L- and H-Tile Transceiver PHY User Guide

ID 683621
Date 1/30/2024
Document Table of Contents KR FEC Blocks

The KR FEC blocks in the Enhanced PCS are designed in accordance with the 10G-KRFEC and 40G-KRFEC of the IEEE 802.3 specification. The KR FEC implements the Forward Error Correction (FEC) sublayer, a sublayer between the PCS and PMA sublayers.

Most data transmission systems, such as Ethernet, have minimum requirements for the bit error rate (BER). However, due to channel distortion or noise in the channel, the required BER may not be achievable. In these cases, adding a forward error control correction can improve the BER performance of the system.

The FEC sublayer is optional and can be bypassed. When used, it can provide additional margin to allow for variations in manufacturing and environmental conditions. FEC can achieve the following objectives:

  • Support a forward error correction mechanism for the 10GBASE-R/KR and 40GBASE-R/KR protocols
  • Support the full duplex mode of operation of the Ethernet MAC
  • Support the PCS, PMA, and Physical Medium Dependent (PMD) sublayers defined for the 10GBASE-R/KR and 40GBASE-R/KR protocols

With KR FEC, the BER performance of the system can be improved.

Transcode Encoder

The KR forward error correction (KR FEC) transcode encoder block performs the 64B/66B to 65-bit transcoder function by generating the transcode bit. The transcode bit is generated from a combination of 66 bits after the 64B/66B encoder which consists of a 2-bit synchronization header (S0 and S1) and a 64-bit payload (D0, D1,…, D63). To ensure a DC-balanced pattern, the transcode word is generated by performing an XOR function on the second synchronization bit S1 and payload bit D8. The transcode bit becomes the LSB of the 65-bit pattern output of the transcode encoder.

Figure 203. Transcode Encoder

KR FEC Encoder

FEC (2112,2080) is an FEC code specified in Clause 74 of the IEEE 802.3 specification. The code is a shortened cyclic code (2112, 2080). For each block of 2080 message bits, another 32 parity checks are generated by the encoder to form a total of 2112 bits. The generator polynomial is:

g(x) = x32 + x23 + x21 + x11 + x2 +1

KR FEC Scrambler

The KR FEC scrambler block performs scrambling based on the generation polynomial x58 + x39 +1, which is necessary for establishing FEC block synchronization in the receiver and to ensure DC balance.

KR FEC TX Gearbox

The KR FEC TX gearbox converts 65-bit input words to 64-bit output words to interface the KR FEC encoder with the PMA. This gearbox is different from the TX gearbox used in the Enhanced PCS. The KR FEC TX gearbox aligns with the FEC block. Because the encoder output (also the scrambler output) has its unique word size pattern, the gearbox is specially designed to handle that pattern.