AT&T DEFINITY 7200 series - Manual (Page 235)

SYNCHRONIZATION OF DIGITAL FACILITIES B-27
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. . . . . . . . . . . . . . SECONDARY (BACKUP) FREQUENCY REFERENCE
DIGITAL TRANSMISSION FACILITY
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PRIMARY FREQUENCY REFERENCE
Figure B-20. Minimized Synchronization from One Node
External-Reference Selection Rules
There are seven rules for selecting external synchronization references. These rules are described next.
RULE 1:
Nodes within the externally referenced sublevel may not use as a reference source a facility from a node
within the internally referenced sublevel (refer to figure B-11, External and Internal Reference Levels, for
an explanation of these terms).
RULES 2 THROUGH 7:
Rules 2 through 7 are the same for both internal- (that is, SCS, external stratum-3 clock, and HAC) and
external-reference selections.