Whether attenuators are required depends on the transmit power at the Tx end and the receive power range allowed at the Rx end. If the transmit power at the Tx end is beyond the receive power range allowed at the Rx end, attenuators are required. The path loss is small in short-distance transmission. If optical modules with a long transmission distance (generally more than 40 km) are used for short-distance connection, it is recommended that you use attenuators to reduce the receive power at the Rx end.