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CUSTOMER NETWORK SITE-A SITE-B Optical Power Budget Unbalanced Root Cause Analysis and Workaround

Publication Date:  2017-01-13 Views:  637 Downloads:  7
Issue Description

Customer Optical Transport Network spans across West Europe including backbone, aggregate and access/customer layers utilizing OptiX OSN 8800 and OptiX OSN 1800 devices. Within the network somewhere in Madrid, Spain there is a link between SITE A and SITE B where the optical power budget is highly out of product specifications and unbalanced. In particular the EDFA amplifiers have reached maximum output power levels reporting relative alarms and as a result the capacity of the link has been dramatically decreased from 24 OCH End-Of-Life to only 7 OCH. This creates major discrepancy due to all upcoming expansions involving specific link are blocked

Handling Process

The fault rectification process is comprised of the following steps:

1. Acquire Network Data:

  • U2000 logical data and optical power measurements
  • Onsite physical data and optical power measurements (optical power meter, optical spectrum analyser)

2.  Process Network Data:

  • cross-check U2000 logical data (e.g. logical fibres) with onsite physical data (e.g. actual connections) and validate correct data
  • cross-check U2000 optical power measurements (e.g. MCA readings) with onsite optical power measurements (e.g. OSA readings) and validate correct data
  • process validated data

3. Root Cause Analysis:

  • output root cause analysis based on data process

4. Solution/Workaround Proposal

  • output solution/workaround based on root cause analysis
Root Cause

Major root cause was the mis-design of the link in the various network phases:

  • during initial deployment EDFA amplifiers were added only on the transmit direction
  • during expansions new channels inserted where suffering excessive attenuation due to multiple insertion losses of the many optical components 

As a result the existing EDFA amplifiers were had to be tuned in very high output power levels in order for the transmitted channels to be received at the far ends above the lower sensitivity thresholds. Root Cause Analysis was reviewed by customer 3rd Line O&M and approved    


Due to high urgency a Workaround was initially proposed as a temporal remedy in order to reduce the high output power levels of the EDFA amplifiers and allow small-scale expansions in the near future. As a final Solution it had been proposed the redesign of the link with the addition of EDFA amplifiers also in the receive direction to cope with the excessive attenuation values

The basic goals and limitations of the proposed Workaround were:

  • to improve current optical performance by reducing link's optical power budget by around 4dB
  • to achieve a 3dB margin from the lower sensitivity threshold of the line cards
  • to not affect live network at any time
  • to not produce any new alarms

The basic corrections of the proposed Workaround were:

  • basic adjustments of the aggregate signal by modifying the MR8V VOA of the multiplexed signal 
  • fine tuning of the channels by modifying the MR8V VOAs of each channel  

Proposed Workaround was reviewed by customer 3rd Line O&M and approved. Implementation was done during maintenance window (low traffic hours) and all goals were successfully met


Such major fault can be prevented with correct design from the initial deployment. When designing a DWDM link a number of factors needs to be properly considered e.g. EOL link capacity. In addition during capacity expansions it needs to be verified that each added channel can effectively traverse the existing hardware components (suffering insertion losses) but keeping its optical power levels within product specifications or regeneration needs to be introduced