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MULTIPLEXED COMMUNICATION 1-27
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Facilities using RBS carry digital data at 56 kbps. They can be used to carry voice, voice-grade data, or
data that uses only 56 kbps of the available 64-kbps bandwidth (for example, mode-1 data).
24th-Channel Signaling
24th-channel signaling permits the channels to use their full 64 kbps bandwidth capacity. This type of
signaling provides 23 64-kbps channels for voice or data transmission, with the 24th channel reserved for
signaling only.
The format used to carry signaling information requires a 24-frame superframe. ESF meets this
requirement. For D4 framing to meet this requirement, however, two D4 frames are linked together to form
the required 24-frame superframe.
There are three variations of 24th-channel signaling:
• AT&T proprietary signaling
• DMI BOS (bit-oriented signaling)
• ISDN-PRI (sometimes called DMI MOS, or DMI message-oriented signaling)
Table 1-8, 24th-Channel Signaling Arrangement, depicts the frame number and signaling channel
relationship used for DMI BOS (many other relationship rotations are possible).
Table 1-8. 24th-Channel Signaling Arrangement — DMI BOS
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Channels 24th Channel 24th Channel Channels 24th Channel 24th Channel
1-23 D4 ESF 1-23 D4 ESF
Frame No. Frame No. Frame No. Frame No. Frame No. Frame No.
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1 1 13 13 1 1
2 2 14 14 2 2
3 3 15 15 3 3
4 4 16 16 4 4
5 5 17 17 5 5
6 6 18 18 6 6
7 7 19 19 7 7
8 8 20 20 8 8
9 9 21 21 9 9
10 10 22 22 10 10
11 11 23 23 11 11
12 12 24 24 12 12
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AT&T proprietary signaling was the first type of 24th-channel signaling provided by System 75 and
System 85. It was developed to carry DCP-formatted data (in digital form) between System 75 and/or
System 85 switches. AT&T proprietary signaling is described in the initial release of the DMI technical
specification.
For AT&T proprietary signaling, a complete set of signaling information is sent every 24 frames. This 24-
frame period is not synchronized to the 12-frame superframe format of D4 framing or to the 24-frame
superframe format of ESF framing. Each signaling byte contains the equivalent of a channel identification