AT&T MERLIN LEGEND Installation & Maintenance Manual page 91

System Description
In addition, the source of synchronization can be factory set to
"loop clock reference source" (the clock is synchronized to the
external endpoint) or set to "local clock reference source" (the
clock is free running). This setting must be made for the primary,
secondary, and tertiary synchronization modules.
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; ISDN-PRI
always uses common-channel signaling (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
1200 baud), but facilities using RBS cannot accurately transmit
digital data because digital data uses high-speed data rates,
such as 64 kbps. Therefore, the channel is limited to voice and
analog voice-band data applications.
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DS1 Capabilities