AT&T MERLIN LEGEND Communications System PBX System Planning Manual page 79

Trunks
Signaling Mode
CSU Setting for T1
Emulation
3-24 Trunk Options
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 least significant bit of every
sixth frame of each DSO 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.
Planning Form Instructions - Continued
13. Under the "Signaling Mode" heading of Form 3c, in all columns that
describe T1 modules:
Check "RBS" to retain the factory setting of robbed-bit signaling.
Check "CCS" to change the factory setting to common-channel
s i g n a l i n g .
Use these instructions only if the 100D module type of service is T1 and
outside trunks are to be emulated.
For loop-start and ground-start emulation with T1, specify whether the type of
equipment provided by the local telephone company central office is foreign
exchange (the factory setting) or special access.
Planning Form Instructions - Continued
14. Under the "Channel Service Unit" heading of Form 3c, in all columns that
describe T1 modules used for trunk emulation:
Check "Foreign Exchange" to retain the factory setting.
Check "Special Access" to change the factory setting.