Your goal should be to try and get the receive levels on your optics within the thresholds given by the optic/manufacturer. This applies to longer distance DWDM and CWDM optics (40k, 80k, 120k).
Notes/Key:
- 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
- Example: https://www.flexoptix.net/en/sfp-plus-zr-transceiver-10-gigabit-stm64-sm-1550nm-80km-23db-ddm-dom.html
Calculating Loss:
- Record the TX power of your optic.
- Record the receive min and max of your optic.
- Record the OTDR test results that give total dB loss.
- If you don't have OTDR test results, estimate based on the known distance of the fiber.
- Record the estimated connector loss if it's not included in the OTDR (how many patch cables/connectors)
- Estimate if a pad is needed:
-(connector loss) -(OTDR OR distance estimated loss) -(pad loss) = -(target RX level dB away from TX level)
- Ship the pad
Example:
- I'm transmitting at +1dB
- I'm using a FlexOptix SFP+ ZR 1550, min rx -23dB max rx -8dB
- I don't have an OTDR test but the CAN said the fiber distance is 27km.
- 0.4 dB loss per kilometer for 1550nm at 27km, 10.8dB of loss estimated
- I'm using two patch cables, one on each side, ~1dB
- -(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 - 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