AT&T DS1/DMi/ISDN-PRI manual Optimal Diverse Routing

SYNCHRONIZATION OF DIGITAL FACILITIES 5-23

Facility rank ordering is generally based on limited technical and operational information. It is recommended that facility selections be based on local field experience where available.

In figure 5-14,Improper Use of Backup Facilities, only node C has both a primary and a secondary frequency reference. Node C derives its primary source from node B and its secondary timing source from node D. This configuration is optimal because if either node B, D, or the interconnecting facilities should fail, node C would still receive timing traceable to node A from the other node.

RULE 4:

Where possible, the routes for all primary and secondary synchronization facilities should be diverse.

EXAMPLE FOR RULE 4

Figure 5-15, Optimal Diverse Routing, explains the optimal configuration when rule 4 is used. Figure 5-16,Less Than Optimal Diverse Routing, shows a less than optimal application of rule 4.

DIGITAL TRANSMISSION FACILITY

PRIMARY FREQUENCY REFERENCE

SECONDARY (BACKUP) FREQUENCY REFERENCE

Figure 5-15.Optimal Diverse Routing

Figure 5-16,Less Than Optimal Diverse Routing, shows node C deriving both primary and secondary timing from node D, via the two separate T1 facilities. If node D should fail, node C would no longer receive timing that is traceable to node A. Here, node C would lose synchronization and begin to introduce slips into the network.