What is FEC (Forward Error Correction) in high-speed optics?

FEC (Forward Error Correction) is a technique that adds redundant bits to transmitted data, allowing the receiver to correct errors without retransmission.

Why FEC is Needed

• Higher data rates increase raw bit error rates
• Copper: signal degrades over distance
• Optical: dispersion, noise, low signal margins
• Standards require BER better than 10^-12
• Native BER for 25G+ often 10^-5 to 10^-8 without FEC

FEC Types

• FC-FEC (Fire Code): Firecode BASE-R, low overhead, low latency
• RS-FEC (Reed-Solomon): Better correction, higher latency, RS(528,514) for 25G
• KP4/KR4: Various standards for different speeds

Where FEC is Used

Impact on Latency

• FC-FEC: adds ~80ns
• RS-FEC: adds ~100-250ns depending on speed
• Usually negligible except HFT/real-time

Configuration Challenges

• Both ends must use matching FEC type
• Mismatch = link never comes up or unstable
• Auto-negotiation helps but isn't universal
• Some optics have fixed FEC, others configurable

Troubleshooting FEC Issues

• Check for FEC symbol errors (indicate near-limit performance)
• Uncorrected codewords = data corruption
• Monitor via FEC statistics in switch CLI
• Excessive corrections indicate marginal link

FEC and Distance

FEC often enables longer distances by tolerating lower SNR. E.g., 25G LR at 10km relies on RS-FEC.