FEC ( forward error correction ) is used for correct errors in data transmission.

The idea is the sender encodes the message with redundant information in the form of an ECC ( error correcting code). The redundancy allows the receiver to detect a limited number of errors that may occur in the message, and often to correct these errors without re-transmission.

This function has become increasingly important in optical fiber communication systems to meet growing demand of higher bandwidth and longer transmission distance for telecom operators and system integrators.

Although FEC has benefits for increasing transmission distance, such as 100G QSFP28 ER4 module, without FEC normally transmission distance is 30km, with FEC support can reach 40km. It also has dis-advantage, inevitably cause some delay in data packet transmission in the process of error correction. So it is not recommend for all 100G QSFP28 transceiver to open this function.

Below points need pay attention for FEC:

1. Implementation: Host device need support FEC function

Some optical transceiver modules itself has FEC function, such as CFP module. But majority optical modules, like 100G QSFP28, not have this function, it is reply on the host device, like switch to support FEC configuration. If equipment not support this then can’t implement FEC function. Not all switch are designed to support FEC, this point need be noted.

2. Both Ends of the Link need support FEC function then can link up

The FEC function of the port is part of the auto-negotiation. When auto-negotiation of the port is enabled, the FEC function is determined by negotiation at both ends of the link. If the FEC function is enabled at one end, the other end should also enable it, otherwise, the port link is not up.

3. Not every optical transceiver module suggest to use FEC

It will have transmission delay in the process of error correction. So not necessary to for optical transceiver modules. According to IEEE standard protocol, it is not recommended to enable FEC when using QSFP28-LR4 transceivers. For longer transmission distance, 100G QSFP28 ER4 and ZR4 recommend enable this function.