Optical: Calculating Loss for Pads/Attenuators

• Standard estimates:
  • 0.4 dB loss per kilometer for 1550nm
  • 0.3 dB loss per connector
• Command line to see reported RX high and low levels and current levels:

  • Juniper: 

    show interfaces diagnostics optics xe-0/0/1

  • Cisco: 

    show interfaces gigabitEthernet 0/1 transceiver detail

  • Brocade: 

    show optic thresholds 1/1/1

• The optic vendor should also have the rx and tx thresholds documented:
  • Example: https://www.flexoptix.net/en/sfp-plus-zr-transceiver-10-gigabit-stm64-sm-1550nm-80km-23db-ddm-dom.html

    • RECEIVE MIN. -23.00dBm
      RECEIVE MAX. (RECEIVER OVERLOAD) -8.00dBm
      TRANSMIT MIN. 0.00dBm
      TRANSMIT MAX. 5.00dBm

Calculating Loss:

1. Record the TX power of your optic
2. Subtract the OTDR results (loss in dB)
   
   1. If you don't have OTDR test results, subtract the estimated loss based on distance
3. Subtract the connector loss
4. Record the result
5. Is the result within your optic's receive min -> max? If not, add the appropriate pad

Example:

1. I'm transmitting at +1dB
2. I don't have an OTDR
   1. Subtract 0.4 dB loss per kilometer for 1550nm at 27km, subtract 10.8dB
3. I'm using two patch cables, one on each side, subtract ~1dB
4. Result: −10.8
5. I'm within the -8dBm to -23dBm receive specification for the optic, I probably won't need to ship a pad