The bit errors occur in a DWDM 10G network due to the incorrect PMD.
Symptom
After the fiber of the OTM site (node CLS1 to node CLS2) is replaced, the MSBBE bit errors and OTU_LOF alarm occur on the OTM site.
Procedure
- The optical power is measured and adjusted to the reference value. After the adjustment, bit errors and the alarm are cleared. Half an hour later, the bit errors and alarm occur again.
- Add or remove the DCM to extend the distance from 5 km to 10 km. Then, the bit errors and alarm are cleared. One hour later, the bit errors and alarm occur again.
- Then, the customer checks the features of the new fiber (chromatic dispersion and polarization mode dispersion). The check shows that the chromatic dispersion (CD) in the fiber is normal but the polarization mode dispersion (PMD) is excessively high. The problem is solved when the customer replaces the fiber.
Reference Information
PMD coefficient relation is given in Table 1.
Table 1 PMD coefficient relation
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Bit Rate(Gbit/s)
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Max PMD(ps)
a
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PMD Coefficient (ps.km)
a
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2.5
|
40
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< 2.0
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10
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10 (with no FECb)
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< 0.5
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40
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2.5
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< 0.125
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a: Standards suggest a value for 10 Gbit/s only. Values at other data rates are nonetheless accepted by the industry.
b: Most long-haul instruments perform forward error correction (FEC).
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