AT&T DEFINITY 7200 series - Manual (Page 229)

SYNCHRONIZATION OF DIGITAL FACILITIES B-21
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RULE 2:
A node may use a facility as a backup only if the node at the other end is not using that same facility as a
backup.
EXAMPLE FOR RULE 2
Figure B-13, Proper Use of Backup Facilities, illustrates an application of rule 2 and proper use of a
backup facility between nodes A and C.
B
C D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DIGITAL TRANSMISSION FACILITY
. . . . . . . . . . . . . . . . .
PRIMARY FREQUENCY REFERENCE
SECONDARY (BACKUP) FREQUENCY REFERENCE
A
Figure B-13. Proper Use of Backup Facilities
Figure B-14, Improper Use of Backup Facilities, shows the same network arrangement but with improper
use of the backup facility between nodes A and C. This improper arrangement would form a timing loop if
node B should fail (nodes A and C would switch to their backup, which is each other).
RULE 3:
The transmission facilities associated with those nodes meeting Rules 1 and 2 and having the greatest
availability must be selected for primary and secondary synchronization facilities.