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. Record the receive min and max of your optic.
3. Record the OTDR test results that give total dB loss. 
   1. If you don't have OTDR test results, estimate based on the known distance of the fiber.
4. Estimate if a pad is needed:

   1. -(connector loss) -(OTDR OR distance estimated loss) -(pad loss) = -(target RX level dB away from TX level)

5. Ship the pad

Example:

1. I'm transmitting at +1dB
2. I'm using a FlexOptix SFP+ ZR 1550, min rx -23dB max rx -8dB
3. I don't have an OTDR test but the CAN said the fiber distance is 27km.
   1. 0.4 dB loss per kilometer for 1550nm at 27km, 10.8dB of loss estimated
4. -(connector loss) -(OTDR OR distance estimated loss) -(pad loss) = -(target RX level dB away from TX level)
   -(1) -(10.8) -(pad loss) = -(13)
   -(pad loss) = -1.2
5. The above estimate shows I'm within 1.2dB of getting a receive level of -12dB, I probably don't need to have a pad installed