Forward Error Correction (FEC) in 100G Data Transmission
Updated at Dec 14th 20241 min read
As the demand for bandwidth increases, the acceptance of errors and delays decreases. Data communication system designers are constantly searching for ways to boost bandwidth. They also aim to improve transmission quality. Though not new, one solution has proven highly effective: Forward Error Correction, a technique that has been used for years to enable efficient, high-quality data communication over noisy channels.. With the growing need for greater data transmission capacity and extended distances, let’s explore how FEC is transforming optical networks today.
What Is FEC and How Does It Work?
Forward error correction (FEC full form in networking) is a digital signal processing technique used to enhance data reliability. It introduces redundant data, called error-correcting code, before data transmission or storage. The term "FEC" stands for "Forward Error Correction," a crucial method in networking and telecommunications that ensures data integrity over potentially unreliable or noisy communication channels. FEC enables the receiver to fix errors without needing a reverse channel to request the sender to resend data. By embedding redundancy within the transmitted data, FEC improves network efficiency and reduces latency, as retransmissions are minimized.The diagram below provides a simplified overview of how this works.
This feature is very important in optical networks. Signal degradation during transmission can lead to errors at the receiving end. This can confuse a "1" with a "0" or the other way around. If the number of errors in transmission is within the correction capacity (discontinuous errors), the channel decoder will locate and correct the false “0” or “1” to improve the quality of the signal.

In optical communication systems, if an error arises in the link, the signal cannot be transmitted further. FEC technology involves encoding the signal to be transmitted into a code with a certain error correction capability at the FEC encoder at the transmitting end. The FEC decoder at the receiving end decodes the received sequence. If the number of errors generated during transmission is within its error correction capability (non-continuous errors), the errors are located and corrected.
FEC operates using different FEC modes, such as in-band and out-of-band modes, to ensure error correction across diverse network conditions. In-band FEC inserts the error-correcting code directly into the data being sent. Out-of-band FEC uses separate channels for this. This gives flexibility based on the system design and performance needs. This process effectively reduces error rates, enhances signal reliability, and extends the potential transmission distance.

Forward Error Correction Types & Features
At present, the practical FEC technologies for SDH (Synchronous Digital Hierarchy) and DWDM (Dense Wavelength Division Multiplexing) are mainly as follows: In-band FEC(coding gain is small 3-4dB)、Out-of-band FEC(5-6dB) and Enhanced FEC (EFEC).
FEC reduces the number of transmission errors, extends the operating range, and reduces the power requirements for communications systems.
FEC increases the throughput of the effective system, even with the extra check bits added to the data bits, by eliminating the need to re-transmit data corrupted by random noise.
FEC independently increases the reliability of data at the receiver.
Ethernet FEC is essential for high-speed networks, particularly in 100G systems. It uses two main encoding types: hard-decision and soft-decision. Hard-decision is simpler but provides less error correction, while soft-decision offers better correction, especially in challenging environments.
FEC improves performance by reducing errors without requiring costly upgrades, extending transmission distances (up to 30-40% more on 100G links with SD-FEC), and cutting down on retransmissions, saving bandwidth. However, it can increase latency due to overhead bytes and requires matching FEC types at both ends, which can complicate configurations, especially with equipment from different vendors. Despite these challenges, FEC is crucial for reliable, high-speed data transmission.
Application of Forward Error Correction in 100G Networks
In fiber-optic networking, Forward Error Correction (FEC) is used to address optical Signal-to-Noise Ratio (OSNR), one of the key parameters that determine how far a wavelength can travel before it needs regeneration. FEC is especially important at high-speed data rates, where advanced modulation schemes are required to minimize dispersion and signal correspondence with the frequency grid.
Without FEC, 100G transport would be limited to extremely short distances. To implement long-haul transmission (> 2500 km), the system gain must be further improved by approximately 2 dB. FEC's upgrade from hard-decision to soft-decision fills this performance gap.
As the push for ever-higher transmission rates has continued, soft-decision Forward Error Correction (SD-FEC) schemes have grown in popularity. Although these can require a byte overhead of around 20% — nearly three times as large as the original RS coding scheme — the gains they produce in the context of high-speed networking are substantial. FEC, which results in a 1 to 2 dB gain on a 100G network, for instance, translates to a 20% to 40% increase in reach.
Matters Needing Attention to Forward Error Correction in 100G Networks
What to consider when configuring FEC in 100G networks? It is suggested to pay attention to the following tips.
FEC Implementation Methods in 100G Networks
Some special modules have their own FEC functions, such as the FS 100G QSFP Single Lambda module. While other 100G QSFP28 optical modules mainly rely on FEC function configuration on the device to realize error correction, such as 100G switches.
Switch Compatibility with FEC
The configuration of FEC on 100G switches can be achieved only if the switch supports it, and not all switches do so.
Whether to Enable FEC on 100G QSFP28 Transceivers
The FEC function is not just an advantage, the process of correcting error code will inevitably cause some data packet delay. Therefore, not all 100G QSFP28 transceivers need it. According to IEEE standard protocol, it is generally not recommended to enable FEC when using QSFP28-LR4-100G transceivers. Since the technology of 100G QSFP28 optical modules varies from company to company, the situation is not exactly the same. The following table explains whether it is recommended to enable FEC when using the FS 100G QSFP28 optical modules.
Transceiver Type | Description | With FEC |
QSFP28-LR4-100G | 1310nm 10km Transceiver Module for SMF | No |
QSFP28-PSM4-100G | 1310nm 500m Transceiver Module for SMF | No |
QSFP28-SR4-100G | 850nm 100m MTP/MPO Transceiver Module for MMF | Yes |
QSFP-DR-100G | Single Lambda 1310nm 500m Transceiver Module for SMF | Yes |
QSFP-LR-100G | Single Lambda 1310nm 10km Transceiver Module for SMF | Yes |
QSFP28-CWDM4-100G | 1310nm 2km Transceiver Module for SMF | Yes |
QSFP-4W10-100G | 1310nm 10km Transceiver Module for SMF | Yes |
QSFP-ER4L-100G | 1310nm 40km Transceiver Module for SMF | Yes |
QSFP-ER4L-100G | 1310nm 40km Transceiver Module for SMF | Yes |
The FS 100G QSFP28 optical module uses high-quality components and rigorous testing to ensure reliable, uninterrupted performance with FEC enabled, reducing network interruptions. FEC also extends transmission distances under poor signal quality and minimizes delays by reducing retransmissions. Built for perfect compatibility and original quality, the module supports multi-vendor devices and ensures seamless integration. With FS Box real-time re-coding and professional testing reports, FS helps customers quickly adapt to their network, simplifying deployment and ensuring stable, efficient operation.
FEC Function Consistency at Both Ends of the Link
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 is not up.
FEC Configuration and Stacking Compatibility
Configuring the FEC command is not supported if the port is already configured as a physically stacking port. Conversely, ports that have been configured with FEC commands do not support configuring as a physical stacking member.
By addressing these considerations, you can ensure efficient FEC configuration and maintain the performance and reliability of your 100G network.
Conclusion
FEC plays a crucial role in maintaining the reliability and performance of fiber optic networks, especially as speeds increase to 40G and 100G. With higher speeds, poor optical signal-to-noise ratio (OSNR) becomes a common challenge.FEC ensures data integrity and reduces errors caused by such conditions.
For optimal performance and seamless network operation, FS’s 100G optical modules, equipped with advanced FEC technology, offer reliable solutions for modern network demands. Discover FS’s range of 100G optical modules to enhance your network today.