High latency is a problem for performance-critical networks in applications such as high-performance computing (HPC), data center interconnection, machine learning, financial trading, and others. The availability of low-latency FEC allows high-speed Ethernet to be better suited to these applications, especially for HPC networks where other interconnect technologies are more critical than Ethernet.
"Five years ago, only HPC developers were concerned about low latency, but today latency sensitivity has reached many more mainstream applications," says Rob Stone, chair of the 25G Ethernet Consortium's technical working group. "With this new specification, the consortium is improving the largest source of packet processing latency, thereby enhancing the performance that high-speed Ethernet brings to these applications.".
FEC is a major source of latency in a switched network, and the new specification reduces FEC latency by approximately half. This will have a significant impact on overall physical layer latency, particularly for hyperscale data center networks consisting of a large number of nodes with multiple hops between servers.
The specification allows NEMs to optionally use an abbreviated codeword variant of FEC - RS(272, 257+1, 7, 10) which replaces the IEEE 802.3cd and 802.3bs FEC standard.
The abbreviated codeword contains 272 x 10⁶ bits instead of the originally specified 544 x 10⁶ bits. Nothing else changes in the process of distributing symbols from the encoder output to the FEC lanes in the new FEC, but that process is implemented more quickly due to the reduced codeword.
"The value of having a standard extends to ensuring interoperability across multiple vendors and the ecosystem," says Tim Lustig, president of marketing for the 25G Ethernet Consortium. "To ensure compliance with specifications and interoperability, testing will be conducted in future trials that the consortium regularly performs at the University of New Hampshire Interoperability Laboratory (UNH-IOL).".
The original FEC standards are designed to maximize data integrity and minimize data packet retransmissions due to packet loss or corruption during transmission. Simulations conducted by the consortium using the new FEC standard support copper cable lengths up to 2 m and fiber optic cable lengths up to 30 m. To ensure high data integrity, the consortium recommends that the new FEC be used only in engineered data connections.
