AT&T MERLIN LEGEND Release 3.0 Programming Summary Manual page 124

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NOTE:
If you are adding PRI facilities to an existing system, certain values must be
set correctly. To inspect or change these values, see "DS1 Facilities." Do not
start these procedures until you have checked the following:
Type of DS1 Facility must be set to PRI.
Frame format must be specified correctly.
Zero code suppression must be specified correctly.
Clock synchronization source must be set to loop (derived from the T1
line).
The settings for frame format and zero-code suppression must be consistent
with the options selected when the PRI connection was ordered.
If you are using ARS in connection with PRI, make sure you select voice, data,
or voice and data, as appropriate, when you perform the ARS "Voice and/or
Data Routing" procedure found in "Automatic Route Selection."
9-2 PRI Facilities