AT&T Telephone DS1/DMi/ISDN-PRI Uživatelský manuál stáhnout pdf (Strana 114)

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SYNCHRONIZATION 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.

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