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

5-24SYNCHRONIZATION OF DIGITAL FACILITIES

DIGITAL TRANSMISSION FACILITY

PRIMARY FREQUENCY REFERENCE

SECONDARY (BACKUP) FREQUENCY REFERENCE

Figure 5-16.Less Than Optimal Diverse Routing

RULE 5:

Obtaining both primary and secondary synchronization facilities from within the same transmission cable should be minimized.

RULE 6:

The total number of cascade node connections from the referenced node should be minimized.

EXAMPLE FOR RULE 6

Figure 5-17,Excessive Cascading, shows excessive cascading in that node B derives timing from a source three transmission facilities away.

Figure 5-18,Minimized Cascading, shows less cascading in that node B derives timing from a source two transmission facilities away. Excessive cascading is undesirable because of intermediate link vulnerability. For example, an intermediate link failure in node C of figure 5-17,Excessive Cascading, would cause node B to lose timing. Such a failure increases the number of slips between nodes A and B. However, the same failure in Node C of figure 5-18,Minimized Cascading, does not affect node B’s synchronization.