The preamble field is used for synchronization and must contain seven bytes with the pattern 10101010. If a collision is detected during the transmission of the preamble or start-of-frame delimiter fields, the transmission of both fields is completed.
For transmission, this field is always automatically inserted by the AXI Ethernet Lite MAC core and should never appear in the packet data provided to the AXI Ethernet Lite MAC core. For reception, this field is always stripped from the packet data. The AXI Ethernet Lite MAC design does not support the Ethernet 8-byte preamble frame type.