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Connected OTU unit of DWDM induces OSN 7500 SL64 board report Bit_Error

Publication Date:  2012-07-25 Views:  124 Downloads:  0
Issue Description
In a case, the singal diagram is as follow:

from the diagram we can see this STM-64 link was beared by one WDM Och.the SDH equipment was OSN 7500, WDM equipment was BWS 1600G.
once the SL64 of SITE A report B1,B2 BIT_ERROR,also SITE B SL64 report SM_BDI alarm.

Alarm Information
The SL64 of SITE A report B1,B2 BIT_ERROR,also SITE B SL64 report SM_BDI alarm.however, the perfermance of both LWFS boards WDM site were good, no any BIT_ERROR. 

Handling Process
1,in SITE A :from LWFS client side make a hardware loop to SL64(with 7db attenu. to protect the optical module ), check 15 minutes the BIT_ERROR the fault of both SL64 and the patchcord which connect between WDM and SDH were excluded.
2,in SITE B:do the same operation as SITE A, also the SM_BDI alarm in the SL64 of this site disappeared, it proved that both the SL64 and patchcord in this site were also ok.
3, in SITE A:make a self loop of the LWFS client side(also with 7db attenu.)check the perfermance of the RX/TX port, some BIT_ERROR were found then. so obviously, this board was fault.
4,change the LWFS board in SITE A, then check 15 minutes perfermance of this link again. there is no BIT_ERROR any more. the problem solved. 
Root Cause
there are many reasons such as input power low,dispersion or the fault of the the board can casue bit_error,in this case,however ,because of the WDM side no bit_error and all the perfermance such as Inputpower and BEFFEC were we can conclude the WDM link was good. that means also the outdoor fiber was ok, query input power of SL64 of SITE A, -13.5,was relatively low, SL64 input power in SITE B, -9.8, perfect.
so we can located the fault must be occured in the client side of WDM or the board of SL64 itself was suspected to be fault,the possibilities were as follows:
1,SITE B :sending side problem,  SL64 board sending fault or LWFS client side(RX/TX) receiveing fault;
2,SITE A: receiving side problem, SL64 board receiveing fault or LWFS client side(RX/TX) sending fault;
solve the problem of WDM or SDH link, loopback is a very useful method.