modulated data - modems). In every 6th and 12th frame the least significant bit in each
channel (DS0) is "robbed" for channel signalling; these are called A and B bits. These least
significant bits in these channels can be at logic 0 or 1 depending on what type of signaling
protocol is being carried. Signalling protocols may be 4Wire E & M (immediate start, wink-start,
delay dial), FX (foriegn exchange, loop or ground start), automatic ringdown, etc. The protocol
defines the logic condition during seizure, call setup, busy, and idle conditions.
Bell Core publishes a document call "Notes on the Network" that provides a very informative
technical explaination of signalling protocols; inclusive of Extended Superframing and IDSN.
Regards,
JLS
John Fricks
<Stal...@mediasoft.net> provided a very good response to John Finley's
request for description of T1 robbed bit signalling:
>
> Rob bit signalling is used on T1 carrier lines to transition on-hook and off-hook conditions
> for the 24 voice calls on the T1 line. The T1 is call a DS-1 with D4 framing. A T1 operates at
> 1.544 mbits and uses 8kbits for D4 framing, or every 125 microseconds a framing bit occurs to
> bound your 24 channels on the T1. Each channel (called a DS0) uses 8 bits in what is called a
> PCM format (pulse coded modulation)to digitally "step" the analog value of sample voice (or
> analog modulated data - modems). In every 6th and 12th frame the least significant bit in each
> channel (DS0) is "robbed" for channel signalling; these are called A and B bits. These least
> significant bits in these channels can be at logic 0 or 1 depending on what type of signaling
> protocol is being carried. Signalling protocols may be 4Wire E & M (immediate start, wink-start,
> delay dial), FX (foriegn exchange, loop or ground start), automatic ringdown, etc. The protocol
> defines the logic condition during seizure, call setup, busy, and idle conditions.
>
I would add a few historical notes:
T1 is the first commercial pulse code modulation (PCM) system designed (by
Bell Labs) for twisted-pair copper cables, introduced in 1962.
DS1 (and DS0, DS2, DSn) is digital signal level one of the Bell System
digital hierarchy, described in 1976. Not all DS1 signals are on T1
systems.
The D4 channel bank was introduced in 1977.
Robbed-bit signalling may be used on a DS0 channel in many cases besides T1
carrier. There are still many interoffice digital trunks and even more
remote access systems which use DS0 channels with robbed-bit signalling.
Such trunks and channels have a 56kb/s limit when used for digital data.
Increasingly, interoffice trunks and remote access systems are capable of
clear-channel (64kb/s) digital data. Common-channel signalling is used in
these cases. Deployment is very widespread due to ISDN and wideband
services offerings.
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John Fricks
Email: John_...@nt.com
Nortel Inc.
The opinions expressed in this news posting are those of the
poster and not necessarily those of Nortel.
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