AT&T DS1/DMi/ISDN-PRI manual Proper Use of Backup Facilities

SYNCHRONIZATION OF DIGITAL FACILITIES 5-21

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 5-13, Proper Use of Backup Facilities, illustrates an application of rule 2 and proper use of a backup facility between nodes A and C.

DIGITAL TRANSMISSION FACILITY

PRIMARY FREQUENCY REFERENCE

SECONDARY (BACKUP) FREQUENCY REFERENCE

Figure 5-13.Proper Use of Backup Facilities

Figure 5-14,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.