Low latency spec for 50GbE tweaks forward error correction

The 25 Gigabit Ethernet Consortium has completed a low-latency forward error correction (FEC) specification for 50 Gbps, 100 Gbps and 200 Gbps Ethernet networks.

The new spec cuts FEC latency approximately in half by using a shortened codeword FEC variant – RS (272, 257+1, 7, 10) that replaces the IEEE 802.3cd and 802.3bs standard FEC.  The shortened codeword contains 272 x 10-bit symbols rather than the 544 x 10-bit symbols originally specified. Nothing else changes in the symbol distribution process from the output of the encoder to the FEC lanes in the new FEC, but that process is implemented more quickly due to the shortened codeword.

This will have a significant impact on overall physical layer latency, in particular for hyperscale datacenter networks comprised of a large number of nodes, with multiple hops between servers.

“Five years ago, only HPC developers cared about low latency, but today has latency sensitivity has come to many more mainstream applications,” said Rob Stone, technical working group chair of the 25G Ethernet Consortium. “With this new specification, the consortium is improving the single largest source of packet processing latency, which improves the performance that high-speed Ethernet brings to these applications.”

The specification is available at https://25gethernet.org/ll-fec-specification