AT&T 7100 series, 7500 series, 2500 series, 7200 series Figure B-13.Proper Use of Backup Facilities

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

A

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C

D

DIGITAL TRANSMISSION FACILITY

PRIMARY FREQUENCY REFERENCE

. . . . . . . . . . . . . . . . . SECONDARY (BACKUP) FREQUENCY REFERENCE

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.