QUESTION: Why is a timing loop so bad, and why is it so difficult to fix?
ANSWER: Timing loops are inherently unacceptable because they preclude having the affected network elements (NE) synchronized to the PRS. The clock frequencies are traceable to an unpredictable unknown quantity; that is, the hold-in frequency limit of one of the affected NE clocks. By design, this is bound to be well outside the expected accuracy of the clock after several days in holdover, so performance is guaranteed to become severely degraded.
The difficulty in isolating the instigator of a timing loop condition is a function of two factors: first, the cause is unintentional (a lack of diligence in analyzing all fault conditions, or an error in provisioning, for example) so no obvious evidence exists in the network's documentation;- and second, there are no sync-specific alarms, since each affected NE accepts the situation as normal. Consequently, trouble isolation must be carried out without the usual maintenance tools, relying on a knowledge of the sync distribution topology and on an analysis of data on slip counts and pointer counts that is not usually automatically correlated.
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