AT&T MERLIN LEGEND Release 2.0 Equipment And Operations Manual page 155

Lines and Trunks
Backup synchronization in the event of a maintenance failure can be provided
by programming the second and third installed modules as secondary and
tertiary synchronization.
In addition, the source of synchronization can be factory set to "loop clock
reference source" (the clock is synchronized to the external endpoint—the
factory setting) or set to "local clock reference source"
Signaling Mode
Signaling is the process of communicating channel-state information (such as
dialing) from endpoint to endpoint. Two types of signaling can be used in T1
transmission: robbed-bit signaling (RBS) and common-channel signaling (CCS).
Choosing a signaling mode pertains only to T1 service; PRI always uses CCS
(23 B-channels and 1 D-channel).
Robbed-Bit Signaling
RBS replaces ("robs") the least significant bit of every sixth frame of each DS0
channel with signaling information. (RBS is also called in-band signaling. since
signaling information is embedded in the least significant bit of every sixth 8-bit
word.)
RBS is appropriate for voice and voice-grade data (up to 19.2 bps), but facilities
using RBS cannot transmit digital data at 64 kbps because this bit-robbing
corrupts data. Digital data at 56 kbps may be possible in certain applications
by using 7-bit words.
Common-Channel Signaling
CCS is an format that places the signaling bits for channels 1 through 23 into
the 8-bit word of the 24th channel. This restricts DS1 from using the 24th
channel for voice or data transmissions. D4 framing does not preclude the use
of CCS, but CCS is not compatible with D4 channel banks because the D4
channel banks only recognize RBS. Coupled with B8ZS coding, CCS can
support digital data up to 64 kbps per channel.
Recommended Framing Formats and Signaling
Modes
ESF framing should be used to take advantage of its improved maintenance,
diagnostic, and testing capabilities (the ESF T1 CSU is required to interface with
the network). If the transmission between two systems is voice-only, RBS
should be used for all 24 communication paths. For voice transmission, both
ZCS and B8ZS line coding can be used to satisfy the ones-density requirement;
the preferred line-coding format is B8ZS, which is needed for 64 kbps digital
data.
DS1 Facilities
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