Overview
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