AT&T Telephone DS1/DMi/ISDN-PRI Uživatelský manuál stáhnout pdf (Strana 111)

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101

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.

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