AT&T MERLIN LEGEND Key System Planning page 74

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Signaling Mode
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Planning Form Instructions—Continued
10. If installing more than one 100D Module, decide which Module, if any,
provides the primary synchronization.
If Module 1 provides clock synchronization, check "Primary" in the
first column (the factory setting).
If Module 2 (or Module 3) provides clock snychronization, check
"Primary" in the column that describes that 100D module.
If the synchronization source is other than through a 100D module
check "None".
If assigning backup synchronization:
Check "Secondary" in the column that describes the 100D module
providing secondary synchronization.
Check "Tertiary" in the column that describes the 100D module
providing tertiary backup.
11. Under the "Source" subheading in each column:
Check "Loop" to retain the factory setting. The system uses the clock
of the far-end connection.
Check "Local" to change the setting. (Clock is free running).
12. Under the "Activation" subheading in each column:
Check "Active" if loop clock synchronization is taking place.
Check "Not Active" if the clock is free-running.
Note: Normally, the primary synchronization source is set to "active" and
other modules are set to "not active".
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).
Note: The signaling mode setting does not apply to ISDN-PRI.
Robbed-Bit Signaling. RBS replaces ("robs") the Ieast 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, 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.
Common-Channel Signaling. CCS is an 'out-of-band' signaling format that
places the signaling bits for channels 1 through 23 into the 8-bit word of the
24th channel. The 24th channel, therefore, cannot be used for voice or data
transmissions.