- 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).
- Always pad optics on the the rx ports (one pad on the rx port of the hub interface, one pad on the rx port of the cpe interface).
Notes/Key:
- Standard estimates:
- 0.4 dB loss per kilometer for 1550nm <- is this high? DanP says .25dB loss per km at 1550
- 0.5 dB loss per kilometer for 1310
- 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
- Subtract the OTDR results (loss in dB)
- If you don't have OTDR test results, subtract the estimated loss based on distance
- Subtract the connector loss
- Record the result
- Is the result within your optic's receive min -> max? If not, add the appropriate pad
Example:
- I'm transmitting at +1dB
- I don't have an OTDR, but the CAN says the distance is 27km
- Subtract 0.4 dB loss per kilometer for 1550nm at 27km, subtract 10.8dB
- I'm using two patch cables, one on each side, subtract ~1dB
- Result: −10.8
- I'm within the -8dBm to -23dBm receive specification for the optic, I probably won't need to ship a pad