QUESTION:
A customer is using a private network of TDMs. They have clock slip problems on a number of leased lines which are provided as unframed E1s. The Customer TDMs provide clock. Diagnostics of the E1s show no error conditions. How should we go about diagnosing a 'clocking problem' with the TDMs?
ANSWER:
Too often when unframed E1s/DS1s are employed from a multitude of carriers, one side or both of the Path Terminating Elements of the E1 are untraceable to PRS (unframed E1s can imply an E1 capable transport facility but no existing Path Terminating Element. The PTEs are typically a private network provider/customer owned equipment.)
When leased lines are employed with privately owned network elements, both ends are typically set to loop recovered timing thus they chase their tails. To avoid this "timing-loop", at least one side must be set to internal or provided a separate timing reference. Generally speaking, it is advisable that this reference be Stratum-1-traceable (traceable to a PRS). Customer Premise equipment typically has a "low quality" oscillator, Stratum-4 at the best. Traceability to a PRS removes many of the drawbacks associated with an inexpensive oscillator, in particular stability, not to mention frequency offset. The issue that needs to be examined and determined is whose clock is traceable to PRS. If the network is complex, it may be the case that there is no single "master" and thus slips would be observed. Making the master PRS-traceable provides the requisite accuracy and stability, and also provides the freedom of having multiple "masters", provided all are PRS-traceable.
Testing should consist of measuring each side of the E1 as referenced to a known PRS or PRS traceable source (avoid using your own network element or the leased E1 as the reference). One source of a PRS traceable E1 could be an E1 carrying conventional voice service from the local phone company (not a third party service). Typically, the local phone company operates at PRS and thus their E1 can serve as a reference to help identify which side(s) of the leased E1 is not at PRS.
If neither side of the E1 is at PRS but side 1 is stable with regard to side 2, one of the privately owned network elements should be connected to a PRS/SASE as an external reference. (If element does not support an external SASE interface, employ an Line Retiming Unit.)
If Side 1 and side 2 are operating at separate frequencies and neither is at PRS, establish one end to an external PRS (as described above) and set the far end element to Loop-timed.
If one side of the E1 is at PRS AND CAN BE VERIFIED TO MAINTAIN TRACEABILITY TO A PRS, establish both NEs as loop recovered (this is equivalent to the "Centralized Clock Interface" defined in G.703 and can be achieved using a Line Retiming Unit). If the slips do not stop, examination of the network and the employed NEs will be required.
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