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The bit error Constantly changing due to PMD

Publication Date:  2012-07-25 Views:  76 Downloads:  0
Issue Description
As the network topology of  Attachment:
When we finish the system debug,Ashgabat to Mary direction has bit error,and this bit error is constantly changing,after we cleaning the patch-cord and changing the fix attenuation,system performance is good,the bit error before FEC of two wavelength is 0 .but the next day,we found the system performance become bad and began to constantly changing,and some time it will appear OTU2_LOF. The performance of system bit error as following:(the following is bit error performance before FEC).
05/15/2012 15:00:00      4.850e-008(14.538118487400094)
05/15/2012 15:15:00      8.690e-007(13.591508203489331)
05/15/2012 15:30:00      2.290e-008(14.754717642259422)
05/15/2012 15:45:00      1.450e-007(14.201939669094795)
05/15/2012 12:00:00      1.400e-001(0.6215709188337215)
05/15/2012 12:15:00      1.890e-004(11.015997336181623)
05/15/2012 12:30:00      1.780e-003(9.290013644042697)
05/15/2012 13:45:00      8.290e-004(9.952483776893814)



Alarm Information
BEFFEC_EXC
OTU2_LOF.

Handling Process
1. we make the loop in FIU to Ashgabt and Mary direction from every OLA site, and in Tejen to Ashagat and in 
      RSS22  to Mary direction, The performance is Constantly changing. it shows that this two section of fiber
 2. we change the tejen and RSS22 site to REG site and the problem solved.

Root Cause
 1. We use the Spectrum analyzer to test  the OSNR of the system,and the OSNR is 21dB, bigger than the  
       design value, so the OSNR is normal.
 2. We check every optical amplifier‘s input and output power, it is ok, so exclude the reason of non-liner.
 3. We use the 100G board LTX to test the CD dispertion. it is about Owed to fill 10km,it is ok,so exclude the 
        reason CD dispertion.
 and the 100G board performance is also Constantly changing. so we think it is the reason of PMD. In 10G 
 system, the PMD must be smaller than 20ps, if it is bigger, the cost of OSNR will become very much, about  
 more than 5 dB. 
 PMD is the fundamental property of optical components and fibers by which light wave signal is split into two  
 polarization modes that travel at different speed. The difference between the propagation time of two  
 polarization modes is called differential group delay (DGD).
 As the picture 1 we can see that PMD will change with the time. and as the picture 2, we can see that  In 10G 
 system, when the system is bigger than 20ps, the  the cost of OSNR will become very much.


Suggestions
Usually, we think the PMD has no effect on 10G system, but actually, the 10G system's tolerance is 20ps from end to end, if it is more bigger, the system can't work normally.

END