The RS-FEC feature of the 1/10/25G Subsystem provides error correction capability according to IEEE 802.3 Clause 108 or Schedule 3 of the 25G Ethernet Consortium.
The feature requires the insertion of PCS alignment markers as defined in IEEE 802.3 Table 82-2. Inputs are provided for the alignment markers and also for the value of words between alignment markers.
It is possible to bypass the RS-FEC function by means of the enable signals. This will bypass the RS-FEC function and connect the PCS directly to the transceiver, with the benefit of reduced latency. Refer to 25G IEEE 802.3by Reed-Solomon Forward Error Correction LogiCORE IP Product Guide (PG217) (registration required) for the latest latency performance data in the various bypass modes, defined as follows:
- FEC Bypass Correction: The decoder performs error detection without correction, (see IEEE Std 802.3by section 184.108.40.206. The latency is reduced in this mode (see 25G IEEE 802.3by Reed-Solomon Forward Error Correction LogiCORE IP Product Guide (PG217) (registration required) for latency figures).
- FEC Bypass Indication: In this mode there is correction of the data
but no error indication. An additional signal,
rx_hi_ser, is generated in this mode to reduce the likelihood that errors in a packet are not detected. The RS decoder counts the number of symbol errors detected in consecutive non-overlapping blocks of 8192 codewords (see IEEE Std 802.3by section 220.127.116.11). The latency is reduced in this mode.
- Decoder Bypass: The RS decoder can be bypassed by setting the IEEE Error indication Low when the correction bypass and indication bypass are High.