184.108.40.206. Preamble Insertion
In the TX datapath the MAC prepends an eight-byte preamble to the client frame. If you turn on Enable link fault generation, this MAC module also incorporates the functions of the reconciliation sublayer (RS).
The source of the 7-byte preamble (including a Start byte) and 1-byte SFD depends on whether you turn on Enable preamble passthrough in the parameter editor.
If the preamble pass-through feature is enabled, the client provides the eight-byte preamble (including the 0xFB Start byte and final 1-byte SFD) on l1_tx_data. The client is responsible for providing the correct Start byte (0xFB) and an appropriate SFD byte. If the preamble pass-through feature is disabled, the MAC inserts the standard Ethernet preamble in the transmitted Ethernet frame.
Note that a single parameter in the F-Tile Low Latency 50G Ethernet Intel® FPGA IP parameter editor turns on both RX and TX preamble passthrough.