220.127.116.11. 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 (comprising seven bytes of preamble, and final 1-byte SFD) on a dedicated preamble bus, l2_tx_preamble[63:0]. In this case, 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.
When l2_tx_startofpacket is asserted, l2_tx_preamble[63:0] contains the preamble data and l2_tx_data[127:0] contains the first 16 bytes of frame data, starting from destination address.
Note that a single parameter in the 50GbE parameter editor turns on both RX and TX preamble passthrough.
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